Mcl-1 is required for melanoma cell resistance to anoikis

摘要

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 50–70% 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 pro-apoptotic proteins, BimEL and Bad. BH3-only proteins act, at least in part, by sequestering pro-survival B-cell lymphoma (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. Utilizing RNA interference, we show that depletion of myeloid cell leukemia-1 (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 to melanocytes and up-regulated by the B-RAF-MEK-ERK1/2 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 non-adherent 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.