ACK1/TNK2 kinase: Molecular mechanisms and Emerging Cancer Therapeutics

Abstract

Activated Cdc42-associated kinase 1 (ACK1), encoded by TNK2 gene, is a cytoplasmic non-receptor tyrosine kinase whose aberrant activation positively correlates with cancer severity. Recent research has revealed functional relevance of this oncokinase- an epigenetic regulator driving cancer progression in multiple malignancies. Despite ACK1 being an attractive target for therapeutic intervention, incomplete knowledge of its diverse signaling mechanisms and lack of specific inhibitor has challenged its clinical success. Here, we summarize the recent breakthroughs on ACK1 regulation and its cellular signaling, and shed light on its immunomodulatory role balancing T cell activation. We provide a comprehensive consolidation of the pre-clinical, proof-of-concept studies employing potent ACK1-targeting small-molecule inhibitors that are expected to enter clinical trials for cancer patients.

ACK1 kinase structure and activation

Activated Cdc42-associated kinase 1 (ACK1) is an atypical class VIII oncogenic non-receptor tyrosine kinase encoded by the TNK2 gene, located on human chromosome 3q29 [1]. Containing 1038 amino acids, it is a widely expressed cytoplasmic protein with high expression in bone marrow and lymphoid tissues, as well as the brain, muscles, gastrointestinal tract, liver, and skin (https://www.proteinatlas.org/). The protein structure indicates the presence of eight discrete domains (Figure 1A) [2, 3]. The studies on functional relevance define ACK1 as a regulatory kinase transducing signals from multiple RTKs including MER proto-oncogene tyrosine kinase (MERTK), insulin receptor, platelet derived growth factor receptor (PDGFR), human epidermal growth factor receptor 2 (HER2), and epidermal growth factor receptor (EGFR), to the cytosolic or nuclear effector proteins; and thus can be activated by an array of ligands including growth arrest-specific protein 6 (Gas6), insulin-like growth factor (IGF), insulin, PDGF, heregulin and EGF, [3-5]. Other known ACK1 activating receptors include the muscarinic M3 receptor and integrins [2]. Though deregulated ACK1 has been consistently observed in neurological (Box 1), autoimmune disorders (Box 2) and cancers [1, 6-9], the plethora of signaling mechanisms in which ACK1 plays a central role remains largely unknown.

Figure 1. Domain Architecture of ACK1 and role in T cell regulation.

(A) Domain Architecture of ACK1 and its phosphorylation sites. Abbreviations: SAM, Amino-terminal sterile α motif; NE, nuclear export signal; SH3, Src homology domain; CRIB, Cdc42/Rac interactive binding domain; CBD, Clathrin interacting domain; MHR, Mig6 homology region; UBA, Ubiquitin-association domain. Phosphorylation sites are numbered in red.

(B) In the basal state of T cells, ACK1 phosphorylates CSK at Tyr18 and PAG at Tyr227, Tyr317, and Tyr417. The phosphorylation of CSK leads to the inhibitory phosphorylation of LCK (pY505-LCK) and hampers T cell activation. Upon genetic or pharmacological inhibition of ACK1, unphosphorylated CSK allows LCK to gain activating phosphorylation (pY394-LCK) that subsequently relays a series of signals to proximal kinases LAT and ZAP-70 to activate T cells. During resistance to immune checkpoint blockade therapy, the reactivation of ACK1/pY18-CSK signaling causes suppression of T cell activation. Abbreviations: PAG, glycosphingolipid-enriched microdomains 1; CSK, C-terminal SRC kinase; LCK, lymphocyte-specific protein tyrosine kinase, ZAP-70, Zeta-chain-associated protein kinase 70. PD-1, Programmed cell death protein 1; CTLA-4, cytotoxic T-lymphocyte-associated protein 4. Figure created with BioRender.

Box 1. ACK1 in Neuronal Circuitry.

ACK1 is expressed in cerebral cortex, hippocampus, cerebellum and olfactory bulb, and its enriched expression dictates migration and maturation of neurons during brain development. During neuritogenesis, ACK1 exhibits an increased kinase activity in response to neurotrophins and interaction with Trk receptors, which promotes neuritic outgrowth and branching [64].

Neuron specific interacting partners of ACK1 identified by proteomic screening of adult and postnatal mice brain include adhesion regulators (NCAM1, neurabin-2), synapse mediators (SynGAP, GRIN1 and GRIN3) and protein kinase (CAMKII). Colocalization with CAMKII in neurons indicates the role of ACK1 in physiological plasticity and neurotransmission [65]. ACK1 also regulates the terminal differentiation of neuronal progenitors upon recruitment by CDC42b, a splice variant of CDC24 (Rho-family small GTPase) to “cut-off” mTOR function, controlling the multipotency and pro-survival influence in neurons [66, 67].

Dopamine (DA) neurotransmission via dopamine transporter (DAT) controls multiple cognitive functions [68]. Deregulation of DA trafficking including the incidence of mutant DATs and associated signaling causes a spectrum of neurological disorders [69]. ACK1 acts as a clathrin-dependent endocytic brake causing DAT stability at the plasma membrane upon protein kinase C (PKC) activation and its inactivation enhances DAT internalization. Constant activation of ACK1 counteracts the gain-of-function phenotype created by the attention-deficit/hyperactivity disorder (ADHD) DAT encoding variant, R615C. This offers a potential platform for therapeutic intervention. [62]. A parallel mechanism involving the neuronal GTPase, Rit2, contributes to PKC-activated DAT internalization and converges with ACK1 function at the transporter independent of each other [70].

Evaluation of systemic RNA interference protein-3 (SID-3), a C. elegans (animal model for progressive dopaminergic neurodegeneration) ortholog for TNK2, showed inability of NEDD4 (neuronal precursor cell-expressed developmentally down-regulated-4) to degrade ACK1 or aberrant ACK1, induced by single nucleotide polymorphisms. This overactivation mechanism contributed to neurodegeneration in Parkinson’s disease patients [71]. ACK1 inhibitor or N-aryl benzimidazole 2 (NAB2, NEDD4 activator) provides significant neuroprotection. Further, ACK1 mutations have also been reported to be the potentially causative event for onset of infantile epilepsy in autosomal recessive conditions [72]. This finding warrants further investigation.

Box 2. Genetic variations in ACK1 and its role in immunity.

