Niacin deficiency, poly(ADP-ribose),p53 and chromosomal instability in rat bone marrow cells in vivo

This thesis is an investigation of the effects of altered dietary niacin status on the response of rat bone marrow to the chemotherapy drug etoposide (ETO). NAD+ is the substrate for the synthesis of poly(ADP-ribose) (pADPr) by poly(ADP-ribose) polymerase (PARP), an enzyme involved in DNA repair, recombination, apoptosis, and p53 expression/function. This thesis characterizes the impact of niacin status on p53 expression/function and the extent of DNA damage and chromosomal instability before and after treatment with ETO. Male Long-Evans rats were fed niacin deficient (ND), or pair-fed niacin replete (PF) or nicotinic acid supplemented (4g/kg diet) (NA) diets for 3 weeks and then dosed with ETO (1--25 mg/kg) or vehicle (CON). Niacin deficiency (ND-CON) decreased bone marrow NAD+ concentrations 3.5-fold and prevented pADPr accumulation, while supplementation (NA-CON) increased NAD+ levels 2.4-fold and pADPr 5-fold, relative to PF-CON. p53 protein detected in niacin deficient bone marrow was over-expressed (2.9-fold) and appeared as two unique isoforms which were insensitive to further ETO-induced upregulation. In addition, expression of downstream p53-target genes was misregulated in niacin deficient bone marrow and apoptotic efficiency and cell cycle arrest were impaired following ETO treatment. Niacin deficiency alone caused a dramatic increase in spontaneous DNA damage and chromosomal instability while supplementation did not cause any adverse genotoxicity. Niacin deficiency enhanced the formation of DNA strand breaks following treatment with ETO, relative to PF-ETO and NA-ETO rats. Supplementation did not afford any added protection against the genotoxic actions of ETO above levels of niacin adequacy. These data demonstrate that niacin is required for the maintenance of genomic stability in vivo, in a tissue that is sensitive to niacin depletion and impaired pADPr metabolism. Niacin supplementation may help to protect the bone marrow cells of nutritionally compromised cancer patients from the genotoxic action of chemotherapy drugs.