Differentiation of human melanoma cells through p38 MAP kinase is associated with decreased retinoblastoma protein phosphorylation and cell cycle arrest.

The retinoblastoma protein (pRB), the product of the retinoblastoma gene, is a key regulator of the cell cycle, affecting apoptosis, proliferation and differentiation. Dysregulation of pRB is implicated in the pathogenesis of many cancers, including malignant melanoma. Recently we demonstrated that alpha-melanocyte-stimulating hormone (alpha-MSH)-induced activation of p38 mitogen-activated protein (MAP) kinase leads to differentiation of B16 murine melanoma cells. The current study assesses the ability of alpha-MSH to activate p38 MAP kinase in COLO 853 human melanoma cells and determines whether this is linked to modulation of pRB activity. Treatment of COLO 853 cells with alpha-MSH induced time- and concentration-dependent increases in the phosphorylation of p38 MAP kinase, which corresponded with its ability to induce melanogenesis and inhibit cell growth. SB 203580, a selective inhibitor of p38 MAP kinase, blocked both the alpha-MSH-induced melanogenic response and inhibition of cell growth. Cell cycle analysis using flow cytometry revealed that treatment of COLO 853 cells with alpha-MSH for 72 h led to an increase in the proportion of cells in the G(1) phase and a marked reduction in the amount of phosphorylated pRB. Both of these effects were reversed by pre-treatment of cells with SB 203580. In summary, we have demonstrated for the first time that the alpha-MSH-induced differentiation of COLO 853 human melanoma cells proceeds via a p38 MAP kinase-mediated pathway and is associated with decreased pRB phosphorylation and accumulation of cells in the G(1) phase.