Identification and functional characterization of a novel unspliced transcript variant of HIC-1 in human cancer cells exposed to adverse growth conditions.

The wild-type p53 gene has been widely implicated in the regulation of hypermethylated in cancer-1 (HIC-1) transcription, a master growth regulatory gene with multiple promoters and, consequently, multiple alternatively spliced transcripts. We investigated the role of p53 (wild type and mutant, both endogenous and exogenous) in modulating the various HIC-1 transcripts. We discovered a novel unspliced HIC-1 transcript, identified as "f" in leukocytes and in the human cell lines U87MG (wild-type p53), U373MG (mutant p53), MCF-7 (wild-type p53), HeLa (p53 degraded by HPV18-E6 oncoprotein), and Saos-2 (p53 null). This transcript is initiated from a new transcription start site and has an intervening stop codon that would result in a possibly truncated 22-amino-acid polypeptide. When U87MG (wild-type p53) and MCF-7 cells (wild-type p53) were exposed to adverse growth conditions of serum starvation or treated with the chemotherapeutic agent cisplatin, cells underwent apoptosis and cell cycle arrest accompanied by increase in p53 and HIC-1 transcript levels. Although the increase of the HIC-1-spliced transcripts followed the increase of p53, increase in f transcript coincided with declining p53 and HIC-1 transcript and protein levels. Moreover, the levels of HIC-1 f transcript were not induced by exogenously transfected wild-type p53 in p53-mutated U373MG and p53-null Saos-2 cells, unlike the spliced transcripts that code for full-length HIC-1 protein. These findings suggest a working model wherein the status of f transcript, which is not under direct transcriptional control of wild-type p53, may influence the level of HIC-1 protein in cancer cells.