PYRIDOXAL 5'-PHOSPHATE MODULATES EXPRESSION OF CYTOSOLIC ASPARTATE-AMINOTRANSFERASE GENE BY INACTIVATION OF GLUCOCORTICOID RECEPTOR

The level of mRNA for cytosolic aspartate aminotransferase (cAST) in t he liver of vitamin B-6-deficient rats was found to be 7-fold higher t han that of the control rats. The administration of hydrocortisone to adrenalectomized vitamin B-6-deficient rats induced expression of hepa tic cAST mRNA and the induction was suppressed by the simultaneous adm inistration of pyridoxine. Since the 5' regulatory region of the rat c AST gene contains several. sequences showing homology to glucocorticoi d-responsive elements, we synthesized an oligonucleotide probe of gluc ocorticoid-responsive element sequence and assayed the binding activit y of liver nuclear extract to the oligonucleotide by gel mobility shif t analysis. We found that the binding activity of nuclear extract prep ared from the liver of vitamin B-6-deficient rats was far greater than that of the control rats, indicating that the DNA-binding activity of glucocorticoid receptor was enhanced by vitamin B-6 deficiency. We fu rther found that preincubation of the nuclear extract from the vitamin -deficient liver with pyridoxal 5'-phosphate brought about a rapid and extensive decrease in the binding of the extract to the glucocorticoi d-responsive element. Congeners of pyridoxal phosphate, such as pyrido xamine 5'-phosphate, pyridoxal, pyridoxamine and pyridoxine, did not s how an inhibitory effect. These observations suggest that pyridoxal 5' -phosphate modulates cAST gene expression by inactivating the binding activity of glucocorticoid receptor to glucocorticoid-responsive eleme nts.