Nuclear-targeted deleted in liver cancer 1 (DLC1) is less efficient in exerting its tumor suppressive activity both in vitro and in vivo

RhoGTPase Signalling in Cancer Progression and Dissemination

Rho GTPases are a family of small G proteins that regulate a wide array of cellular processes related to their key roles controlling the cytoskeleton. On the other hand, cancer is a multi-step disease caused by the accumulation of genetic mutations and epigenetic alterations, from the initial stages of cancer development when cells in normal tissues undergo transformation, to the acquisition of invasive and metastatic traits, responsible for a large number of cancer related deaths. In this review, we discuss the role of Rho GTPase signalling in cancer in every step of disease progression. Rho GTPases contribute to tumour initiation and progression, by regulating proliferation and apoptosis, but also metabolism, senescence and cell stemness. Rho GTPases play a major role in cell migration, and in the metastatic process. They are also involved in interactions with the tumour microenvironment and regulate inflammation, contributing to cancer progression. After years of intensive research, we highlight the importance of relevant models in the Rho GTPase field, and we reflect on the therapeutic opportunities arising for cancer patients.

Nuclear DLC1 exerts oncogenic function through association with FOXK1 for cooperative activation of MMP9 expression in melanoma

A Rho GTPase-activating protein (RhoGAP), deleted in liver cancer 1 (DLC1), is known to function as a tumor suppressor in various cancer types; however, whether DLC1 is a tumor-suppressor gene or an oncogene in melanoma remains to be clarified. Here we revealed that high DLC1 expression was detected in most of the melanoma tissues where it was localized in both the nuclei and the cytoplasm. Functional studies unveiled that DLC1 was both required and sufficient for melanoma growth and metastasis. These tumorigenic events were mediated by nuclear-localized DLC1 in a RhoGAP-independent manner. Mechanistically, mass spectrometry analysis identified a DLC1-associated protein, FOXK1 transcription factor, which mediated oncogenic events in melanoma by translocating and retaining DLC1 into the nucleus. RNA-sequencing profiling studies further revealed MMP9 as a direct target of FOXK1 through DLC1-regulated promoter occupancy for cooperative activation of MMP9 expression to promote melanoma invasion and metastasis. Concerted action of DLC1–FOXK1 in MMP9 gene regulation was further supported by their highly correlated expression in melanoma patients’ samples and cell lines. Together, our results not only unravel a mechanism by which nuclear DLC1 functions as an oncogene in melanoma but also suggest an unexpected role of RhoGAP protein in transcriptional regulation.

Mechanotransduction in talin through the interaction of the R8 domain with DLC1

The mechanical unfolding of proteins is a cellular mechanism for force transduction with potentially broad implications in cell fate. Despite this, the mechanism by which protein unfolding elicits differential downstream signalling pathways remains poorly understood. Here, we used protein engineering, atomic force microscopy, and biophysical tools to delineate how protein unfolding controls cell mechanics. Deleted in liver cancer 1 (DLC1) is a negative regulator of Ras homolog family member A (RhoA) and cell contractility that regulates cell behaviour when localised to focal adhesions bound to folded talin. Using a talin mutant resistant to force-induced unfolding of R8 domain, we show that talin unfolding determines DLC1 downstream signalling and, consequently, cell mechanics. We propose that this new mechanism of mechanotransduction may have implications for a wide variety of associated cellular processes.

GAP-independent functions of DLC1 in metastasis

Metastases are responsible for most cancer-related deaths. One of the hallmarks of metastatic cells is increased motility and migration through extracellular matrixes. These processes rely on specific small GTPases, in particular those of the Rho family. Deleted in liver cancer-1 (DLC1) is a tumor suppressor that bears a RhoGAP activity. This protein is lost in most cancers, allowing malignant cells to proliferate and disseminate in a Rho-dependent manner. However, DLC1 is also a scaffold protein involved in alternative pathways leading to tumor and metastasis suppressor activities. Recently, substantial information has been gathered on these mechanisms and this review is aiming at describing the potential and known alternative GAP-independent mechanisms allowing DLC1 to impair migration, invasion, and metastasis formation.

