αB‐crystallin is mutant B‐RAF regulated and contributes to cyclin D1 turnover in melanocytic cells

Summary The serine/threonine kinase, B‐RAF, is frequently mutated in melanoma and is required for cell proliferation. Proteasomal turnover of cyclins and cyclin‐dependent kinase inhibitors via E3 ubiquitin ligases regulates cell cycle progression. We previously showed that B‐RAF regulates Cks1, a co‐factor for the F‐box protein Skp2. Recently, a second F‐box protein cofactor was identified, αB‐crystallin, that binds Fbx4 and promotes cyclin D1 degradation. Here, we demonstrate that αB‐crystallin is down‐regulated in mutant B‐RAF melanoma cells compared to melanocytes in a B‐RAF and MEK‐dependent manner. In a subset of lines, MEK inhibition was sufficient to up‐regulate αB‐crystallin protein levels; whereas in other lines combined MEK and proteasome inhibition was required. αB‐crystallin knockdown partially stabilized cyclin D1 in melanocytes. Expression of αB‐crystallin in mutant B‐RAF melanoma cells did not promote cyclin D1 turnover under normal conditions, but did enhance turnover following etoposide‐induced DNA damage. Together, these data show that αB‐crystallin is highly expressed in melanocytes contributing, in part, to cyclin D1 turnover. Furthermore, αB‐crystallin is down‐regulated in a B‐RAF‐dependent manner in melanoma cells and its re‐expression regulates cyclin D1 turnover after DNA damage.