Deregulated ACK1, as documented in the COSMIC (Catalogue Of Somatic Mutations In Cancer) and cBioPortal dataset from 217 studies, reports TNK2 copy number variation (CNV) gain and overexpression (Table 1) that has been associated with cancer incidence. ACK1 overexpression results in its kinase activation, often scored by assessing phosphorylation at Tyr284 (pY284-ACK1). Concurrent with gene amplification, immunohistochemical staining of breast, pancreatic and prostate cancers exhibit increased ACK1 activation that positively correlates with disease progression, with highest expression in the metastatic stage [6, 8, 9, 49, 51]. In contrast, ACK1 mutations are uncommon with a somatic mutation frequency of 0.9% in the TNK2 gene. Of 702 mutations identified, 541 were missense mutations, while 134 were truncating mutations. D495Rfs*, Q508Sfs*, P633Afs* P711Afs* and Q831Rfs* are some of the truncating mutations located at the C-terminus of the kinase domain that cause frameshifts, primarily in colorectal cancers. One of the truncating mutations, P633fs* increases ACK1 oncogenic activity by disrupting the MHR and UBA domains hindering autoinhibition or lysosomal degradation of the kinase respectively [73]. Out of the four somatic mutations - R34L, R99Q, E346K, M409I identified in ACK1, E346K showed ability to enhance kinase activation [49]. In addition, 5 distinct TNK2 gene fusions have been identified, however their physiological significance is yet-to-be deciphered (Table 1).

Catalytically inactive (and with reduced activity) ACK1 mutations have been identified in systemic lupus erythematosus (SLE) patients, where abnormal T cell activation and cytokine production are considered to be the primary cause for initiating and maintaining autoimmune reactions [74]. Pediatric SLE patients with loss-of-function mutations in ACK1 (A156T, K161Q, and R247H) exhibit severe forms of the disease [74], validating the role of ACK1 as a crucial negative regulator of T cell activation. Thus, in the physiological setting ACK1 activation and inhibition has distinct and opposing consequences. Precisely how ACK1 activity is kept in check in healthy immune cells is unclear. Further, in addition to CSK, ACK1 might potentially have other targets in immune cells, the significance of which remains to be investigated [52].

Similar to ACK1, MERTK, a member of the RTK family is associated with cancer and autoimmunity [75] and causes improper clearance of apoptotic cells by macrophages [76]. Interestingly, MERTK is also an upstream activator of ACK1 [76] that binds to the MHR causing a sequential phosphorylation at Tyr859 and 860 prompting the significance of ACK1-MERTK interaction during normal and hyperactivated immune signaling [4]. Further, colon cancer samples with high ACK1 indicate that its overexpression may hinder the function of antigen presenting cells, along with reduced immune infiltration in the tumor microenvironment [37]. The antigen presenting cells - macrophages and dendritic cells (DCs) perform antigen processing, followed by phagocytosis and cytokine production, constituting the first line of innate immune defense [77]. Toll-like receptors (TLRs) are highly conserved membrane bound antigen recognition receptors on the macrophages and DCs whose activation is indispensable for establishing inflammation or auto-immunity [78, 79]. Under conditions that mimic chronic inflammation using endotoxin and chemically induced or genetically modified lupus-prone mouse models of autoimmunity, ACK1 is activated particularly in TLR4, TLR7 and TLR9 pathways. Pharmacological inhibition of ACK1 alleviates TLR-mediated autoimmune conditions, signifying the pivotal contribution of ACK1 in orchestrating autoimmune diseases [80]. Further studies to elucidate the roles of ACK1 in immune cells will further add to the understanding of molecular mechanisms driving the gradual progression of several diseases, particularly autoimmune disorders.

ACK1 has been extensively studied in cancers and identified to be a pro-survival kinase. Erroneous ACK1 activation occurs by at least five distinct mechanisms, including (i) receptor tyrosine kinase amplification or ligand dependent activation leading to ACK1 phosphorylation, (ii) frameshift or other somatic autoactivating mutations in ACK1, (iii) ACK1/TNK2 gene amplification, (iv) TNK2 gene fusions (Table 1), and (v) transcription factor, HOXB13-mediated upregulation [10]. These findings also spurred the development of ACK1 small-molecule inhibitors as potential anti-cancer agents. However, the lack of substantial evidence of ACK1’s involvement in the pathogenesis of several cancer types and its downstream signaling substrates has challenged the progress of ACK1 inhibitors as the standard of care for cancers. The mechanism of action of ACK1 inhibitor, (R)-9b has recently been uncovered using X-ray crystallography (PDB ID is 7KP6). It was demonstrated that when the kinase domain is in the active conformation with helix αC (the structural element of the kinase regulating activation) rotated towards the active site, a salt bridge is formed between E177 and the catalytic lysine (K158) [11]. The activation loop was also shown to be folded underneath helix αC (a structural element involved in regulating kinase activity), in a conformation capable of substrate peptide binding similar to that observed for other active kinases such as, Src and Insulin Receptor Kinase. Moreover, the aspartate of the Asp-Phe-Gly (DFG) motif, which forms part of the ATP-binding site and coordinates magnesium binding, was deeply embedded into the active site and formed a hydrogen bond with the side chain of S136 at the turn of the phosphate-binding loop [11]. The change in the DFG motif and helix αC are the two main events needed for the activation of the kinase and elucidation of these motifs in the active form of ACK1 could pave way to design next generation of small molecule inhibitors targeting the ACK1 kinase activity.

Table 1:

Frequency of copy number variations, amplification, fusions in ACK1/TNK2 gene (see footnote^*) and phosphorylation sites of ACK1 and its substrates

Copy Number Variations
Cancer Type	Gain (%)	Overexpression (%)
Cervical cancer	17	26
Lung cancer	12.7	9.3
Upper aerodigestive tract cancer	10.2	18.2
Esophageal cancer	3.3	13.6
Ovarian cancer	6.3	27.8
Endometrial cancer	2.9	11
Gene Amplification or Gain
Cancer Type	Frequency (%)
Lung squamous cell carcinoma	30
Esophageal adenocarcinoma	19
Urothelial carcinoma	17
Ovarian serous cystadenocarcinoma	16.3
Cervical squamous cell carcinoma	14.3
Head and neck squamous cell carcinoma	13.1
Pancreatic cancer	11
Metastatic prostate adenocarcinoma	6.5
Gene fusions
Fusion	Cancer Type	Reference
TNK2-MELTF	Seminoma	[81]
PLXNA1-TNK2	Cutaneous melanoma	[82]
TNK2-DLG1	Gastric adenocarcinoma	[83]
JAG1-TNK2	Breast invasive ductal carcinoma	[84]
TERF2-TNK2	Uterine endometrioid cancer	[84]
Phosphorylation sites of ACK1 and its interacting proteins
Interacting Protein	Phosphorylation sites	Reference
AKT	Y176	[49]
AR	Y267, Y363	[14, 15]
Wwox	Y287	[6]
Histone H3	Y54	[13]
Histone H4	Y88	[1]
Wiskott-Aldrich syndrome protein	Y256	[2]
CSK	Y18	[11]
PAG	Y227, Y317, Y417	[11]
ACK1 phosphorylation sites	Y40, Y193, Y232, Y242, Y284, Y423, Y518, Y635, Y859, Y860, Y992	[1, 2, 4, 11, 28, 43, 49]

^*The percentages of copy number variations and gene amplification were obtained from datasets in cBioPortal and COSMIC database from an unbiased search for ACK1/TNK2 across multiple cancer datasets deposited without any exclusion criteria.

