PROTEIN KINASE-C IN MOUSE KIDNEY - EFFECT OF THE HYP MUTATION AND PHOSPHATE DEPRIVATION

To test whether protein kinase C plays a role in the regulation of renal brush border membrane phosphate transport and mitochondrial vitamin D metabolism, we examined the activity, distribution and endogenous substrates of protein kinase C in renal subcellular fractions derived from two mouse models exhibiting perturbations in both renal functions. The X-linked Hyp mouse is characterized by reduced phosphate transport and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) synthesis relative to normal, whereas the phosphate-deprived mouse exhibits elevated phosphate transport of vitamin D hormone synthesis. Protein kinase C activity was higher in renal cytosol of Hyp mice, when compared to normal littermates (358 ± 11 vs. 244 ± 31 pmol 32P/mg prot/min, P < 0.02), whereas genotype differences in brush border membrane and mitochondrial kinase were not apparent. Phosphate deprivation of normal mice elicited a 50% reduction in brush border membrane protein kinase C (from 819 ± 56 to 460 ± 48 pmol 32P/mg prot/min, P < 0.03), an increase in mitochondrial kinase (from 57 ± 7 to 87 ± 10 pmol 32P/mg prot/min, P < 0.03), and no change in cytosolic kinase activity. Phosphate deprivation of Hyp mice led to an increase in mitochondrial protein kinase C (from 72 ± 7 to 98 ± 9 pmol 32P/mg prot/min, P < 0.03) and no change in either brush border membrane or cytosolic kinase activity. Phosphate deprivation elicited an adaptive increase in the apparent V(max) for brush border membrane phosphate transport in both normal (from 518 ± 54 to 1288 ± 126 pmol 32P/mg prot/6s, P < 0.001) and Hyp mice (from 253 ± 30 to 962 ± 62 pmol 32P/mg prot/6s, P < 0.001) with no change in apparent Km. Circulating levels of 1,25(OH)2D were increased (2.2-fold) in normal mice and decreased (0.4-fold) in Hyp mice fed the low phosphate diet. Genotype and dietary effects on protein kinase C-dependent phosphorylation of endogenous proteins in renal subcellular fractions are also described. Although it is not clear whether protein kinase C influences the expression of the renal abnormalities in phosphate transport and vitamin D metabolism in mutant Hyp mice, our data suggest that protein kinase C may play a role in the renal adaptive responses to phosphate deprivation in normal mice.