The main function of K
vitamins is to act as co-factors for
gamma-glutamyl carboxylase. However, they have also recently been shown to inhibit cell growth. We have chemically synthesized a series of K
vitamin analogs with various side chains at the 2 or 3 position of the core
naphthoquinone structure. The analogs with short thio-
ethanol side chains are found to be more potent
growth inhibitors in vitro of various tumor cell lines. Cpd 5 or [2-(
2-mercaptoethanol)-3-methyl-1,4-
naphthoquinone] is one of the most potent. The anti-proliferation activity of these compounds is antagonized by exogenous
thiols but not by non-
thiol antioxidants. This suggests that the growth inhibition is mediated by sulfhydryl arylation of cellular
glutathione and
cysteine-containing
proteins and not by oxidative stress. The
protein tyrosine phosphatases (PTP) are an important group of
proteins that contain
cysteine at their catalytic site.
PTPs regulate mitogenic signal transduction and cell cycle progression. PTP inhibition by Cpd 5 results in prolonged
tyrosine phosphorylation and activation of several
kinases and
transcription factors including EGFR, ERK1/2, and Elk1. Cpd 5 could activate ERK1/2 either by signaling from an activated EGFR, which is upstream in the signaling cascade, or by direct inhibition of ERK1/2
phosphatase(s). Prolonged ERK1/2 phosphorylation strongly correlates with Cpd 5-mediated growth inhibition. Cpd 5 can also bind to and inhibit the Cdc25 family of dual specific
phosphatases. As a result, several Cdc25 substrates (Cdk1, Cdk2, Cdk4) involved in cell cycle progression are
tyrosine phosphorylated and thereby inhibited by its action. Cpd 5 could also inhibit both normal liver regeneration and
hepatoma growth in vivo.
DNA synthesis during rat liver regeneration following partial
hepatectomy, transplantable rat
hepatoma cell growth, and
glutathione-S-transferase-pi expressing hepatocytes after administration of the chemical
carcinogen diethylnitrosamine, are all inhibited by Cpd 5 administration. The growth inhibitory effect during liver regeneration and transplantable
tumor growth is also correlated with ERK1/2 phosphorylation induced by Cpd 5. Thus, Cpd 5-mediated inhibition of
PTPs, such as Cdc25 leads to cell growth arrest due to altered activity of key cellular
kinases involved in signal transduction and cell cycle progression. This prototype K
vitamin analog represents a novel class of
growth inhibitor based upon its action as a selective PTP antagonist. It is clearly associated with prolonged ERK1/2 phosphorylation, which is in contrast with the transient ERK1/2 phosphorylation induced by growth stimulatory
mitogens.