Orthovanadate, an inhibitor of protein tyrosine phosphatases, acts more potently as a promoter than as an initiator in the BALB/3T3 cell transformation.

The phosphorylation and dephosphorylation of proteins are critical in cellular signal transduction. Phorbol esters and okadaic acid, which affect protein phosphorylation, are potent promoters in mouse skin carcinogenesis and cell transformation in vitro. Orthovanadate inhibits protein tyrosine phosphatases and causes hyperphosphorylation of cellular proteins. We have performed two-stage transformation assays using BALB/3T3 cells to determine the major activity of orthovanadate (1-10 microM) for transformation. This chemical acted as a weak initiator because its initiating treatment produced a significant, though small, number of transformed foci in the presence of promoting treatment by 12-O-tetradecanoylphorbol-13-acetate (TPA) but not in the absence of TPA. Promoting treatment by orthovanadate markedly enhanced the transformation of the cells pretreated by a subthreshold dose of 3-methylcholanthrene (MCA) but not of non-pretreated cells. Superiority of promoting over initiating activity of orthovanadate was confirmed by an assay carried out in the reversed treatment sequence (orthovanadate and then MCA), where the transformation frequency was conspicuously decreased compared with the regular treatment sequence. The transformed foci in the cultures treated by orthovanadate, following MCA treatment, continued to grow in normal medium, showing cell proliferation independent of orthovanadate. Orthovanadate, in addition to TPA and okadaic acid, will be a useful reagent for studying the signaling cascades responsible for tumor promotion.