Though ACK1 interactome has been studied [12], the unclear signaling circuit urges the need for identifying proteins that could fill the missing link for effective therapeutic implication. Furthermore, ACK1 has multiple tyrosine phosphorylation sites, the functional significance of which are yet to be fully deciphered, including its autoactivation. However, the latest findings of ACK1’s epigenetic activity leading to transcriptional upregulation of key cancer-driving genes has opened its functional diversity [13]. This review will give an overview of ACK1-regulated tumor promoting molecular mechanisms providing insights into the novel substrates and how the pleotropic effects of ACK1 inhibition can overcome therapy-induced resistance. Also, we discuss ACK1’s immunomodulatory function by elaborating on the interplay with C-terminal Src kinase (CSK) immunokinase in T cells. Further, we provide an update on the pre-clinical studies and advances in ACK1-targeting small molecule inhibitors that show promise towards clinical trials in the near future as next-generation of therapeutics.

ACK1 signaling in cancers

ACK1 has emerged to be an important oncokinase and an epigenetic writer in multiple tumors regulating various disease promoting signaling events. Recent studies uncovered multiple ACK1 substrates, including two histones, H3 and H4, AKT, a protein that plays a key role in cancer growth and glucose metabolism (Figure 2A), and androgen receptor (AR), a critical player in prostate cancer, and (Figure 2B), opening new avenues for targeted therapies.

Figure 2. ACK1 signaling in cancers.

(A) ACK1 relays cellular signals from multiple RTKs which subsequently alters cellular processes that contribute towards cancer cell survival and progression. Activated ACK1 phosphorylates AKT at Tyr 176, which gets activated and translocated to the nucleus resulting in FOXO displacement and suppression of FOXO-targeted genes. (B) In prostate cancer, ACK1 promotes AR phosphorylation and nuclear translocation, where ACK1 deposits pY88-H4 epigenetic marks in the cell cycle genes (CCNB1/2) and AR enhancer promoting AR and AR-V7 transcription causing drug resistance and tumor progression. In addition, AR suppresses the expression of NXTAR (a long non-coding RNA) in cancer. Consistently, NXTAR-N5, an oligonucleotide that binds to AR upstream regulatory sequences, hinders the transcriptional outcome of AR. In prostate cancer and leukemia, ACK1 also phosphorylates SHP2 which translocates to the nucleus and erases pY54-H3 epigenetic marks enhancing AKT, AR, PTPN11 (SHP2) transcription. (C) Hepatocellular carcinoma and prostate cancer promote ACK1 expression through increased HOXB13 binding leading to metastasis. ACK1 also promotes ubiquitination and degradation of WWOX. (D) In HER2 and hormone receptor positive tumors, ACK1 phosphorylates KDM3A, an ER co-activator, suppressing its activity dampens HOXA1 expression and development of chemoresistance, which is restored upon ACK1 inhibition. Activation of estrogen signaling promotes SIAH2, a E3 ubiquitin protein ligase 2 mediated ACK1 degradation. (E) ACK1 inhibition disrupts ACK1-ATP5F1A interaction in the mitochondria inducing mitophagy, followed by autophagy and mitigates tumor growth by interfering the metabolic pathways. Abbreviations: FOXO, Forkhead box O; CCNB1/2, Cyclin B1 or Cyclin B2; KDM3A, Lysine demethylase 3A; ER, Estrogen Receptor, SIAH2, Seven in Absentia Homolog. Figure created with BioRender.

Prostate cancer

Prostate cancers (PCs) are initially responsive to androgen deprivation therapies. However in most cases, disease recur and progress to highly metastatic Castrate Resistant Prostate Cancer (CRPC) stage. Multiple studies have expanded our understanding of ACK1s role in regulating AR, a key player in PC progression to CRPC, both at the transcriptional and the posttranslational level. The transcriptional control was first uncovered with ACK1 causing phosphorylation of AR at Tyr267- and Tyr363 promoting a distinct transcription program (Figure 2C) [14, 15]. Subsequent proteomics studies have shown acetylation of AR at Lys609 (acK609-AR) that was dependent on ACK1 mediated AR Tyr-phosphorylation, which lead to dual-modified AR nuclear translocation and its recruitment to the TNK2 and AR enhancers causing their transcriptional upregulation [8].

ACK1 also has a distinct epigenetic role that contributes to global transcriptional regulation. ACK1 phosphorylates histone H4 at Tyr88 (Figure 2B) [1]. These pY88-H4 epigenetic marks are deposited in the AR gene enhancer and subsequently promote the recruitment of the H3K4-methyltransferase, MLL2 (KMT2D) and its interacting protein, WD40 repeat (WDR5). This chromatin modifying complex deposits transcriptionally activating H3K4 trimethylation marks. Consequently, high AR and AR-V7 expression facilitates PC progression to the drug-resistant CRPC stage [1]. The mechanism of ACK1-mediated AR regulation revealed another layer of complexity, phosphorylation of Src homology-2 domain-containing protein tyrosine phosphatase-2 (SHP2) by ACK1. Phosphorylated SHP2 (pSHP2) translocated to the nucleus in complex with AR to bind the chromatin, including AR exons 1 & 2 [16]. It erased the repressive epigenetic mark, phosphorylated Tyr54 of histone H3 (pY54-H3), triggering the AR transcriptional program (Figure 2B). Prostate tumors with high pSHP2 and pACK1 activity exhibits low levels of pY54-H3 marks, and high AR expression, correlating with PC disease severity [16]. These data demonstrate the complex interplay of ACK1 kinase and its substrate, SHP2 phosphatase in regulating the epigenetic landscape that culminates in transcriptional activation.

