The Tumor Promoter 12-O-Tetradecanoylphorbol-13-Acetate and the ras Oncogene Modulate Expression and Phosphorylation of Gap Junction Proteins

Abstract

Gap junctional intercellular communication is inhibited in response to tumor promoters and oncogene transformation, suggesting that loss of this function is an important step in tumor formation. To elucidate the molecular mechanisms responsible for this inhibition, we examined the expression of gap junction proteins and mRNA in mouse primary keratinocytes after treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and/or ras transformation. During normal cell growth, keratinocytes express the α₁ (connexin 43) and β₂ (connexin 26) proteins. Within 5 min of TPA treatment, the α₁ protein became rapidly phosphorylated on serine residues and its expression was dramatically reduced by 24 h. The β₂ protein, after an initial increase in expression, was also significantly reduced 24 h after treatment with TPA. ras transformation caused changes similar to those induced by TPA. The α₁ protein underwent an increase in serine phosphorylation, although its expression declined only slightly, while β₂ expression was greatly reduced. The effects of TPA and ras on α₁ expression were additive; treatment of ras-transformed cells with TPA resulted in increased α₁ phosphorylation, with greatly decreased protein levels, much lower than those generated by either agent alone. These data provide a likely explanation for the similar and synergistic inhibition of gap junctional intercellular communication by phorbol esters and ras.