Retinoic acid nuclear receptors and tumor promotion: decreased expression of retinoic acid nuclear receptors by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate.

Abstract

During studies to determine the mechanism of tumor promotion by 12-O-tetradecanoylphorbol-13-acetate (TPA), we found that TPA downregulates mouse epidermal retinoic acid nuclear receptors (RAR), a superfamily of nuclear steroid/thyroid receptors implicated in mediating effects of retinoic acid (RA). Application of TPA to mouse skin decreased the binding of [3H]RA to RAR from mouse epidermal nuclear extracts. In this experiment, 20 nmol of TPA was applied to mouse skin and 3.5 h later binding of [3H]RA to RAR was analyzed by chromatography on a size-exclusion column. TPA treatment resulted in an approximately 67% decrease in the specific binding of [3H]RA to RAR. In a more detailed time course, application of 20 nmol of TPA to mouse skin led to 20, 36, 92 and 0% decrease in the binding of [3H]RA to mouse epidermal RAR at 2, 4, 12 and 72 h after treatment respectively. Okadaic acid, an inhibitor of protein phosphatases 1 and 2A but a mouse skin tumor promoter, also inhibited the binding of RA to RAR. RAR alpha and RAR gamma, but not RAR beta mRNA, could be detected in mouse epidermis. In addition, RA nuclear receptor RXR alpha was also expressed in the mouse epidermis. As determined by Northern blot analysis of total as well as poly(A)+ RNA, application of 10 nmol of TPA to mouse skin led to decreased expression of RAR alpha, RAR gamma and RXR alpha mRNA at 3.5 h after treatment. The effect of TPA on the attenuation of RAR expression was specific. Specific binding of RA to RAR was decreased when TPA-induced expression of the c-fos, c-jun and ornithine decarboxylase gene was increased. Downregulation of RAR(s) may be an essential component of the mechanism of mouse skin tumor promotion.