Identification of ACK1’s epigenetic nature accelerated studies focusing on its therapeutic intervention which intriguingly led to the identification of a long noncoding RNA (lncRNA), annotated as NXTAR (LOC105373241) located adjacent to AR gene. It is expressed at low levels in most PCs, and acts as a novel PC-suppressing lncRNA by inhibiting AR/AR-V7 expression [17]. NXTAR binds upstream of the AR promoter and reduces AR (and AR-V7) expression, promoting Enhancer of zeste homolog 2 (EZH2) recruitment. Significantly, AR bound to NXTAR promoter, and suppression of AR transcription by ACK1 inhibitor, restores NXTAR expression. Thus, reinstatement of NXTAR may be another outcome of ACK1 inhibition, contributing to CRPC suppression. Furthermore, a novel oligonucleotide derived from NXTAR exon 5, NXTAR-N5 suppressed AR/AR-V7 expression and compromise PC cell proliferation, opening a new therapeutic avenue for AR antagonism [17].

Concomitant delineation of ACK1-AR downstream signaling led to the detection of a new AR interacting protein, SRA stem-loop interacting RNA binding protein (SLIRP). Under low androgen conditions, the noncoding RNA, SRA improves SLIRP binding to androgen responsive elements of AR target genes. However, this interaction is disrupted by high ACK1 activity or presence of androgen or heregulin [18]. Such studies explain the previously unknown role of ACK1 in the assembly of protein complexes and the regulation of functionally significant non-coding RNAs. However, the precise mechanism of coregulation or context-dependent repression by ACK1-regulated proteins needs to be fully elucidated.

In addition to active signaling in fully differentiated cancer cells, ACK1 also contributes to the stemness of cells upon androgen insufficiency, catalyzing tumor progression and dissemination. Prostate cancer stem cells (PCSCs), a subpopulation of cells with a CD44⁺PSA^−/lo phenotype, rely on androgen-independent survival mechanisms and exhibit high tumorigenicity. Kinome analysis of PCSC has indicated that ACK1 depletion decreases the abundance of CD44⁺ cells and compromises tumor growth in castrated mice [19]. As PCSCs in the tumor niche promote therapy diffraction, the identification of ACK1 as a vulnerable signaling target for therapeutic intervention could compromise the stem cell-driven PC recurrence upon development of potent ACK1 selective inhibitors.

Tumors elicit high energy production through metabolic phenotype switching to meet their extensive pro-survival needs [20]. ACK1 phosphorylates ATP synthase F1 subunit alpha (ATP5F1A) at Tyr243 and Tyr246, thus inhibiting its binding to its physiological inhibitor, ATP synthase inhibitory factor subunit 1 (ATP5IF1), and increasing complex V stability and energy production [21]. ACK1 inhibition induces mitochondrial stress, which leads to the recruitment of lysosomes causing mitophagy, and consequently to the activation of caspase-based apoptotic signaling, and cancer cell specific clearance (Figure 2E). Prostates from transgenic mice bearing auto-activating E346K point mutation in Tnk2 exhibited increased pY-ATP5F1A levels, mitophagy loss and formation of prostatic intraepithelial neoplasias (PINs) [21]. These data have provided insights into ACK1’s involvement in mitochondrial energy regulation in cancer cells, its impact on tumor initiation and have widened the scope of ACK1 inhibitor treatment that prevent cancer progression by targeting multiple dysregulated pathways simultaneously.

Breast cancer

Most HER2-negative, hormone receptor-positive breast cancer (BC) exhibit high sensitivity to CDK4/6 inhibitors such as palbociclib, which interferes with the cell cycle and consequently controls tumor growth [22-24]. However, most patients with BC eventually develop resistance after showing an initial treatment response [22, 25]. Immunohistochemical staining has revealed enhanced ACK1 activation in most BC subtypes, regardless of their hormone receptor status [9]. Delineation of the underlying molecular mechanism indicated the regulation of cell cycle by ACK1. ACK1 deposits pY88-H4 epigenetic marks on the cell cycle genes CCNB1, CCNB2, and cell division cycle 20 (CDC20), thus promoting their transcription [9]. CDC20 is required for anaphase and chromosome separation, whereas CCNB1 and CCNB2 are essential for early mitosis events [26, 27]. Furthermore, pharmacological inhibition of ACK1 causes cell cycle arrest in G2/M phase, leading to inhibition of palbociclib-resistant BC growth [9]. In addition, phosphoproteomic profiling shows ACK1 activation in most triple negative breast cancers (TNBC) cell lines and tumors [9, 28]. Genetic ablation of ACK1 substantially prolongs the TNBC cell line doubling time, decreases vascularization and xenograft tumor size compromising tumor aggressiveness [28].

Efforts to understand the status of ACK1 in BC has unraveled miRNA-mediated regulation of ACK1. LINC00963, a lncRNA upregulated in multiple cancers including BC, antagonizes the repression of ACK1 by the microRNA miR-324-3p. ACK1 knockdown inhibits LINC00963-mediated breast tumorigenesis, thereby overcoming radioresistance [29]. Therefore, ACK1 is a pivotal player in the progression to drug-resistance in BC, and overcoming its activity can be a novel therapeutic option for patients with BC, including those who have developed resistance to CDK4/6 inhibitors.

Lung cancer

Lung cancers exhibit highest levels of TNK2 gene amplification, especially in squamous cell carcinoma - a type of non-small cell lung cancer (NSCLC) (Table 1). Heat shock protein 90 (HSP90α/β) interacts with ACK1, and Hsp90 inhibitor treatment suppresses ACK1 activity [3]. ACK1 catalyzes the phosphorylation and nuclear accumulation of STAT1 and STAT3, and inhibition of HSP90 suppresses p-STAT3 in lung adenocarcinoma preventing disease progression [30]. Though we lack direct evidence for ACK1-induced signaling, ACK1 inhibition in NSCLC cell lines affects mitogen-activated protein kinase (MAPK), PI3K/AKT, and Wnt pathways, thereby increasing NSCLC sensitivity to AKT or MEK inhibitors [31]. ACK1 also contributes to NSCLC progression by binding to fibroblast growth factor receptor (FGFR) and promoting subsequent AKT phosphorylation and activation [32]. Additionally, ACK1 contributes to the development of resistance to osimertinib (an EGFR inhibitor) in NSCLC cells bearing mutant EGFR [7]. These studies suggest the importance of ACK1 kinase in the progression of lung cancer to drug-resistant stage. The concurrence of multiple pathways being affected upon ACK1 perturbation complicates the therapeutic intervention in lung cancers. Further studies aimed at identifying the specific conditions supporting or dampening ACK1-mediated coregulation of the signaling mechanisms will be beneficial to understand the disease etiology, particularly therapy refraction and will facilitate the use of ACK1 inhibitor, or its combination with AKT inhibitor to be an effective therapeutic option.

