1. ¹Division of Dermatology, Department of Clinical Molecular Medicine, Kobe University Graduate School of Medicine, Kobe, Japan
2. ²Biosignal Research Center, Kobe University, Kobe, Japan

Correspondence: Dr Masahiro Oka, Division of Dermatology, Department of Clinical Molecular Medicine, Kobe University Graduate School of Medicine, 7-5-1, Kusonoki-cho, Chuo-ku, Kobe 650-0017, Japan. E-mail: oka@med.kobe-u.ac.jp

Received 27 January 2007; Revised 2 April 2007; Accepted 17 April 2007; Published online 12 July 2007.

Abstract

The hepatocyte growth factor (HGF) signaling pathway was examined in human normal melanocytes and three malignant melanoma cell lines. HGF-induced activation of c-Met, its receptor-tyrosine kinase, 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. The electrophoretic mobility of Gab1, the scaffolding adapter protein that couples activated c-Met and PI3K, was slower in the melanocytes 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)-II into the melanoma cells, which is expressed in melanocytes but absent in melanoma cells, resulted in serine and threonine phosphorylation of Gab1 and also prevented tyrosine phosphorylation of Gab1 and its association with PI3K. Furthermore, the introduction of PKC-II 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 PKC-II, and its loss is critical for melanoma cells to gain invasive potential.