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Although protein kinase C (PKC) has been implicated in cell cycle progression, cell proliferation, and tumor promotion, the precise roles of specific isoforms in these processes is not clear. Therefore, we constructed and analyzed a series of expression vectors that encode hemagglutinin-tagged wild type (WT), constitutively active mutants (⌬NPS and CAT), and dominant negative mutants of PKCs ␣, ␤1, ␤2, ␥, ␦, ⑀, , , and .

In a recent study on head and neck squamous cell carcinoma (HNSCC) cells we found that epigallocatechin-3-gallate (EGCG), a major biologically active component of green tea, inhibited activation of the epidermal growth factor receptor (EGFR) and related signaling pathways. Since activation of EGFR signaling pathways is associated with angiogenesis, we examined the effects of EGCG on vascular endothelial growth factor (VEGF) production by YCU-H891 HNSCC and MDA-MB-231 breast carcinoma cell lines, because we found that both of these cell lines display autocrine activation of transforming growth factor-alpha (TGF-alpha)/EGFR signaling and produce high levels of VEGF. Treatment with EGCG inhibited the constitutive activation of the EGFR, Stat3, and Akt in both cell lines. These changes were associated with inhibition of VEGF promoter activity and cellular production of VEGF. Mechanistic studies indicated that inhibition of Stat3, but not mitogen-activated protein kinase kinase (MEK)1 or phosphatidylinositol 3'-kinase (PI3K), significantly decreased VEGF promoter activity. However, the inhibitory effects of a dominant negative Stat3 on VEGF expression was not as strong as that produced by EGCG. An analysis of alternative pathways indicated that EGCG strongly inhibited the constitutive activation of NF-kappa B in both cell lines, and an NF-kappa B inhibitor strongly inhibited VEGF production. These results suggest that EGCG inhibits VEGF production by inhibiting both the constitutive activation of Stat3 and NF-kappa B, but not extracellular-signal-regulated kinase (ERK) or Akt, in these cells. Therefore, EGCG may be useful in treating HNSCC and breast carcinoma because it can exert both antiproliferative and antiangiogenic activities.

We have studied the role of phosphorylation in the activation of metal-regulatory transcription factor-1 (MTF-1) and metallothionein (MT) gene expression. We showed that MTF-1 is phosphorylated in vivo and that zinc stimulates MTF-1 phosphorylation 2-4-fold. Several kinase inhibitors were used to examine the possible involvement of kinase cascades in the activation of MTF-1. Metal-induced MT gene expression was abrogated by protein kinase C (PKC), c-Jun N-terminal kinase (JNK), phosphoinositide 3-kinase, and tyrosine-specific protein kinases inhibitors, as assayed by Northern analysis and by cotransfection experiments using a metal regulatory element-luciferase reporter plasmid. The extracellular signal-activated protein kinase and the p38 kinase cascades did not appear to be essential for the activation of MT gene transcription by metals. By using dominantnegative mutants of PKC, JNK, mitogen-activated kinase kinase 4 (MKK4), and MKK7, we provide further evidence supporting a role for PKC and JNK in the activation of MTF-1 in response to metals. Notably, increased MTF-1 DNA binding in response to zinc and MTF-1 nuclear localization was not inhibited in cells preincubated with the different kinase inhibitors despite strong inhibition of MTF-1-mediated gene expression. This suggests that phosphorylation is essential for MTF-1 transactivation function. We hypothesize that metal-induced phosphorylation of MTF-1 is one of the primary events leading to increased MTF-1 activity. Thus, metal ions such as cadmium could activate MTF-1 and induce MT gene expression by stimulating one or several kinases in the MTF-1 signal transduction pathway.

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