Colon and gastric cancers

Tumor necrosis factor alpha (TNF-α) promotes ACK1 activation, which in turn activates AKT leading to Caco-2 cells, a colon carcinoma cell line proliferation [33]. siRNA mediated ACK1 knockdown or pharmacological inhibition disrupts TNF-α-mediated pro-survival. In addition to prostate and breast cancers, ACK1-AKT signaling was also reported in gastric cancers. Studies deciphering the impact of ACK1-mediated AKT activation showed that it also upregulates POU2F1, a transcription factor that directly binds to Ecdysoneless protein, encoded by ECD gene [34]. The human ortholog of the Drosophila ecd gene is responsible for epithelial to mesenchymal transition during gastric cancer progression. Additionally, ECD and POU2F1 levels are directly proportional to ACK1 levels in gastric cancer samples emphasizing its clinical implications [34]. Further, ACK1-mediated ECD regulation also induces polyubiquitination and degradation of p53, tumor suppressor [35]. Together, these data uncover the role of ACK1 as not only a prognostic marker, but also a therapeutic target in colorectal neoplasms [36, 37]. The limited data on the ACK1 status in these cancers has hindered the application of ACK1 inhibitors for therapy. Expanding studies on understanding the concomitant mechanisms regulated by ACK1 in multiple colon and gastric cancer models might substantiate its therapeutic relevance in targeting the aggressive forms of the disease including metastatic colon cancer.

Hematologic malignancies

ACK1 interacts with the protein tyrosine phosphatase, SHP2, whose activating mutations have been implicated in multiple malignancies including acute myeloid leukemia and juvenile myelomonocytic leukemia. The negative feedback loop in which ACK1 phosphorylates SHP2, and phospho-SHP2 dephosphorylates ACK1, maintains a fine balance in downstream MAPK signaling [38]. In addition, SHP2 epigenetic activity which includes erasing of pY54-H3 marks, regulates the expression of multiple genes, including AKT and PTPN11 (SHP2) itself [13]. Therefore, ACK1/SHP2 signaling may have a role in both solid tumors [13] and hematological malignancies. Additionally, ACK1 is a downstream substrate of CSF3R, the receptor for colony-stimulating factor that regulates the granulocyte growth and differentiation and 59% of chronic neutrophilic leukemia and chronic myeloid leukemia cases possess activating mutations in CSF3R [39]. Since mutant CSF3R preferentially activates ACK1, patients with chronic neutrophilic leukemia carrying activating CSF3R mutations might potentially respond to ACK1 inhibitors. Though these data are indicative of contribution of ACK1 signaling in hematologic malignancies, we need studies focusing on mutation-driver tools that can correlate and establish ACK1 hyperactivation with the conventional mutation-activated oncogenes to utilize ACK1-targeted therapy for leukemias.

Activated ACK1 is also directly correlated with tumor progression in other malignancies including hepatocellular carcinoma (HCC) [40, 41], osteosarcoma [42] and gliomas [43]. Together, these data demonstrate the crucial role of ACK1 kinase in various malignancies with distinct signaling outcomes, opening the doors for ACK1 inhibitors as a distinct class of anti-cancer drugs.

ACK1 in immune modulation

Antigen encounters by the immune system trigger signaling cascades causing innate or adaptive responses for antigen clearance. Protein kinases are critical mediators of this intracellular signaling that participate in every stage of active immune surveillance [44]. Expanded mechanistic studies to identify druggable immune targets of the human kinome have revealed ACK1 as a critical immune kinase controlling the functionality of immune cells, primarily in T cells (Figure 1B), and autoimmunity (Box 2). Although the role of ACK1 in malignancies is well established, its importance under normal physiological conditions is unclear. During T cell activation, CD4 and CD8 molecules bind to lymphocyte-specific protein tyrosine kinase (LCK), followed by intracellular T cell receptor (TCR)-associated CD3 ζ-chains phosphorylation, which recruits zeta-chain-associated protein kinase 70 (ZAP70) to initiate signal transduction [45] (Figure 1b, active state). Recently, ACK1 has been identified as the upstream kinase that phosphorylates pY227, pY317, and pY417, in the CSK-binding protein (Cbp), also known as PAG, and CSK at Tyr18, which enhances CSK function, resulting in restriction of T-cell activation [11, 46] (Figure 1b). Significantly, samples from CRPC patients who were treated with immune check point blockade (ICB) therapy exhibited reactivation of ACK1/pY18-CSK signaling, suggesting that ICB insensitivity is fueled by activated ACK1 and regulation of its effector, CSK [11]. ACK1 genetic ablation or pharmacological inhibition significantly compromises growth of ICB-resistant prostate cancer denoting that ACK1 is a crucial negative regulator of T cell activation. These data suggest the possibility of a new therapeutic modality where ACK1 inhibitor can be combined with ICB to sensitize solid tumors which are often refractive to ICB. In addition, LCK and ACK1 inhibition individually rescue the T-acute lymphoblastic leukemia cells from resistance to BH3 mimetics, further suggesting that a combination of ACK1 and BCL2/xL inhibition might increase the clinical efficacy and decrease the relapse of leukemia [47].

Studies focusing on delineating the infiltrated immune cells landscape in cancer to develop prognostic scores for successful therapy have also indicated the role of ACK1 in immune function. Comprehensive immune profile analysis of normal and cancer sample cohorts from The Cancer Genome Atlas (TCGA) for lung cancer has revealed TNK2 copy number association with immune infiltration, particularly in CD8⁺, CD4⁺ T cells and neutrophils [48]. Despite growing evidence of ACK1 as a prime signaling molecule in T cells, the immune functions of this kinase have not been fully elucidated. Follow-up studies regarding the influence of ACK1 on T-cell proximal signaling, including TCR activation-mediated endocytosis and recycling are needed. Further delineation of ACK1’s functionality in other immune cells such as B cells, antigen presenting cells and natural killer cells might reveal its signaling partners and support the development of prognostic immune signatures, thereby expanding the scope of ACK1-targeted immune therapy.

Inhibitors of ACK1

Evidence of ACK1’s role in the development of clinical conditions initiated specific investigation as a potential target for drug discovery and treatment. The emergence of large-scale profiling platforms has enabled identification of suitable small molecule inhibitors targeting ACK1’s functionality (Table 2), which holds a promise for entry into clinical trials.

