Down-Regulation of Protein Kinase C by Parathyroid Hormone and Mezerein Differentially Modulates cAMP Production and Phosphate Transport in Opossum Kidney Cells

Abstract

We examined the effects of prolonged exposure to parathyroid hormone (PTH) and the protein kinase C (PKC) activator mezerein (MEZ) on cyclic adenosine monophosphate (cAMP) production, PKC activity, and Na ؉ -dependent phosphate (Na/Pi) transport in an opossum kidney cell line (OK/E). A 5 minute exposure to PTH stimulated, while a 6 h incubation reduced, cAMP production. Na/Pi transport was maximally inhibited under desensitizing conditions and was not affected by reintroduction of the hormone. MEZ pretreatment (6 h) enhanced PTH-, cholera toxin (CTX)-, and forskolin (FSK)-stimulated cAMP production, suggesting enhanced Gs␣ coupling and increased adenylyl cyclase activity. However, PKA-and PKC-dependent regulation of Na/Pi were blocked in MEZ-treated cells. The PTH-induced decrease in cAMP production was associated with a reduction in membrane-associated PKC activity while MEZ-induced increases in cAMP production were accompanied by decreases in membrane and cytosolic PKC activity. Enhanced cAMP production was not accompanied by significant changes in PTH/PTH related peptide (PTHrP) receptor affinity or number, nor was the loss of Na/Pi transport regulation associated with changes in PKA activity. The results indicate that down-regulation of PKC by PTH or MEZ differentially modulates cAMP production and regulation of Na/Pi transport. The distinct effects of PTH and MEZ on PKC activity suggest that agonist-specific activation and/or down-regulation of PKC isozyme(s) may be involved in the observed changes in cAMP production

Mining the National Cancer Institute's Tumor-Screening Database: Identification of Compounds with Similar Cellular Activities

“…Within this large portion of the map are compounds that function as protein kinase C activators, in subregion P 13 , at k6.9, shown as pink hexagons near the top of Figure 8B. These compounds include the phorbols (mezerein, huratoxin, and prostratin), 83 gni- dimacrin, 84 and cytoblastin. 85 Adjacent to k6.8 is the previously described class of protein phosphatase 2A inhibitors, fostriecin and cytostatin.…”