MORC2 Interactome: Its Involvement in Metabolism and Cancer

Abstract

Microrchidia 2 (MORC2) is an emerging chromatin modifier with a role in chromatin remodeling and epigenetic regulation. MORC2 is found to be upregulated in most cancers, playing a significant role in tumorigenesis and tumor metastasis. Recent studies have demonstrated that MORC2 is a scaffolding protein, which interacts with the proteins involved in DNA repair, chromatin remodeling, lipogenesis, and glucose metabolism. In this review, we discuss the domain architecture and cellular and subcellular localization of MORC2. Further, we highlight MORC2-specific interacting partners involved in metabolic reprogramming and other pathological functions such as cancer progression and metastasis.

Introduction

Cancer is the leading cause of mortality worldwide (Ferlay et al. 2020). Genome instability is one of the hallmarks of cancer development (Hanahan and Weinberg 2011). Genome regulation and surveillance are monitored by a set of proteins known as chromatin regulators (Swygert and Peterson 2014). In distinct cancer types, deregulated expression or altered function of these genome regulators is associated with tumorigenesis (Morgan and Shilatifard 2015; Farooqi et al. 2020; Ghasemi 2020). And also, the epigenetic mechanisms like DNA methylation, histone modifications, and noncoding RNAs are playing a pivotal role in tumorigenesis (Dawson and Kouzarides 2012; Sharma et al. 2010). Epigenetic drugs against protein domains involved in chromatin regulation have shown promising results as cancer therapeutics (Dawson and Kouzarides 2012; Yao et al. 2020). Chromatin regulators can be broadly classified as enzymes catalyzing chromatin modification (Kouzarides 2007) and for chromatin remodeling (Sahu et al. 2020). The chromatin remodelers help to open the chromatin in order to drive the process of gene expression (Fig. 1) (Nair and Kumar 2012). One group of ubiquitously expressed chromatin regulators is the Microrchidia (MORC) protein family, which belongs to an evolutionarily conserved CW-type zinc finger (ZF-CW) nuclear protein superfamily. MORC family proteins are strongly involved in chromatin remodeling and epigenetic regulation (Perry and Zhao 2003; Iyer et al. 2008; Li et al. 2013). The morc gene was first identified in a mouse by its spontaneous autosomal recessive mutation. This mutation results in the arrest of spermatogenesis in meiosis and reduced testicular mass (Watson et al. 1998). Four convergent and least characterized MORC family members: MORC1 (CT33 or ZCW6), MORC2 (ZCWCC1, ZCW3, KIAA0852, CMT2Z or AC004542.C22.1), MORC3 (ZCWCC2, ZCW5, KIAA0136 or NXP2), and MORC4 (ZCWCC2, ZCW4, DJ75H8.2, or FLJ11565) have been reported in humans (Li et al. 2013; Hong et al. 2017)). The common structural features of the MORC family of proteins include GHKL (gyrase, hsp90, histidine kinase, and MutL)-ATPase domain, ZF-CW domain, a nuclear localization signal (NLS), and coiled-coil domains (Li et al. 2013). Based on their ZF-CW domain architectures, MORC1 and MORC2 are grouped under subfamily I; MORC3 and MORC4 are assigned under subfamily IX (Perry and Zhao 2003). Emerging evidence reveals that MORC proteins have been associated with cancer development. In this review, we focus on the MORC2 protein and its associated functions in cancer.

Fig. 1.

Chromatin structure in a eukaryotic cell. Chromatin is composed of histone octamer, and DNA exists in two major forms, heterochromatin and euchromatin. Chromatin remodelers play an essential role in the interconversion of two chromatin forms

MORC2 expression

MORC2 is a ubiquitously expressed protein in human cells and tissues and was found to be upregulated in most cancers including the lung, kidney, prostate, esophagus, breast, liver, colon, stomach, ovary, pancreas, skin, endometrium, nasopharynx, and bladder (Ding et al. 2018). MORC2 also shows high expression levels in the mouse ovary, testis, and brain (Wang et al. 2010). MORC2 expression levels were more elevated in high-grade cancer tissues and associated with poor overall survival and unfavorable pathological conditions in breast cancer and non-small cell lung cancer (Ding et al. 2018).