Table 2:

ACK1 inhibitors used as a single agent or combinations against cancers

Inhibitor	Cancer Type	Dosage	Mechanism & Outcome	Ref
(R)-9b	Castration Resistant Prostate Cancer	Mice - 50 mg/kg of body weight	Reverses pY88-H4 epigenetic mark reducing AR and AR-V7 levels in turn mitigates tumor growth in enzalutamide resistant cells.	[1]
		Mice – 24 mg/kg and 48 mg/kg of body weight	Curbs AR Y267 phosphorylation and subsequent K609 acetylation which inhibits enzalutamide resistant tumor growth.	[8]
		Mice – 24 mg/kg of body weight	Inhibits enzalutamide resistant tumor growth by inhibiting nuclear translocation of SHP2 causing subsequent increase in pY54-H3 epigenetic marks that reduces AR and PSA expression.	[13]
		Mice – 24 mg/kg of body weight	Inhibits tumor formation of prostate cancer stem cells and induces apoptosis overcoming radio-resistance.	[19]
		Mice – 24 mg/kg of body weight	Sensitizes and compromises growth of immune check point blockade therapy resistant tumors.	[11]
	Breast cancer	Mice – 24 mg/kg and 36 mg/kg of body weight	Dampens cell cycle genes causing G2/M arrest and inhibits palbociclib-resistant breast tumor growth and metastasis.	[9]
	Triple Negative Breast Cancer	Cell line – 0.2 to 2 μM	Decreases ACK1-mediated AKT Tyr176-phosphorylation and promotes cell death.	[28]
AIM-100	Pancreatic cancer	Cell line - 7 to 8 μM	Suppresses AKT tyrosine phosphorylation causing cell cycle arrest leading to apoptosis.	[6]
		Cell line – 5 μM	Suppresses TNF-alpha-mediated aberrant cell proliferation and apoptosis.	[33]
Dasatinib	Juvenile myelomonocytic leukemia (JMML)	Cell line – 100 nM	Blocks MAPK signaling and decreased disease severity in PTPN11-mutant bearing JMML.	[38]
AIM-100 /Dasatinib	Hepatocellular cancer	Cell line	Targets ACK1-induced PTEN/AKT/mTOR signaling and inhibits cell proliferation and invasion.	[40]
Onalespib (AT13387)	Lung cancer	Cell line - 0.5 to 1 μM	Inhibits HSP90 which compromises ACK1-STAT signaling and promotes apoptosis.	[30]
AG1296	Glioma	Cell line - 10 μM	Inhibits glioma progression targeting ACK1-AKT Signaling activated by PDGFR-β.	[43]
GNF-7 and its derivative	Acute myeloid leukemia (AML)	Mice – GNF-7 - 8 mg/kg of body weight; derivative - 15 mg/kg of body weight	Inhibits growth of NRAS mutation driven AML.	[55]
21a (EGFRL858R/T790M inhibitor)	Non-small cell lung carcinoma	Mice – 20 to 60 mg/kg of body weight	Overcomes osimertinib resistance and inhibits tumor growth	[59]
10zi	Lung cancer	Cell line – 1 to 5 μM	Inhibits EGFR-inhibitor resistant cells targeting ACK1-AKT	[60]
Combinations
AIM-100 and Imatinib	Gastrointestinal stromal tumors	Cell line – AIM-100 - 2.5 μM and imatinib – 0.5 μM	Causes cell cycle arrest and induces apoptosis.	[53]
Dasatinib and NWP-0476	T-acute lymphoblastic leukemia (T-ALL)	Mice –Dasatinib - 30 mg/kg of body weight and NWP-0476 - 50 mg/kg of body weight	Induces apoptosis in leukemic blasts and circulating T-ALL cells in PDX mice model.	[47]
Dasatinib and MK-2206	Non-small cell lung carcinoma	Cell line – 0.1 to 2.5 μM	Dasatinib increases the sensitivity of cells to AKT inhibitor resulting in synergistic effect improving cell death.	[31, 58]
Sunitinib and GDC-0068	Non-small cell lung carcinoma	Cell line – Sunitinib - 1 to 5μM and GDC-0068 - 0.2 to 5μM	Combined inhibition of ACK1 and AKT induces apoptosis in KRAS mutant tumors.	[58]
Dasatinib and Panobinostat (LBH589)/Entinostat (MS-275)	Acute and chronic myeloid leukemia (AML/CML)	Cell line – Dasatinib – 50nM and 5 μM MS-275 or 100 nM LBH589	Induces cell cycle arrest and apoptosis.	[56]
Dasatinib and Chloroquine	Lung cancer	Mice – Dasatinib - 20 mg/kg of body weight and CQ 30 mg/kg of body weight	Inhibits protective autophagy-like response and compromises tumor growth	[57]

(R)-9b

The ACK1 kinase targets AKT by its phosphorylation at Tyr176 [49]. Use of a peptide derived from this site to screen a kinase inhibitor library has led to the identification of (R)-9b, a potent ACK1 inhibitor with a half-maximal inhibitory concentration (IC₅₀) of 13 nM [1, 9]. The X-ray crystal structure of the ACK1 kinase domain with (R)-9b showed its binding to the ATP-binding site and formed two hydrogen bonds in the hinge region – a binding mode to be consistent with the type I kinase inhibitors [11].

(R)-9b, also known as (R)-9bMS (for its mesylate salt version) sequentially curbs AR Y267-phosphorylation and K609-acetylation in enzalutamide resistant PC, which compromised CRPC xenograft tumor growth in preclinical mouse models [8]. (R)-9b also successfully mitigates PCSC sphere formation and tumor growth, and subsequently overcomes radioresistance [19]. Further, the reversal of pY88-H4 epigenetic marks by (R)-9b decreases AR and AR-V7 levels, sensitizes enzalutamide-resistant PC cells, and consequently mitigates CRPC growth [1]. Enzalutamide-resistant CRPCs with high acK13-HOXB13 exhibit increased ACK1 levels and are significantly sensitive to (R)-9b [10]. Intriguingly, (R)-9b dampens CSK Y18-phosphorylation, and consequently causes activation of T cells and promotes inhibition of ICB-resistant PC [11]. Another consequence of AR suppression by (R)-9b is the recruitment of the acetyl transferase, GCN5 and deposition of H3K14 acetylation marks. This enhances NXTAR expression that inversely corelates with AR levels thereby reinstating ACK1-NXTAR feedforward signaling circuitry [17]. (R)-9b also serves as a “mitocan”, targeting the mitochondrial function and inducing mitophagy, followed by autophagy, thus restricting prostate tumor growth [50]. In BC cells, (R)-9b not only causes G2/M arrest by suppressing key cell cycle genes, but also inhibits CXCR4 expression, causing the regression of palbociclib-resistant BC growth and metastasis [9]. Further, a preclinical study in lung carcinoma cells has indicated that (R)-9b inhibits the growth of EGFR mutant expressing NSCLC cell line that has acquired resistance to a third generation EGFR inhibitor, osimertinib [7], offering a scope to overcome drug resistance by using a single agent or in combination with other earlier generation cancer drugs. Overall, (R)-9b may be the most promising drug-like molecule for treating multiple malignancies with demonstrated ability to be applied as a “dual” inhibitor with tumor suppressing activity and ability to activate host immune cells. A phase I clinical trial of (R)-9b is expected to be initiated for patients with prostate cancer in 2025 (IND #167907).

