Functional characterization of the tumor-suppressor MARCKS in colorectal cancer and its association with survival.

The myristoylated alanine-rich C-kinase substrate (MARCKS) acts as a tumor suppressor in a variety of human neoplasms. In colorectal cancers (CRCs), MARCKS has been shown to be a preferential target of mutational inactivation in tumors following the microsatellite instability (MSI-H) pathway but little is known about its impact on intestinal carcinogenesis. To investigate the relevance of MARCKS inactivation in more detail, we analyzed 926 MSI-typed CRCs for MARCKS expression by immunohistochemistry and studied the functional consequences of MARCKS depletion in colorectal cancer cell lines. We found that loss of MARCKS expression was not restricted to MSI-H cancers but also occurred in microsatellite stable (MSS) tumors, where it was associated with an adverse outcome regarding overall survival, cancer-specific and disease-free survival (P=0.002, P=0.0018, P=0.0001, respectively; univariate analysis). In MARCKS-positive MSS colon cancer cell lines (SW480 and SW707) small interfering RNA (siRNA)-mediated knockdown of MARCKS conferred resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. This was accompanied by the downregulation of the TRAIL receptors DR4 and DR5 at the cell surface and activation of AKT signaling. Inhibition of AKT signaling and transient overexpression of wild-type MARCKS, but not of MARCKS lacking the effector domain (ED), abolished the anti-apoptotic effect. In conclusion, our data show that inactivation of MARCKS is common in CRCs and is associated with adverse outcome in MSS cancers. The finding that MARCKS acts as a mediator of apoptosis in MSS CRC cells adds a novel tumor-suppressing function to the so far established roles of MARCKS in cell motility and proliferation and can explain the prognostic effect of MARCKS depletion in MSS CRC.