MORC2 primary structure

MORC2 gene is located on the reverse strand at the region q12.2 of chromosome 22. Among 13 transcripts, 2 isoforms have complete coding sequence: MORC2 isoform-1 (1032 amino acids) and MORC2 isoform-2 (970 amino acids). MORC2 protein isoform-2 lacks 62 amino acid residues from the N-terminal region, which are present in its canonical form. Irrespective of the difference, both isoforms of MORC2 are fully functional.

MORC2 like other MORCs have conserved domains such as (i) GHKL-type ATPase which is a split ATPase composed of GHKL and S5 domain (Douse et al. 2018); (ii) ZF-CW, reader domain which has not been well characterized, functionally (Andrews et al. 2016; Liu et al. 2016); and (iii) coiled coils, which known to play a role in protein-protein interactions (Xie et al. 2019). These structural domains can be present as a single copy, such as ZF-CW, or multiple copies, such as coiled coils (Fig. 2). Although coiled coils are present in multiple copies, each single domain has its own unique interactome and associated function. For instance, coiled coil 1 of MORC2 binds DNA and plays a vital role in chromatin interaction (Douse et al. 2018), whereas the C-terminal coiled coil facilitates its dimerization, which is essential for its role in DNA damage repair and cell survival (Xie et al. 2019). Along with the conserved domains, MORC2 has several unique domains such as the chromo-like domain and the proline-rich domain.

Fig. 2.

Schematic representation of MORC2 domain architecture and its domain-specific interacting partners. Conserved ATPase domain in brown (1–469 a.a.) is a split module composed of GHKL (1–278 a.a.) and S5 (323–469 a.a.) domains. PHD-X/ZF-CW or zinc finger domain is represented as ZF (490–544 a.a.) in purple and multiple coiled coils in gray as CC-1 or coiled coil-1 (282–362 a.a.), CC-2 or coiled coil-2 (547–584 a.a.), CC-3 or coiled coil-3 (741–761 a.a.), CC-4 or coiled coil-4 (966–1016 a.a.), and CC-5 or coiled coil-5 (1024–1032 a.a.). Two unique domains, proline-rich domain (PRD) (601–734 a.a.) and chromo-like domain (CLD) (790–854 a.a.), are shown in green and blue, respectively. Solid lines indicate MORC2 region required for interaction with its interacting partners PARP1, NAT10, C/EBPα, HSPA8, LAMP2A, CTNND1, TASOR and MPP8, PAK1, and ACLY

MORC2 localization

MORC2 can be localized to both the nucleus and cytoplasm as it is containing the nuclear localization signal, NLS (657-781 a.a), and the cytoplasm localization signal or nuclear export signal, NES (481-657 a.a.). However, MORC2 is predominantly localized to the nucleus compared to the cytoplasm as the NLS predominates over the NES (Wang et al. 2010). Within the nucleus, it is highly involved with the regulation of gene transcription (Guddeti et al. 2021), chromatin remodeling (Li et al. 2012), and DNA damage response (Zhang and Li 2019). Apart from the nuclear functions, MORC2 also regulates cytosolic functions like lipogenesis and adipogenesis (Sánchez-Solana et al. 2014).

Protein degradation in eukaryotes is largely carried out by the autophagy-lysosome system and the ubiquitin-proteasome system. Autophagy regulates the degradation of long-lived proteins and organelles in lysosomes whereby the ubiquitin-proteasome system targets short-lived regulatory proteins (Ravid and Hochstrasser 2008). Autophagy in mammalian cells basically consists of three types – microautophagy, macroautophagy, and chaperone-mediated autophagy (CMA) (Mizushima et al. 2008). In CMA, proteins having a KFERQ-like motif are recognized by HSPA8 which brings about selective degradation of the protein. MORC2 amino acid analysis showed the presence of two putative KFERQ-like motifs in its structure, and it has been found to be degraded in lysosomes by CMA (Yang et al. 2020). Serum deprivation promotes MORC2 translocation from the nucleus to cytoplasm where a cytosolic lysosomal protein HSPA8 and LAMP2A interact with MORC2 at regions 352-356 a.a. and 628-632 a.a., respectively, facilitating lysosomal degradation of MORC2 by CMA (Yang et al. 2020). Basically, the transcriptional and chromatin regulatory role of MORC2 associated with oncogenesis is attributed to its subcellular localization, from the cytoplasm to nucleus (Wang et al. 2010).

