Targeting pseudokinase TRIB3 brings about a new therapeutic option for acute promyelocytic leukemia

ABSTRACT

Pseudokinase tribbles (Trib) family, Trib1 and Trib2, but not Trib3, act as oncogene to drive acute leukemia by destabilizing the myeloid transcription factor CCAAT/enhancer-binding protein α (C/EBPα) and inhibiting myeloid differentiation. A recent study identifies pseudokinase TRIB3 as an important factor in acute promyelocytic leukemia (APL) progression and therapy resistance. Targeting TRIB3 may provide a novel therapeutic approach for APL.

Acute promyelocytic leukemia (APL) is the distinct subtype of acute myeloid leukemia (AML). In APL driven by the t(15;17) translocations, promyelocytic leukemia/retinoic acid receptor α (PML-RARα) fusion is the major oncogenic player to initiate APL through recruitment of co-repressors and the histone deacetylase (HDAC)-complex to repress genes implicated in myeloid differentiation. However, when PML-RARα is expressed in transgenic mice, it takes several months for PML-RARα initiating a typical APL, which suggests that other genetic/epigenetic changes are required for progression to full transformation. For instance, multiple common gene mutations and other genetic alterations, as well known for fms like tyrosine kinase 3 (FLT3) activation or Myc amplification synergistically facilitates APL progression.^1,2 Moreover, PML-RARα function is significantly affected by PML-RARα posttranslational modifications. Therefore, searching for new cooperating leukemogens, which regulate PML-RARα expression or its posttranslational modifications, may further reveal the role of PML-RARα in the pathogenesis of APL.

The members of mammalian Tribbles family, including TRIB1, TRIB2, and TRIB3, are classified as pseudokinases and act as adaptor molecules to regulate a diversity of intracellular activities.³ Recent work indicates that Trib1 and Trib2 function as the important modulators of haematopoietic development and oncogenic factors of acute leukemias. For instance, Trib1 or Trib2 overexpression alone is able to induce AML through either the degradation or inactivation of transcription factor CCAAT/enhancer-binding protein α (C/EBPα). However, although recent study shows an increase in TRIB3 expression in AML cells,³ Trib3 overexpression alone is unable to induce leukemia in mouse. Indeed, little is known about the role of TRIB3 in the pathogenesis of leukemia and in haematopoietic development. In our recent published study,⁴ we have provided evidence to indicate that both of the mRNA and protein levels of TRIB3 were upregulated in human APL cells. We further demonstrate that Trib3 knockin promotes whereas Trib3 knockout suppresses Pml-Rarα-driven APL progression in mouse through either enhancing or reducing the protein stability of PML-RARα (Fig. 1). Given that TRIB3 is an endogenous stress sensor, responding positively to a range of stimuli such as inflammatory, metabolic, endoplasmic reticulum (ER) stress and anti-cancer stressors, our work provides new insight into APL pathogenesis: TRIB3, as a new cooperating leukemogen with PML-RARα, may link these different stressors to PML-RARα-driven APL progression.

Figure 1.

Potential role of TRIB3 in PML-RARα-driven leukemogenesis. The elevated intracellular tribbles homolog 3 (TRIB3) expression, in response to a diversity of stresses such as inflammation, metabolism, endoplasmic reticulum (ER) stress and anti-cancer agents, can physically interact with the oncoprotein promyelocytic leukemia/retinoic acid receptor α (PML-RARα) or tumor suppressor promyelocytic leukemia protein (PML), and these interactions suppress the functions of PML and sustain the transcriptional repression of PML-RARα, which promotes PML-RARα-driven acute promyelocytic leukemia (APL) progression (Panel A). Genetic depletion of TRIB3 or pharmacological interruption of the TRIB3 and PML-RARα/PML interaction with a α-helical peptide (Pep2-S160) attenuates APL by restoration of PML/PML-RARα sumoylation, ubiquitylation and degradation, which induces PML-RARα transcriptional activation, and results in the PML nuclear bodies (PML NBs) assembly and tumor protein p53 (p53) mediated senescence (Panel B). Thus, TRIB3 is critically involved in the pathogenesis of PML-RARα-driven APL. Targeting TRIB3, particularly the TRIB3/PML-RARα interaction, is a potential therapeutic option for the treatment of APL.

