Transcriptional regulation of LAMB3 by p53

by Jani, Meghna

The p53 tumor-suppressor plays a very important role in the prevention of cancer and it is known that about 50% of all human tumors possess p53 mutations. Although mutations in p53 are most prevalent in human cancers, inactivation of wild-type p53 occurs through many different mechanisms that are independent of p53 mutation or deletion. In an effort to determine novel p53 target genes, our lab employed a microarray method in which p53 was re-activated by RNAi mediated knockdown of Hdm2 and HdmX in MCF7 human breast cancer cell line, harboring wild-type p53 and elevated levels of Hdm2 and HdmX. Gene expression profiling of the RNAi treated MCF7 breast cancer cells led to the identification of Laminin-beta 3 (LAMB3) as a potential transcriptional target of p53. To test this hypothesis, I carried out validation experiments which confirmed the earlier microarray experimental results. Interestingly, I also found four p53 half binding sites downstream of the LAMB3 promoter. DNA-damage and overexpression studies established that p53 activation unexpectedly did not lead to any transcriptional increase in LAMB3 expression. In contrast, serum deprivation and senescence experiments demonstrated that LAMB3 expression paralleled p21 (a well-known p53 target involved in cell cycle arrest) expression. Taken together, the experimental results suggest that LAMB3 may be a p53 regulated gene, but is not a classical p53 target.