MORC2 in cancer

Induced expression of MORC2 in cancer cells is associated with proliferation, invasion, migration, metastasis, and tumorigenesis (Pan et al. 2018; Zhang et al. 2018; Liao et al. 2019). MORC2 overexpression is implicated in radioresistance, chemoresistance, and endocrine resistance in cancer progression (Pan et al. 2018; Liu et al. 2020; Yang et al. 2020). Emerging studies revealed that MORC2, through its ATPase-dependent chromatin remodeling activity, plays an important role in DNA damage response (DDR) by enabling the recruitment of DNA repair proteins to the sites of DNA lesions (Li et al. 2012; Zhang and Li 2019; Xie et al. 2019). Recent studies demonstrated that MORC2, by regulating glucose metabolism and lipogenesis, is also involved in cancer metabolic reprogramming (Sánchez-Solana et al. 2014; Guddeti et al. 2021).

MORC2 interacting partners and their associated functions in tumorigenesis

Although MORC2 is associated with chromatin remodeling, it does not have the ability to directly catalyze the modification. The function of MORC2 is attributed to its domain organization, where distinct domains interact with modifiers to modulate the function of MORC2 or its downstream targets. Known interacting partners were summarized in Fig. 2 and Table 1.

Table 1.

MORC2 interacting partners and the functional outcome of the interaction in cancer

MORC2 PPIs	Mechanism	Functional outcome	Type of cancer	Reference (PMID)
MYC	Transcriptional regulation of LDHA expression and activity	Reprogramming of glucose metabolism	Breast cancer	Guddeti et al. 2021
ACLY	Increased ACLY activation	Enhanced lipid synthesis and adipocyte differentiation	Breast cancer	Sánchez-Solana et al. 2014
PAK1	Phosphorylated MORC2 confers increased ATPase activity to remodel chromatin	DNA repair	Cervical cancer and breast cancer	Li et al. 2012
PARP1 NAT10	Activation of chromatin remodeling activity of PARylated MORC2 upon DNA damage and stabilization of PARP1 by acetylation	DNA repair and cell survival	Breast cancer	Zhang and Li 2019
PRKACA HSPA8 LAMP2A	Stabilized MORC2 upon induction with anti-estrogens exerts an oncogenic effect	Endocrine resistance to cancer cells	Breast cancer	Yang et al. 2020
HDAC4	Transcriptional regulation of CAIX enzyme	Growth and survival	Gastric cancer	Shao et al. 2010
NAT10 SIRT2 PP1y	Enhanced acetylation of MORC2 by NAT10 upon DNA damage facilitates dephosphorylation of H3T11P and transcriptional repression of CDK1 and Cyclin B1 to arrest cell cycle at the G2 checkpoint	Ensures cell survival by G2/M cell 1cycle arrest upon DNA damage	Breast cancer	Liu et al. 2020
DNMT3A	Transcriptional repression of Hippo signaling regulators, NF2 and KIBRA	Cancer stemness and proliferation	Liver cancer	Wang et al. 2018
HDAC1	Transcriptional repression of p21	Cell proliferation	Gastric cancer	Zhang et al. 2015
PAK1	Phosphorylated MORC2 promote the expression of cyclin D-CDKs, pushing cells from G1 to S phase	Cell proliferation	Gastric cancer	Wang et al. 2015
C/EBPα	Sumoylation mediated degradation of C/EBPα by MORC2	Cell proliferation	Gastric cancer	Liu et al. 2019b
EZH2 HSF1	Transcriptional silencing of ArgBP2 expression	Cell proliferation, invasion and migration	Gastric cancer	Tong et al. 2015, 2018
SIRT1	Transcriptional downregulation of NDRG1	Cell invasion, migration, and metastasis	Colorectal cancer	Liu et al. 2019a
hnRNPM	Enhanced interaction mediates the splicing switch from epithelial to the mesenchymal isoform of CD44	Cell invasion, migration, and metastasis	Breast cancer	Zhang et al. 2018
CTNND1	Unknown	Cell invasion, migration and metastasis	Breast cancer	Liao et al. 2017

MORC2 role in cancer cell proliferation, invasion, and migration

Upregulated MORC2 inhibits tumor suppressor genes (Tong et al. 2018; Wang et al. 2018; Liu et al. 2019a) or activates oncogenes (Liu et al. 2018) as a coregulator (Table 1) in order to promote cancer cell proliferation, invasion, migration, and metastasis. The regulatory switch of MORC2 depends on the interactome, cell type, and signal; for instance, MORC2 upregulates C/EBPα in adipocytes (Sánchez-Solana et al. 2014) whereas it downregulates C/EBPα (interacting region: 719-843) in gastric cancer cells by different mechanisms (Liu et al. 2019b). This MORC2 regulated C/EBPα expression was found to cause cell proliferation and tumorigenesis (Liu et al. 2019b).

