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1,25-Dihydroxyvitamin D₃ Enhances 12-O-Tetradecanoylphorbol-13-acetate-induced Tumorigenic Transformation and Osteopontin Expression in Mouse JB6 Epidermal Cells¹

Department of Nutrition Sciences [P-L. C., C. W. P.] and Comprehensive Cancer Center [C. W. P.], University of Alabama at Birmingham, Birmingham, Alabama 35294

To study the role of 1,25-dihydroxyvitamin D₃ (calcitriol) in tumor promotion, we used JB6 C141.5a cells, a mouse epidermal cell model of tumor promotion. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) irreversibly induces anchorage-independent growth and tumorigenicity in these cells. Since we previously showed that calcitriol does not transform these cells but inhibits their proliferation, we hypothesize that calcitriol would inhibit TPA-induced transformation. Concurrent treatment of JB6 C141.5a cells with TPA and calcitriol revealed that calcitriol enhanced (1.7- to 10-fold, depending on dose) TPA-induced anchorage-independent growth without enhancing cell proliferation. Furthermore, a more than additive effect on osteopontin mRNA and protein levels was observed with concurrent drug treatment, which yielded a more highly phosphorylated form of osteopontin. These studies suggest coordinate regulation between the signaling pathways for calcitriol and TPA in JB6 C141.5a cells and further implicate expression of phosphorylated osteopontin in tumorigenesis.

¹ This investigation was supported by NIH Grant DE06739 and a Research Career Development Award (DE00247) to C.W.P. and a Clinical Nutrition Research Unit center grant from the National Cancer Institute to the Department of Nutrition Sciences (Dr. C. L. Krumdieck, P.I.).

² To whom requests for reprints should be addressed, at Department of Nutrition Sciences, Webb Nutrition Sciences Building, 1675 University Boulevard, University of Alabama at Birmingham, Birmingham, AL 35294.

Received 3/ 1/93. Accepted 4/ 2/93.

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