DLC1 expression is reduced in human cutaneous melanoma and correlates with patient survival

Deleted in Liver Cancer-1 (DLC1) is a Rho-GTPase-activating protein known to be downregulated and function as a tumor suppressor in numerous solid and hematological cancers. Its expression status in melanoma is currently unknown however, prompting us to examine this. Using immunohistochemistry and tissue microarrays containing a large set of melanocytic lesions (n=539), we examined the expression profile of DLC1 in melanoma progression, as well as the association between DLC1 and patient survival. We detected both cytoplasmic and nuclear DLC1 expression, and found that whereas cytoplasmic DLC1 was significantly downregulated in metastatic melanoma compared with nevi and primary melanoma, nuclear DLC1 expression was significantly down in primary melanoma compared with nevi, and then further down in metastatic melanoma. Loss of cytoplasmic DLC1 was significantly associated with poorer overall and disease-specific 5-year survival rates of all melanoma (P<0.001 and P=0.001, respectively) and metastatic melanoma patients (P=0.020 and 0.008, respectively), and similar results were seen for nuclear DLC1 (P<0.001 for both overall and disease-specific survival for all melanoma patients, and P=0.004 for metastatic melanoma patients). Next, we examined the correlation between cytoplasmic and nuclear DLC1 and found that concomitant loss of both forms was associated with the worst outcome for metastatic melanoma patients (P=0.013 and P=0.008 for overall and disease-specific 5-year survival, respectively). Finally, multivariate Cox regression analysis determined that strong cytoplasmic and nuclear DLC1 expression was a favorable independent prognostic factor for all melanoma (HR, 0.61; 95% CI, 0.42-0.88; P=0.008) and metastatic melanoma patients (HR, 0.42; 95% CI, 0.23-0.77; P=0.005). Although more research still needs to be done on the topic, these preliminary results support the hypothesis that DLC1 is a tumor suppressor in melanoma.

HBV infection and hepatocellular carcinoma

The hepatitis B virus (HBV) plays a dominant role in the 749,000 new cases and 692,000 deaths related to hepatocellular carcinoma (HCC) that are estimated to occur each year worldwide. Chronic infection with HBV is responsible for 60% of HCCs in Asia and Africa and at least 20% of the tumors in Europe, Japan, and the United States. This article discusses the pathogenic role of HBV and the risk of HCC. Tumors almost invariably develop in the context of chronic hepatitis or cirrhosis, which makes early diagnosis the only practical approach to improve prognosis. The treatment options are also discussed.

Promoter methylation of the DLC‑1 gene and its inhibitory effect on human colon cancer

Deleted in liver cancer‑1 (DLC‑1), a candidate tumor suppressor gene which is inactive in liver carcinogenesis, is located at 8p21.3, where deletions are frequently found in several types of human cancer. Promoter hypermethylation is an epigenetic mechanism leading to silencing of the gene expression, which may be the primary cause for the absence of DLC‑1. We investigated the expression of the DLC‑1 gene and the methylation of the DLC‑1 gene in colon cancer cell lines (Caco‑2, LoVo and HT‑29). The data showed that reduced or undetectable levels of DLC‑1 mRNA were found in HT‑29 by reverse transcription-polymerase chain reaction (RT‑PCR). By contrast, the DLC‑1 gene was significantly expressed in Caco‑2 and LoVo cells. These findings were in agreement with the data obtained from western blot analysis. To further determine whether aberrant methylation is a contributing factor to transcriptional inactivation of DLC‑1 in HT‑29, the methylation of promoter was examined using methylation‑specific PCR and sodium bisulfite genomic sequencing in LoVo and HT‑29 cells, which suggests that promoter hypermethylation accounts for silencing of the DLC‑1 gene in HT‑29 cells. Since DLC‑1 is a candidate tumor suppressor gene, we sought to determine whether DLC‑1 expression is associated with cell proliferation in colon cancer cell lines. RNA interference techniques were adopted to inhibit DLC‑1 expression in the LoVo cell line and resulted in inhibition of cell growth and reduced colony formation. Collectively, our observations suggest that hypermethylation is responsible for abrogating the function of the DLC‑1 gene in colon cancer and indicate a role of DLC‑1 in colon carcinogenesis.