The chemopreventive effects of 9-cis retinoic acid and 1alpha-25-dihydroxyvitamin D3 against NNK-induced lung carcinogenesis in A/J mice

While the molecular structures of 9-cis retinoic acid and 1alpha,25-dihydroxyvitamin D3 are very different, the nuclear receptors for these compounds are members of the same superfamily of ligand-activated transcription factors and are known to heterodimerize and modulate transcription of genes involved in cellular growth and differentiation. Previous cell culture work has demonstrated the potential for synergistic effects between retinoids and vitamin D. Whether this synergism exists in vivo has yet to be established. This research examined whether 9-cis retinoic acid and 1alpha,25-dihydroxyvitamin D 3, alone or combined, could exert significant protective effects against lung cancer development in vivo in the A/J mouse model of carcinogen-induced lung cancer. We found that dietary supplementation with 9-cis retinoic acid resulted in a significant decline in tumor mutliplicity, without apparent toxicity aside from slight weight loss at the highest dose. Dietary supplementation with 1alpha,25-dihydroxyvitamin D3 resulted in highly significant declines in both tumor incidence and tumor multiplicity. Although weight loss and symptoms of systemic hypercalcemia were observed at all 1alpha,25-dihydroxyvitamin D3 doses tested, adding 9-cis retinoic acid to the 1alpha,25-dihydroxyvitamin D3 treatment resulted in a highly significant chemopreventive effect without any evidence of toxicity. Therefore, it appears that 9-cis retinoic acid can mitigate the effects of vitamin D toxicity without reducing chemoprevetive efficacy. We further demonstrated that the antagonistic effect of 9-cis retinoic acid is not associated{09}with changes in{09} 1alpha,25-dihydroxyvitamin D3 catabolism, and that the chemopreventive effects of 9-cis retinoic acid and 1alpha,25-dihydroxyvitamin D3 do not appear to be mediated by changes in the expression of relevant nuclear hormone receptors, cell cycle regulators p21 or p27, or insulin-like growth factor binding protein-3. Future research should focus on determining what other molecular mechanisms may be involved in the chemopreventive effects of these agents against lung carcinogenesis in vivo, and should investigate the possibility that 9-cis retinoic acid and 1alpha,25-dihydroxyvitamin D3 may act by decreasing the susceptibility of a small population of bronchioalveolar stem cells to transforming mutations, or by depleting transformed lung cancer stem cells before extensive proliferation and metastasis.