Regulation of Parathyroid Hormone-Related Protein Secretion and mRNA Expression in Normal Human Keratinocytes and a Squamous Carcinoma Cell Line

Abstract

Parathyroid hormone-related protein (PTHrP) has been identified as a causative factor in the pathogenesis of humoral hypercalcemia of malignancy (HHM). The regulation and mechanisms of PTHrP secretion in most normal and malignant cells are unknown. PTHrP secretion, mRNA expression, and transcription were measured in neoplastic human squamous carcinoma cells (A253) and normal human foreskin keratinocytes (NHFK) by radioimmunoassay, RNase protection assay, and transient transfections of the 5′-flanking region of humanPTHrPin a luciferase expression vector. Mechanisms of PTHrP secretion were investigated using chemicals (monensin, colchicine, cytochalasin B, guanosine 5′-[γ-thio]triphosphate (GTPγS)) that interfere with or facilitate intracellular transport. Monensin inhibited PTHrP secretion in both NHFK and A253 cells. Ultrastructurally, monensin caused dilatation of rough endoplasmic reticulum and the formation of numerous cytoplasmic secretory vacuoles in both cell lines. Colchicine decreased PTHrP production in NHFK cells and stimulated PTHrP production and mRNA levels in A253 cells. Colchicine also stimulated transcription of the PTHrP–luciferase reporter gene. Cytochalasin B stimulated PTHrP secretion and mRNA expression in A253 cells, but had no effect in NHFK cells. GTPγS had no effect on PTHrP secretion in either cell line. It was concluded that PTHrP secretion is dependent on the constitutive movement of secretory vesicles to the cytoplasmic membrane and regulation of PTHrP secretion and mRNA expression are altered in squamous carcinoma cells compared to normal human keratinocytesin vitro.