Enhancement of apoptosis induction by the synthetic retinoid MX3350-1 in non-small cell lung cancer cells by p53-dependent pathway

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Retinoids, which are a class of natural and synthetic vitamin A analogs, have shown activities of cancer prevention and cancer therapy against many cell types, including premalignant and malignant lung epithelial cells, both in vivo and in vitro. Most non-small cell lung cancer (NSCLC) cell lines are resistant to the natural retinoic acids due to aberrant retinoid receptor expression. Therefore, synthetic retinoids have been examined for ability to inhibit NSCLC better than retinoic acid. Indeed, some novel synthetic retinoids have shown activity against NSCLC cells that were resistant to retinoic acid. Some of these new retinoids induce apoptosis via retinoid receptor-independent mechanisms. One such retinoid, MX3350-1, was shown to inhibit the growth of human NSCLC in vitro and in vivo (nude mouse xenograft model) with minimal side effects. In this study, we explored the mechanisms by which this retinoid induces apoptosis in NSCLC cells. We found that MX3350-1 was more potent in growth inhibition of 5 NSCLC cell lines with wild-type p53 but not in four cell lines with mutant p53. Further studies with a representative cell line with wt p53 (H460) revealed that MX3350-1 induced apoptosis by activating Caspases 3, 8, and 9 and inducing PARP cleavage. None of these changes was detected in the p53 mutant-containing H596 NSCLC cells. In H460, MX3350-1 increased the level of p53 protein without a corresponding change in mRNA suggesting a post-transcriptional effect. The increase in p53 was accompanied by an increase in phosphorylated forms of p53 [pp53(Ser15) and pp53(Ser46)], translocation of p53 into the cell nucleus, and increase in MDM2 and p21/WAF1/CIP1. In addition, MX3350-1 decreased the level of BCl2 and Bcl-XL, and Bid. Cyclin D1 level also decreased by MX3350-1. Some of these effects may be caused by DNA damage response induced by MX3350-1 that leads to p53 phosphorylation and activation of downstream genes. These results indicate that p53 plays a role in MX3350-1-induced growth inhibition and apoptosis, damage/stress, which leads to p53 phosphorylation. Supported by the VITAL lung cancer program grant from the Department of Defense (W81XWH-04-1-0142).