AIM-100

AIM-100 (4-amino-5,6-biaryl-furo[2,3-d]pyrimidine) was the first small molecule inhibitor developed and used pre-clinically to inhibit ACK1; it has an IC₅₀ of 21.58 nM [15, 51]. It suppresses ACK1- and AR-phosphorylation, including inhibition of autoactivated E346K mutant ACK1 [15, 49]. Furthermore, AIM-100 modulates ACK1/SLP-76 interaction, contributing to T cell activation and mobility of CD4⁺ T cells [52]. Gastrointestinal stromal tumors resistant to imatinib, a multi-kinase inhibitor targeting BCR/ABL, KIT and PDGFR have treatment-dependent ACK1 overexpression [53]. AIM-100 in combination with imatinib has been successful in downregulating multiple proteins involved in epithelial to mesenchymal transition, and subsequently enhancing apoptosis and cell cycle arrest in imatinib-resistant gastrointestinal stromal tumors, through inactivation of PI3K/AKT and MAPK signaling [53]. Despite the potent ACK1 inhibitory activity of AIM-100, researchers have sought alternative ACK1 inhibitors with more favorable chemical properties and higher specificity.

Other Inhibitors

GNF-7 is a well-known, potent type-II kinase Bcr-Abl inhibitor, that also inhibits ACK1 and germinal center kinase (GCK) in preclinical models of NRAS-dependent acute myeloid leukemia and acute lymphocytic leukemia cells [54]. The potency of GNF-7 has spawned efforts to develop GNF-7 derivatives with lower minimum inhibitory concentration and improve the pharmacokinetic profile [55]. The other class of drugs considered for ACK1 inhibition include two histone deacetylase inhibitors (HDACi), panobinostat/LBH589 (pan HDACi) and entinostat/MS-275 (class I HDACi), which induce ACK1 proteolysis, thereby causing apoptosis in acute and chronic myeloid leukemia cells, and disrupting ACK1-STAT3 signaling [56]. These studies open yet another strategy to target ACK1 in hematologic malignancies.

Dasatinib, a multi-kinase inhibitor primarily used as a Src and Abelson kinase (ABL) inhibitor, was initially tested as a potential ACK1 inhibitor. In NSCLC models, AIM-100 or dasatinib induces partial adaptive autophagy, which is improved further if combined with lysosomal degradation blockers chloroquine/bafilomycin A1 [57]. Dasatinib also improves NSCLC sensitivity to selumetinib, a mitogen-activated protein kinase kinase (MEK) inhibitor and MK-2206 (AKT inhibitor) [31]. Treatment of KRAS-mutant NSCLC cells with dasatinib and MK-2206 activates cell cycle arrest in G2 phase and inhibits migration, indicating that ACK1/AKT combined inhibition might have better clinical translation in KRAS-mutant lung cancers [58]. In T-acute lymphoblastic leukemia with elevated BCL-xL dependency, a combination of dasatinib and BCL-2/BCL-xL dual inhibitor, NWP-0476 synergistically curbs growth, thus overcoming resistance to BH3 mimetics [47]. In HCC, AIM-100 or dasatinib treatment perturbs PTEN/AKT/mTOR signaling and weakens cell proliferation and invasion [40]. Overall, both as a single agent or as a combination, ACK1 inhibitor could have a significant impact on future therapy.

Recently, the EGFR and ACK1 dual-targeting compound, 21a, was identified [59]. This compound inhibits EGFR L858R/T790M (IC₅₀ = 23 nM) and ACK1 (IC₅₀ = 263 nM) and has shown antitumor effects and safety in xenograft-bearing mice. In addition, a series of (R)-8-((tetrahydrofuran-2-yl)methyl)pyrido [2,3-d]pyrimidin-7-ones compounds have been developed as ACK1 inhibitors and tested for their efficacy in overcoming resistance to third generation EGFR inhibitors, ASK120067 and osimertinib in NSCLC [60]. Although multiple studies have substantiated the potential of ACK1 inhibitors to target various cancer cells, no compound has entered the clinical trials yet. Though multi-kinase inhibitors that target ACK1 such as dasatinib or GNF-7 have been studied in clinical settings, a major drawback of these inhibitors is that the effects observed might not be due to ACK1 inhibition. Further, multi-kinase inhibitors often exhibit significant unwarranted toxicity due to multi-kinase targeting, including those involved in immune cell activation. Therefore, ACK1-specific compounds like (R)-9b may provide significant clinical advantage such as immune activation, while avoiding drawbacks including off-target toxicity.

Concluding remarks and future perspectives

ACK1 is a protein with diverse functional abilities and the relevance of its interactions with multiple substrates is not fully understood. CD3ε, a TCR component, and LCK formed a condensate structure that promotes LCK-mediated CD3 phosphorylation and the amplification of TCR signaling [61]. Phosphorylated CD3ε in turn recruits CSK, causing the dissolution of the condensate. ACK1 being an interacting partner for CSK, it remains to be seen whether ACK1 has a role to play in the dissolution of TCR/LCK condensate structure, terminating TCR signaling (see outstanding questions). Nevertheless, the involvement of enhanced ACK1 levels in lowering the immune response in cancers is a new and promising finding. But, how ACK1 can orchestrate alteration of immune infiltration and contribute to the dynamic tumor microenvironment is still unknown. The influence of ACK1-mediated epigenetic regulation in immune cells that might cause changes in cytokine signaling during cancer progression will also encourage the use of ACK1 targeted therapy in combination with other drugs to treat immunologically “cold” tumors. Research focusing on delineating ACK1 signaling in diverse immune cells of lymphoid and myeloid origin in multiple cancer platforms will provide more insights and direct appropriate implications of the protein for immune-related therapeutic intervention.

Outstanding questions.

What is the role of ACK1 in various cell types of the immune system including B cells, natural killer cells, and antigen-presenting cells including dendritic and mast cells?

Do ACK1-overexpressing tumors alter the tumor microenvironment, thereby affecting host immune cell infiltration?

