Role and regulation of phosphatidylinositol 3-kinase in melanoma cells

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The signaling pathway of hepatocyte growth factor (HGF) through its receptor-tyrosine kinase c-Met was examined in human normal melanocytes and three malignant melanoma cell lines MeWo, HM3KO, and HMV-I. HGF-induced c-Met activation was observed in both melanocytes and melanoma cells, whereas phosphatidylinositol 3-kinase (PI3K), a downstream target of c-Met, was not activated in the melanocytes but enhanced in the melanoma cell lines. In contrast, extracellular signal-regulated kinase activity, another target of c-Met, was enhanced by HGF in both the melanocytes and melanoma cells. Therefore, the role of Gab1, the scaffolding adapter protein that couples activated c-Met and PI3K, was analyzed in these cells. Gab1 in the melanocytes showed an electrophoretic mobility slower than that in the melanoma cells, and the mobility shifted to that of the melanoma cells after treatment with alkaline phosphatase, indicating that Gab1 is highly phosphorylated on serine and threonine in the melanocytes. Introduction of protein kinase C (PKC) βII into the melanoma cells, which is expressed in melanocytes but absent in melanoma cells, using an adenovirus vector resulted in serine and threonine phosphorylation of Gab1 and also prevented tyrosine phosphorylation of Gab1 and its association with PI3K. Furthermore, the introduction of PKCβII suppressed HGF-induced activation of PI3K, and attenuated the in vitro invasion activity of the melanoma cells. These results indicate that the HGF signaling process from Gab1 to PI3K is negatively regulated by PKCβII, and that the loss of PKCβII is one of the steps for melanoma cells to gain invasive potential.