Carbonic anhydrase IX (CAIX) is a transmembrane isoenzyme that regulates tumor cell growth and survival (Robertson et al. 2004). CAIX was found to play a key role in early gastrogenesis as CAIX knockout mice show increased cell proliferation (Gut et al. 2002). Shao et al. reported that the MORC2-HDAC4 complex is recruited to the CAIX promoter and causes repression of CAIX transcription in gastric cancer cells (Shao et al. 2010). MORC2 also interacts with HDAC1 to form a complex which then gets recruited to the p21 promoter in a p53-independent manner. This results in downregulation of p21 expression leading to cell cycle progression in gastric cancer cells (Zhang et al. 2015). Furthermore, Wang et al. demonstrated that PAK1-mediated MORC2 phosphorylation is one of the lead causes for gastric tumorigenesis (Wang et al. 2015). MORC2 in combination with DNA methyltransferase 3A (DNMT3A) recruits at the promoters of neurofibromatosis 2 and KIBRA, controls their DNA hypermethylation and transcriptional repression, and thereby promoted cancer stemness and tumorigenesis of hepatocellular carcinoma cells (Wang et al. 2018).

In addition to proliferation, MORC2 also regulates the invasion and migratory functions in different types of cancers. The ArgBP2 protein is known for its repressive functions in the proliferation, cell migration, and invasion of gastric cancer cells (Taieb et al. 2008). MORC2 was found to regulate the transcriptional repression of ArgBP2 by forming a complex either with EZH2 or HSF1 and binding to the ArgBP2 promoter, thereby modulating the proliferative, invasion, and migratory functions of cancer cells (Tong et al. 2015, 2018). In addition, MORC2 has been shown to downregulate NDRG1 in association with histone deacetylase sirtuin 1(SIRT1) and activate colorectal cancer metastasis (Liu et al. 2019a). A recent study established MORC2 role in breast cancer invasion and metastasis by interacting with catenin delta 1 (CTNND1) (Liao et al. 2017). Similarly, the interaction between mutant MORC2 (M276I) and hnTNPM (heterogenous nuclear ribonucleoprotein M) stimulated the splicing-based switch of CD44 from the epithelial isoform to the mesenchymal isoform, leading to an epithelial to mesenchymal transition, which is a prerequisite for the metastasis (Zhang et al. 2018).

MORC2 role in metabolism

To meet the continuous energy requirements for tumor growth and progression, cancer cells undergo metabolic reprogramming, which is considered one of the important hallmarks of cancer (Hanahan and Weinberg 2011). Even if there is little to no availability of oxygen, cancer cells can rely on glycolysis rather than mitochondrial oxidative phosphorylation for their energy requirements – with this phenomenon known as the Warburg effect (Heiden et al. 2009). Recently we identified that MORC2 plays a key role in rewiring glucose metabolism to promote breast cancer cell migration (Fig. 3) (Guddeti et al. 2021). We found that MORC2 is a glucose-inducible gene and transcriptional target of c-Myc where c-Myc specifically binds to the E-box sequence (5’-CACGTG-3’) of the MORC2 promoter and regulates the expression of MORC2 under high glucose concentration in cancer cells (Guddeti et al. 2021). Further to this, we noticed a positive correlation between MORC2 expression and the expression of a number of glycolytic enzymes (e.g., hexokinase 1, lactate dehydrogenase A, phosphofructokinase liver type (PFKL) and phosphofructokinase platelet isoform (PFKP) in breast cancer patients. It was found that c-Myc stimulates the expression of MORC2 and consequently interacts with MORC2 in a feed-forward loop mechanism, in order to transcriptionally regulate LDHA (lactate dehydrogenase A) expression (Fig. 3A) and in turn the enzyme activity. LDHA activity was found to be very crucial for cancer cell migration (Guddeti et al. 2021).

Fig. 3.

