Dual specificity phosphatases (DSP) play an important role in control of the cell cycle and signal transduction. We have synthesized a new class of DSP inhibitors. Cpd 5 or [2-(
2-mercaptoethanol)-3-methyl-1,4-
naphthoquinone] is one of the most potent of these. It inhibits DSPs of cells in culture and induces
tyrosine phosphorylation of various DSP substrates, including the Cdc25 target Cdks and it potently inhibits cell growth. In this study, we have evaluated Cpd 5 in vivo for its antitumor and growth inhibitory activity on
carcinogen-altered foci. Cpd 5 inhibited growth of the transplantable rat
hepatoma cell line JM-1 in vitro, with concomitant phosphorylation of the
mitogen-activated protein kinase ERK1/2 but not JNK1/2 or p38. This ERK phosphorylation was associated with growth inhibition, as the ERK phosphorylation inhibitor
PD098059 antagonized both ERK phosphorylation and growth inhibition. JM-1 cell lysates were found to contain ERK1/2-specific
phosphatase(s) that could be inhibited by Cpd 5 and which are thought to be its major targets. Cpd 5 caused significant inhibition of both intrahepatic and subcutaneous (s.c.) growth of transplanted JM-1 cells in male Fischer F344 rats. The treatment was equally effective whether Cpd 5 was administered either as a single, acute dose or chronically as several lower doses. However, toxicity was much lower with chronic treatment. As in JM-1 cells in vitro, ERK1/2 was phosphorylated when rats in vivo were treated with Cpd 5 and
tumor growth inhibition in vivo also was antagonized by treating rats with the ERK1/2 phosphorylation inhibitor
PD098059. A single dose of Cpd 5 also inhibited the formation of
glutathione S-transferase-pi enzyme-altered cells induced by the hepatocarcinogen
N-nitrosodiethylamine. This is the first report of the in vivo activity and growth inhibitory mechanism of a novel class of K
vitamin growth inhibitors that have potent
tyrosine phosphatase activity.