Treatment of APL with all-trans retinoic acid (ATRA) and/or arsenic trioxide (As₂O₃) causes clinical remission of APL patients by accelerating the PML-RARα degradation. PML-RARα degradation induced by ATRA depends on PML-RARα cleavage by differentiation-activated proteases and on recruitment of proteasomes, and As₂O₃ induces PML-RARα degradation through specific sumoylation and subsequent ubiquitylation of its PML moiety.⁵ Activation of autophagy has also been reported to contribute to ATRA/As₂O₃-induced degradation of the oncoprotein PML-RARα.⁶ Although combined ATRA with chemotherapy or As₂O₃ therapy made APL highly curable, resistance to therapy still develops in some patients, which cause the failure of therapy or relapsed APL. So far, resistance to the ATRA therapy is commonly associated with mutations in the RARα moiety of PML-RARα, and As₂O₃ resistance is associated with the PML mutations or highly clustered mutations in the PML moiety of PML-RARα;⁷ both types of mutations impede ATRA/As₂O₃-induced PML-RARα degradation. Overall, targeting PML-RARα degradation seems to be the primary basis and general therapeutic strategy in APL patients with or without acquired resistance to As₂O₃ and/or ATRA therapy.

Recently, we have reported that high TRIB3 expression mediates a reciprocal antagonism between autophagy and ubiquitin-proteasome system (UPS). Silencing TRIB3 restores autophagic flux and UPS activity in tumor cells.⁸ In this study, we found that TRIB3 overexpression prolonged, whereas TRIB3 depletion shortened the half-life of PML-RARα degradation by regulating UPS in APL cells. Mechanistically, TRIB3 inhibits sumoylation of PML-RARα by impeding the association of sumo conjugating enzyme (E2) or sumo ligase (E3) with substrate, and the dissociation of E3 from E2-sumo complexes. Subsequently, the ubiquitylation and degradation of PML-RARα were suppressed by TRIB3 through dismissal of proteasomes, ubiquitin ligase to PML nuclear bodies (PML NBs). Given the role of TRIB3 in protein quality control and TRIB3 affecting therapeutic sensitivity of ATRA/As₂O₃, it is worth to further dissect the molecular mechanism accounting for the TRIB3 regulation of PML-RARα degradation both in therapy-resistant and -sensitive APL cells.

Targeting posttranslational modifications of proteins particularly targeting protein-protein interaction (PPI), to manipulate the protein stability of intracellular key oncoproteins and tumor suppressors, is emerging as a promising anticancer strategy. Indeed, several small-molecule PPI antagonists are being studied in clinical trials for cancer patients.⁹ TRIB3 produces multiple actions in a PPI manner. Our previous study revealed that TRIB3 plays a critical role in the promotion of cancer development and progression by interacting with the signaling molecule SMAD family member 3 (SMAD3) or a selective autophagy receptor, sequestosome-1 (SQSTM1).^8,10 A cell-penetrating α-helical peptide, which disturbs the interaction between TRIB3 and SQSTM1, displays an effective activation of autophagic flux, accelerates the degradation of several tumor-promoting factors, and significantly suppresses tumor growth and metastasis.⁸ In the current study,⁴ the elevated TRIB3 expression promotes APL progression and therapy resistance through interacting with PML-RARα and PML and suppressing the sumoylation, ubiquitylation, and degradation of PML-RARα (Fig. 1A). Pep2-S160, a cell-penetrating α-helix peptide disturbing the TRIB3/PML-RARα interaction, produces potent anti-APL effects alone or in combination with ATRA or As₂O₃, indicating that Pep2-S160 is a promising alterative or adjunct therapy for APL (Fig. 1B). In summary, our studies not only demonstrate that the stress protein TRIB3 promotes the PML-RARα driven APL progression, but also provide a proof-of-concept for targeting TRIB3 itself or the TRIB3/PML-RARα interaction to promote the degradation of the oncoprotein PML-RARα as an attractive therapeutic strategy against APL.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.