VITAMIN-B-6 MODULATES EXPRESSION OF ALBUMIN GENE BY INACTIVATING TISSUE-SPECIFIC DNA-BINDING PROTEIN IN RAT-LIVER

The level of albumin mRNA in the liver of vitamin B-6-deficient rats w as found to be 7-fold higher than that of control rats. Since the tran scriptional activity of the albumin gene, as measured by a nuclear run -on assay, was increased 5-fold in vitamin B-6 deficiency, the higher concentration of albumin mRNA in the liver of vitamin-deficient rats c ould be attributed to the enhanced rate of transcription. The promoter proximal sequences of the albumin gene interact with a number of tiss ue-specific transcription factors including HNF-1 and C/EBP. We determ ined the binding activities of liver nuclear extracts to the HNF-1- an d C/EBP-binding sites by gel mobility-shift assay and found that the a ctivities of the extract prepared from liver of vitamin B-6-deficient rats were greater than those of controls. As the concentrations of C/E BP in nuclear extracts from control and vitamin-deficient rats, estima ted by Western-blot analysis, were essentially the same, the lower bin ding activity of the extract from control liver is probably due to ina ctivation of tissue-specific factors by pyridoxal phosphate and/or its analogues. We therefore examined the effect of pyridoxal phosphate an d its analogues on the binding activity of nuclear extract in vitro an d found that only pyridoxal phosphate effectively inhibited the bindin g. These observations indicate that vitamin B-6 modulates albumin gene expression through a novel mechanism that involves inactivation of ti ssue-specific transcription factors by direct interaction with pyridox al phosphate.