The effects of retinoic acid (RA) and other vitamin A derivatives (retinoids) on physiological processes such as development, differentiation, homeostasis and carcinogenesis are largely mediated by the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs) that are members of the steroid/thyroid hormone receptor superfamily. Epidemiological studies have demonstrated that a diet rich in vitamin A and carotenoids prevents significantly the development of lung cancer. However, the pharmacological effects of RA in the treatment of lung cancer patients are poor. A first explanation for the treatment failure was provided by the recent finding that most lung cancer cell lines express abnormally the RARbeta gene and more importantly that the expression of RARbeta gene cannot be induced by RA treatment. In addition, our preliminary results demonstrate that RARalpha and RARgamma, although being expressed, are both not functional in inducing RARbeta production in these lung cancer cell lines. Thus, the loss of RA response may directly contribute to the tumor progression by altering the growth rate and differentiation program of cancer cells. Recently, we have identified an RA response element (betaRARE) in the RARbeta gene promoter and observed that the abnormal RA response may be the result of an impaired interaction of RARs with the betaRARE. In the studies proposed here, we will first investigate the possibility that mutations in the RAR genes are responsible for the lost RAR activities. Next, we will analyze whether changes in the receptor phosphorylation levels occur in the lung cancer cell lines. Since we found that the RXR is required for efficient DNA binding and function of the RARs, we will include this receptor for both studies. Considering our observation that RXR may not function as RAR coregulators in lung cells tested, we will develop strategies to clone putative lung-specific RAR coregulators. Finally, we will use nude mice to investigate the effects of RA on the tumorigenicity of various lung cancer cell lines including RA- nonresponsive lines with restored RARbeta expression. The results from these studies will enhance our understanding of the biological functions or retinoid receptors during cancer development, and may lead to new probes allowing early detection and diagnosis of human lung cancer patients. The results may also provide a molecular basis for devising treatment strategies and developing more effective retinoids.
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