Mcl-1 is required for melanoma cell resistance to anoikis.

Melanoma is a particularly aggressive tumor type that exhibits a high level of resistance to apoptosis. The serine/threonine kinase B-RAF is mutated in 50% to 70% of melanomas and protects melanoma cells from anoikis, a form of apoptosis induced by lack of adhesion or adhesion to an inappropriate matrix. Mutant B-RAF down-regulates two BH3-only proapoptotic proteins, Bim(EL) and Bad. BH3-only proteins act, at least in part, by sequestering prosurvival Bcl-2 family proteins and preventing them from inhibiting the mitochondrial apoptotic pathway. Several Bcl-2 proteins are up-regulated in melanoma; however, the mechanisms of up-regulation and their role in melanoma resistance to anoikis remain unclear. Using RNA interference, we show that depletion of Mcl-1 renders mutant B-RAF melanoma cells sensitive to anoikis. By contrast, minor effects were observed following depletion of either Bcl-2 or Bcl-(XL). Mcl-1 expression is enhanced in melanoma cell lines compared with melanocytes and up-regulated by the B-RAF-MEK-extracellular signal-regulated kinase 1/2 pathway through control of Mcl-1 protein turnover. Similar to B-RAF knockdown cells, adhesion to fibronectin protected Mcl-1 knockdown cells from apoptosis. Finally, expression of Bad, which does not sequester Mcl-1, further augmented apoptosis in nonadherent Mcl-1 knockdown cells. Together, these data support the notion that BH3 mimetic compounds that target Mcl-1 may be effective for the treatment of melanoma in combinatorial strategies with agents that disrupt fibronectin-integrin signaling.

Cyclin D3 expression in melanoma cells is regulated by adhesion-dependent phosphatidylinositol 3-kinase signaling and contributes to G1-S progression.

D-type cyclins regulate G1 cell cycle progression by enhancing the activities of cyclin-dependent kinases (CDKs), and their expression is frequently altered in malignant cells. We and others have previously shown that cyclin D1 is up-regulated in melanoma cells through adhesion-independent MEK-ERK1/2 signaling initiated by mutant B-RAF. Here, we describe the regulation and role of cyclin D3 in human melanoma cells. Cyclin D3 expression was enhanced in a cell panel of human melanoma cell lines compared with melanocytes and was regulated by fibronectin-mediated phosphatidylinositol 3-kinase/Akt signaling but not MEK activity. RNA interference experiments demonstrated that cyclin D3 contributed to G1-S cell cycle progression and proliferation in melanoma cells. Overexpression of cyclin D1 did not recover the effects of cyclin D3 knockdown. Finally, immunoprecipitation studies showed that CDK6 is a major binding partner for cyclin D3, whereas CDK4 preferentially associated with cyclin D1. Together, these findings demonstrate that cyclin D3 is an important regulator of melanoma G1-S cell cycle progression and that D-type cyclins are differentially regulated in melanoma cells.