Ascorbate up-regulates MLH1 (Mut L homologue-1) and p73: implications for the cellular response to DNA damage

We have found previously that ascorbic acid (vitamin C), as well as acting as a radical scavenger, may modulate the expression of several genes [i.e. fra-1, glutathione S-transferase Pi (GSTpi) and Mut L homologue-1 (MLH1)] in human keratinocytes. In the present paper, we demonstrate that MLH1, as well as its downstream target p73, can be positively modulated by this antioxidant vitamin, indeed, up-regulation of the two mRNAs was observed after just 2 h, and increased further up to 16 h of treatment. Modulation of MLH1 and p73 gene expression improves cellular susceptibility to apoptosis triggered by the DNA-damaging agent cisplatin. Indeed, in ascorbate-supplemented cells, increased cisplatin-induced apoptosis was seen, involving activation of the MLH1/c-Abl/p73 signalling cascade. Our results were further confirmed by studies performed on genetically defined mutants, i.e. mouse embryo fibroblasts derived from knock-out animals for c-Abl or p53, as well as human colon carcinoma cell lines deficient in MLH1. The increased sensitivity to cisplatin observed in ascorbate-loaded cells appeared to be dependent exclusively on MLH1 and c-Abl expression, and independent of p53. These data suggest a potential mechanism accounting for the anti-carcinogenic and anti-cancer activities of vitamin C.