GPR56 Drives Colorectal Tumor Growth and Promotes Drug Resistance through Upregulation of MDR1 Expression via a RhoA-Mediated Mechanism

Abstract

Drug resistance continues to be a major obstacle of effective therapy for colorectal cancer, leading to tumor relapse or treatment failure. Cancer stem cells (CSC) or tumor-initiating cells are a subpopulation of tumor cells which retain the capacity for self-renewal and are suggested to be implicated in drug resistance. LGR5 is highly expressed in colorectal cancer and marks CSCs that drive tumor growth and metastasis. LGR5( þ ) CSCs cells were shown to interconvert with more drug-resistant LGR5( À ) cancer cells, and treatment with LGR5-targeted antibody-drug conjugates (ADC) eliminated LGR5( þ ) tumors, yet a fraction of LGR5( À ) tumors eventually recurred. Therefore, it is important to identify mechanisms associated with CSC plasticity and drug resistance in order to develop curative therapies. Here, we show that loss of LGR5 in colon cancer cells enhanced resistance to irinotecan and 5-fluorouracil and increased expression of adhesion G-protein-coupled receptor, GPR56. GPR56 expression was significantly higher in primary colon tumors versus matched normal tissues and correlated with poor survival outcome. GPR56 enhanced drug resistance through upregulation of MDR1 levels via a RhoA-mediated signaling mechanism. Loss of GPR56 led to suppression of tumor growth and increased sensitivity of cancer cells to chemotherapy and monomethyl auristatin E-linked anti-LGR5 ADCs, by reducing MDR1 levels. These findings suggest that upregulation of GPR56 may be a mechanism associated with CSC plasticity by which LGR5( À ) cancer cells acquire a more drug-resistant phenotype.Implications: Our findings suggest that targeting GPR56 may provide a new strategy for the treatment of colorectal cancer and combatting drug resistance.

Regulation of Integrin Subunit Alpha 2 by miR-135b-5p Modulates Chemoresistance in Gastric Cancer

“…For example, miR-27a was decreased in bladder cancer and restored miR-27a re-sensitized cisplatin resistance by targeting the cystine/glutamate exchanger SLC7A11 (27). miR-340-5p was reduced in breast cancer, and its overexpression inhibited drug resistance to docetaxel by targeting LRG5 via the Wnt/β-catenin pathway (28,29). We previously found that low expression of miR-15b and miR-16 was detected in drug-resistant GC cells, which medicated sensitivity to vincristine (VCR) by directly regulating Bcl-2 (30).…”

Anti-GPR56 monoclonal antibody potentiates GPR56-mediated Src-Fak signaling to modulate cell adhesion

“…GPR56 has been shown to induce activation of the small GTPase RhoA mediated through Gα12/13 to promote cell adhesion (13,20,(24)(25)(26). To examine if mutant GPR56 STP was fully functional, we utilized a GTPase pulldown assay that employs the Rho binding domain of the Rho effector, Rhotekin.…”

“…GPR56 is upregulated in cancers of the breast, lung, ovary, pancreas, colon and glioblastoma (9)(10)(11). Analysis of clinical data revealed a significant correlation between high GPR56 levels and poor outcome in acute myeloid leukemia, ovarian cancer, and colorectal cancer (CRC) (10)(11)(12)(13)(14)(15). In CRC, GPR56 was shown to promote drug resistance and drive tumor growth (12,13,16).…”

“…Analysis of clinical data revealed a significant correlation between high GPR56 levels and poor outcome in acute myeloid leukemia, ovarian cancer, and colorectal cancer (CRC) (10)(11)(12)(13)(14)(15). In CRC, GPR56 was shown to promote drug resistance and drive tumor growth (12,13,16). On the contrary, GPR56 has been shown to be downregulated in metastatic melanoma and inhibitory to melanoma growth and metastasis (17).…”

“…GPR56 has been shown to interact with proteins such as collagen III (24), transglutaminase 2 (TG2) (17), and progastrin (16); however receptor-specific ligands have yet to be fully validated. Several studies have established that GPR56 is coupled to the Gα12/13 class of heterotrimeric G-proteins to promote RhoA activation (13,22,(24)(25)(26). GPR56 overexpression has been shown to activate various signaling pathway response elements, including serum response element (SRE) and serum response factor response element (SRF-RE) that are likely regulated downstream of RhoA (20,21,25).…”