DLC1 exerts oncogenic role through association with FOXK1 to synergistically activate MMP9 expression in metastatic melanoma

Abstract

Melanoma is considered as the most aggressive form of skin cancer, accounting for 72% of skin cancer-related deaths globally. Over 50% of melanoma patients harbor the BRAF mutation, which leads to constitutively activated oncogenic signaling pathways, resulting in the neoplastic transformation of melanocytes into metastatic melanomas. Despite improved overall survival of patients with metastatic melanoma treated with BRAF inhibitors, the cancer can rapidly acquire resistance to the treatment that limits its long-term efficacy. Therefore, there is an urgent need to identify new factors in governing melanoma colonization as alternative therapeutic targets.

Deleted in liver cancer 1 (DLC1), a RhoGTPase-activating (RhoGAP) protein, has been characterized as a tumor suppressor, as its expression was frequently found to be downregulated in various cancer types, and restoration of its expression inhibited tumorigenic growth. DLC1 negatively regulates the activity of RHOA, RHOB, RHOC, and to a lesser extent CDC42 by promoting the hydrolysis of active GTP-bound state into the inactive GDP-bound form. Most of the studies focused on the inhibitory effects of DLC1 on RHOA activity, which is involved in cell proliferation and actin cytoskeletal reorganization for cell migration. A previous report demonstrated that DLC1 inhibited TGF--induced expression of parathyroid hormone-like hormone through suppression of RHO-ROCK signaling to prevent breast cancer bone metastasis. In contrast, studies in other cellular contexts showed that interaction of DLC1 with partner factors contributed to the anti-tumorigenic functions via a RhoGAP-independent mechanism, implying a context-dependent role of DLC1. Apart from its cytoplasmic role in regulating RHO signaling, DLC1 can be localized in the nucleus where it was shown to be less efficient in exerting tumor suppressor activity in hepatocellular carcinoma. Similarly, DLC1 was also found to be localized in both the cytoplasm and nuclei of metastatic melanomas but how this subcellular localization of DLC1 regulates melanoma progression is largely unknown. I n the current study, I found that DLC1 expression was upregulated in the majority of melanoma tissues where it was localized in both the nuclei and the cytoplasm. Functional assays showed that nuclear DLC1 promoted growth and invasion of melanoma cells in a RhoGAP-independent manner. By proteomic analysis, I identified Forkhead box transcription factor, FOXK1, was crucial for DLC1 nuclear translocation and retention to orchestrate oncogenic programs. RNA-sequencing identified Matrix metalloproteinase 9 (MMP9) was commonly downregulated in both FOXK1 and DLC1 knockdown cells. Mechanistically, DLC1 was essential for FOXK1 occupancy to the promoter of MMP9, and both DLC1 and FOXK1 synergistically activate MMP9 expression for melanoma invasion and metastasis. My results provide a new paradigm of the underlying mechanism by which a well-known tumor suppressor DLC1 functions in the nucleus as an oncogene in melanoma.