Does ACK1 form condensates that will delineate the spatial and dynamic regulation of ACK1 effector proteins?

Can ACK1 mRNA overexpression and protein activation facilitate prognosis in distinct cancer types?

As a prognostic gene signature for lower-grade glioma, what is the signaling circuit that ACK1 employs in initiating tumor cell death through paraptosis?

Does ACK1-mediated epigenetic regulation influence ion channel function in the brain, and what is its neurophysiological significance?

Although ACK1 has been established as a kinase that is active in the brain and regulates dopamine signaling [62], there is a lack information on how it contributes to the maintenance of dopamine transporters (DAT). Delineating this mechanism would not only have implications in DAT regulation but would also unravel the role of ACK1 in transmitting signals during induction of chronic pain [62] (see outstanding questions). Similar to DAT regulation, ion channel also plays a crucial role in controlling dopamine signaling. Studies are also lacking on the influence of epigenetic regulation in controlling the ion channels of the brain and whether ACK1 plays a role in the process under normal physiology or contributing to neuronal disorders.

Future attempts to use ACK1 inhibitors to the emerging therapeutic modalities such as bridged proteolysis targeting chimeras (PROTACs) will be exciting to check whether this approach can improve the efficacy and overcome the drawbacks of conventional inhibitor combinations to degrade disease-causing proteins with minimal side effects. PROTACs can utilize a small molecule to target the binding partner, causing its degradation [63]. Applying (R)-9b in this bridged PROTAC strategy might result in not only ACK1 degradation, but also bring ACK1’s binding partners, AR or AKT in the proximity of an E3 ubiquitin ligase to target them for degradation. The phosphorylated AR and AKT are critical players in multiple malignancies including prostate and breast cancers [8, 49], and their selective elimination could significantly improve the clinical outcomes. This protein complex degrader approach might also reduce the chances of developing therapy-induced drug resistance.

A major hurdle faced during early development of ACK1 inhibitors was the lack of knowledge on ACK1 interacting partners, its structure and its functional relevance in various disorders. Focused development of ACK1 inhibitors has led to the development of (R)-9b compound with desired properties such as high specificity, potency and low toxicity. The outcome of (R)-9b `first-in-man’ clinical trial would encourage the combination studies as well as development of next-generation of ACK1-specific inhibitors.

Overall, the findings of cellular signaling mediated by ACK1 kinase has significant implications in multiple malignancies and neuronal disorders, which has opened new avenues for the clinical application of ACK1-antagonists. The immunomodulatory properties of ACK1 inhibitor fulfills a unique niche, that not only suppresses the tumor growth, but also activates host immune system to mount a robust “dual” anti-tumor response. Future clinical studies may expand on the therapeutic potential of ACK1 inhibitors in overcoming not only ICB-resistance, but also resistance for EGFR, CDK4/6 or AR antagonists.

Glossary

AKT

Also known as Protein kinase B (PKB) that comprises of three serine/threonine-specific protein kinases involved in multiple cellular signaling mechanisms such as transcription, apoptosis and cell proliferation.

AR

Androgen receptor, a nuclear receptor which acts as transcription factor regulating prostate growth and development.

Autophagosome

A double walled spherical structure generated during degradation of intracellular contents.

CD44

A cell surface glycoprotein that supports cell adhesion and is a marker for prostatic stem cells.

CSK

C-terminal Src kinase, an enzyme that phosphorylates C-terminal tyrosine residues in SRC-family kinases.

CRPC (Castrate Resistant Prostate Cancer)

Prostate cancer that continues to grow even with low or no androgen/testosterone levels in the body unlike early-stage androgen-dependent prostate cancer.

Chaperone

Proteins assisting the conformational folding or unfolding of macromolecular proteins and complexes.

EGFR

Epidermal growth factor receptor, a transmembrane glycoprotein and member of the protein kinase superfamily that binds to epidermal growth factor (EGF), which induced receptor dimerization and tyrosine autophosphorylation causing cell proliferation.

EZH2

Enhancer of zeste homolog 2, a histone-lysine N-methyltransferase enzyme that is responsible for histone methylation and transcriptional repression.

GCK

Germinal Center Kinase, also known as Mitogen-activated protein kinase kinase kinase kinase 2, is encoded by the MAP4K2 gene and is a member of the serine/threonine protein kinase family. It is activated by TNF-alpha and activates MAP kinases.

HER2

Human epidermal growth factor receptor 2, a member of the EGFR family with tyrosine kinase activity that is involved in a variety of signaling pathways causing cell proliferation and tumorigenesis.

Histones

Proteins that provide structural support for chromosomes and has five types, namely H1, H2A, H2B, H3 and H4.

HOXB13

Homeobox protein that controls normal prostate gland development.

Immune Checkpoint Blockade

A strategy using inhibitors or antibodies to target the checkpoint proteins, PD-1/PD-L1 and CTLA-4, potentiating T cells to effectively kill cancer cells.

Interactome

whole set of molecular interactions especially protein-protein interactions in a cell.

Kinome

Collection of all the protein kinases encoded in a cell.

LCK

Lymphocyte-specific protein tyrosine kinase, a member of Src kinase family important for T-cell receptor (TCR) activation signaling.

Long noncoding RNA

RNA transcripts over 200 nucleotides that is not translated into a protein.

MERTK

MER proto-oncogene tyrosine kinase, a member of the MER/AXL/TYRO3 receptor kinase family that encodes a transmembrane protein.

Mitophagy

Selective elimination of mitochondria through regulated lysosome-dependent degradation.

PDGFR

Platelet derived growth factor receptor, cell surface tyrosine kinase receptor that plays a role in wound healing and tumor progression.

Phagocytosis

An endocytic process adapted by cells to eliminate large particles (≥ 0.5 μm), by engulfing using plasma membrane.

Phosphoproteomic profiling

A method in proteomics that detects, lists, and characterizes proteins with phosphate group as posttranslational modification.

Polyubiquitination

A process of adding ubiquitin to a protein for marking its degradation by the proteasome.

PSA

Prostate-specific antigen is a protein produced by the prostate that participates in the seminal fluid dissolution and low PSA is a marker for prostate cancer stem cells.

SLE (systemic lupus erythematosus)

An autoimmune disorder that affects the body’s own tissues leading to chronic inflammation and tissue damage which cannot be cured.

Small molecule inhibitors

A drug-like molecule with low molecular weight (less than 500 Da) facilitating easy cellular entry.

ZAP70

Zeta-chain-associated protein kinase 70, a tyrosine kinase recruited upon antigen encounter by TCR regulating downstream T cell signaling.