Expression and regulation of parathyroid hormone-related peptide in normaland malignant melanocytes

We examined parathyroid hormone-related peptide (PTHrP) production and regu lation in both normal human melanocytes and in a human amelanotic melanoma cell line (A375). Northern blot and immunocytochemical analysis demonstrate d that both cultured A375 cells and normal human melanocytes express PTHrP, but A375 cells expressed much higher levels of the peptide. PTHrP secretor y rate increased at least 10-fold after treatment with 10% fetal bovine ser um (100.2 +/- 2.8 pmol/10(6) cells vs. basal <15 pmol/10(6) cells) in proli ferating A375 cells but only twofold in confluent cells. Treatment of A375 cells with increasing concentrations of 1,25-dihydroxyvitamin D-3 [1,25-(OH )(2)D-3] or its low-calcemic analog EB-1089 revealed that EB-1089 was 10-fo ld more potent than 1,25-(OH)(2)D-3 on inhibition of both cell proliferatio n and PTHrP expression. Furthermore, inoculation of A375 cells into the mam mary fat pad of female severe combined immunodeficiency mice resulted in th e development of hypercalcemia and elevated concentrations of plasma immuno reactive PTHrP in the absence of detectable skeletal metastases. Our study, therefore, demonstrates a stepwise increase in PTHrP expression when cells progress from normal to malignant phenotype and suggests that EB-1089 shou ld be further evaluated as a therapeutic agent in human melanoma.