TISSUE-SPECIFIC EXPRESSION AND REGULATION BY 1,25(OH)(2)D-3 OF CHICK PROTEIN-KINASE INHIBITOR (PKI) MESSENGER-RNA

The heat-stable protein kinase inhibitor (PKI) protein is a specific a nd potent competitive inhibitor of the catalytic subunit of cAMP-depen dent protein kinase (PKA). Previously, it has been shown that vitamin D status affects chick kidney PKI activity: a 5- to 10-fold increase i n PKI activity was observed in kidneys of chronically vitamin D-defici ent chicks and treatment with 1,25-dihydroxyvitamin D-3 (1,25[OH](2)D- 3) in cultured kidney cells resulted in a 95% decrease in PKI activity . The authors have recently cloned the cDNA for chick kidney PKI and h ave used the coding sequence to study the regulation of PKI mRNA. Nort hern analysis showed the expression of two PKI messages, which are 2.7 and 3.3 kb in size. These mRNAs are expressed in brain, muscle, testi s, and kidney, but not in pancreas, liver, or intestine. PKI mRNA stea dy-state levels are downregulated by 47% in kidneys from vitamin D-rep lete chicks as compared to vitamin D-deficient chicks. PKI mRNA levels in brain, muscle, and testis are not affected by vitamin D status. Tr eatment of primary chick kidney cultures treated with 10(-7) M 1,25(OH )(2)D-3 for 24 h resulted in a 20-30% decrease in PKI mRNA. 1,25(OH),D , treatment does not affect the stability of PKI mRNA as determined by treatment of cell cultures with actinomycin D. This study shows that 1,25(OH)(2)D-3 directly and tissue-specifically downregulates PKI mRNA in the chick kidney.