Abstract
Elevated expression of protein kinase C beta II (PKC beta II) is an early promotive event in colon carcinogenesis (Gokmen-Polar, Y., Murray, N. R., Velasco, M. A., Gatalica, Z., and Fields, A. P. (2001) Cancer Res. 61, 1375-1381). Expression of PKC beta II in the colon of transgenic mice leads to hyperproliferation and increased susceptibility to colon carcinogenesis due, at least in part, to repression of transforming growth factor beta type II receptor (TGF-beta RII) expression (Murray, N. R., Davidson, L. A., Chapkin, R. S., Gustafson, W. C., Schattenberg, D. G., and Fields, A. P. (1999) J. Cell Biol., 145, 699-711). Here we report that PKC beta II induces the expression of cyclooxygenase type 2 (Cox-2) in rat intestinal epithelial (RIE) cells in vitro and in transgenic PKC beta II mice in vivo. Cox-2 mRNA increases more than 10-fold with corresponding increases in Cox-2 protein and PGE2 production in RIE/PKC beta II cells. PKC beta II activates the Cox-2 promoter by 2- to 3-fold and stabilizes Cox-2 mRNA by at least 4-fold. The selective Cox-2 inhibitor Celecoxib restores expression of TGF-beta RII both in vitro and in vivo and restores TGF beta-mediated transcription in RIE/PKC beta II cells. Likewise, the omega-3 fatty acid eicosapentaenoic acid (EPA), which inhibits PKC beta II activity and colon carcinogenesis, causes inhibition of Cox-2 protein expression, re-expression of TGF-beta RII, and restoration of TGF-beta1-mediated transcription in RIE/PKC beta II cells. Our data demonstrate that PKC beta II promotes colon cancer, at least in part, through induction of Cox-2, suppression of TGF-beta signaling, and establishment of a TGF-beta-resistant, hyperproliferative state in the colonic epithelium. Our data define a procarcinogenic PKC beta II --> Cox-2 --> TGF-beta signaling axis within the colonic epithelium, and provide a molecular mechanism by which dietary omega-3 fatty acids and nonsteroidal antiinflammatory agents such as Celecoxib suppress colon carcinogenesis.
Highlights
Cancer has been described as a disease of aberrant signal transduction [1]
To determine whether elevated Protein kinase C (PKC)II levels contribute to colon carcinogenesis, we developed transgenic protein kinase C II (PKCII) mice that express elevated PKCII in the colonic epithelium to levels comparable with those observed in carcinogen-induced colon tumors [6, 7]
A real-time RT-PCR assay for human PKCII mRNA demonstrated that rat intestinal epithelial (RIE)/PKCII cells expressed human PKCII mRNA at levels somewhat lower than those observed in human HEK293 cells, which express abundant endogenous PKCII protein (Fig. 1B)
Summary
Cancer has been described as a disease of aberrant signal transduction [1]. Carcinogenesis is a multistep process characterized by progressive changes in the amounts or activity of proteins that regulate cellular proliferation, differentiation, and survival [1, 2]. The -3 fatty acid eicosapentaenoic acid (EPA), which inhibits PKCII activity and colon carcinogenesis, causes inhibition of Cox-2 protein expression, re-expression of TGF-RII, and restoration of TGF-1-mediated transcription in RIE/PKCII cells.
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