Calcium-Stimulated Parathyroid Hormone-Like Protein Secretion: Potentiation through a Protein Kinase-C Pathway*

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Abstract
The effect of altering intracellular calcium levels on PTH-like protein (PLP) secretion was assessed in the NCI-H727 cell line. Ionomycin stimulated PLP secretion in a dose- and time-dependent manner, with a maximum rate of secretion occurring within minutes. Phorbol esters (phorbol 12-myristate 13-acetate and phorbol 12,13-didecanoate) stimulated PLP secretion at 10(-7)-10(-5) M, whereas forskolin treatment had no effect. When cells were treated initially with phorbol esters, ionomycin-stimulated PLP secretion was potentiated. The kinetics of this effect were rapid (t1/2, less than 1 min), and pretreatment with cycloheximide or actinomycin-D before phorbol ester and ionomycin treatment did not alter PLP secretion. Pretreatment for 24 h with phorbol 12-myristate 13-acetate to desensitize the cells or use of inactive phorbol ester isomer 4 alpha-phorbol 12,13-didecanoate did not potentiate the ionomycin-stimulated PLP secretion. Treatment with exogenously added phospholipase-C caused a dose-dependent increase in PLP secretion. These data indicate that PLP secretion can be stimulated through a phospholipase-C-mediated mechanism.

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