Model depicting the role of MORC2 in cancer metabolism. A Schematic representation of MORC2 regulation of LDHA expression and function: high glucose concentration in the culture medium stimulates the induction of MORC2 expression in cancer cells. Once, MORC2 is induced, it forms a complex with c-Myc and gets recruited to LDHA promoter to induce LDHA expression, which in turn is responsible for cancer cell migration. B A model representing the interaction between MORC2 and ACYL: adipogenic signal promotes the interaction between MORC2 and ACYL in the cytosol leading to the activation of ACYL. This activated ACYL is essential for increased fatty acid synthesis in cancer cells. Bold black arrows represent either the induction of MORC2 or increased activation of ACYL proteins

A recent study found that MORC2 promotes lipogenesis and adipocyte differentiation by posttranslational activation of ATP-citrate lyase (ACLY) enzyme, which catalyzes the formation of acetyl-coA (Sánchez-Solana et al. 2014). Adipogenic signals facilitate MORC2 and ACLY protein-protein interaction where ACYL was found to interact with MORC2 region 63-718 a.a. with the interaction taking place within the cytosol (Table 1), to promote ACLY phosphorylation and, thereby, also promote its activity (Sánchez-Solana et al. 2014). Activated ACLY enzyme results in increased production of acetyl-CoA, which is an essential building block for the fatty acid synthesis (Fig. 3B).

The microRNA (miRNA) sponges contain complementary binding sites to a specific miRNA. miRNA sponges bind to the target miRNAs through these complementary sequences and inhibit the action of miRNAs (Ebert and Sharp 2010). Interestingly in cholangiocarcinoma (CCA) cells, Su et al. identified that circular RNA CircDNM3OS is a sponging miR of miR-145-5p and MORC2 is a specific target gene of miR-145-5p (Su et al. 2021). Their study revealed that in CCA cells, MORC2 expression levels were elevated by circular RNA circDNM3OS via sponging miR-145-5p in order to modulate glutamine metabolism and malignancy. Furthermore, MORC2 upregulation also increased glutamine consumption, its conversion to α-ketoglutarate, and ATP levels in CCA cells (Su et al. 2021). Together, these findings established a promising role for MORC2 in cancer metabolic reprogramming.

MORC2 role in DNA repair

Posttranslational modification of a protein can regulate its specific function; for instance, upon DNA damage, the balance of activity between acetyltransferase NAT10 and deacetylase SIRT2 is shifted to affect acetylation of MORC2 in breast cancer cell lines (Liu et al. 2020). Enhanced acetylation of MORC2 (K767Ac) ensures both resistance to DNA damaging agents and breast cancer cell survival by regulating cell cycle progression at the G2-M checkpoint (Liu et al. 2020). Along with acetylation, MORC2 phosphorylation (Wang et al. 2015; Yang et al. 2020) and PARylation (Zhang and Li 2019) by virtue of its interaction with PAK1 (interacting region: 719-843) and PARP1 (interacting region: 490-718), respectively, have also been shown to play an essential role in maintaining high levels of MORC2 in cancer cells, thereby facilitating its role in DNA repair.

MORC2 role in endocrine resistance

Aberrant activation of estrogen signaling is one of the major route causes for breast cancer progression and pathogenesis (Yager and Davidson 2006). Antiestrogen treatment using drugs like tamoxifen (TAM) and fulvestrant (FUL) was found to be more effective in estrogen-positive breast tumors but resulted in endocrine resistance (Mo et al. 2013; Ignatov et al. 2010; Ignatov et al. 2011; Yang et al. 2020). Yang et al. reported that TAM and FUL stabilize MORC2 in a GPER1 (G protein-coupled estrogen receptor 1)-dependent manner (Yang et al. 2020). The underlying mechanism included activation of PRKACA (protein kinase cAMP-activated catalytic subunit alpha) by GPER1, with phosphorylation of MORC2 at threonine 582 (T582), resulting in its decreased interaction with HSPA8 and LAMP2A. This resulted in MORC2 protection from CMA-mediated lysosomal degradation, leading to the MORC2 stabilization and in turn endocrine resistance (Yang et al. 2020).

Conclusions

MORC2 functions as a scaffolding protein, and its unique domain architecture facilitates protein-protein interactions. These interactions of MORC2 are found to be important for proliferation, invasion, migration, cancer stemness, DNA repair mechanisms, and metabolism in cancer. Current knowledge about MORC2 interacting partners is quite basic and warrants more efforts to identify novel interacting partners for MORC2. This will help in developing new therapeutic strategies to disrupt the interactions of MORC2 with specific protein partners in tumor cells for effective targeting of cancers.

Author contribution

RKG, NC, and PSB wrote this review.

Date availability

Not applicable

Declarations

Conflict of interest

The authors declare no competing interests.