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

We examined parathyroidhormone-related peptide (PTHrP) production and regulation in bothnormal human melanocytes and in a human amelanotic melanoma cell line(A375). Northern blot and immunocytochemical analysis demonstrated thatboth cultured A375 cells and normal human melanocytes express PTHrP,but A375 cells expressed much higher levels of the peptide. PTHrPsecretory rate increased at least 10-fold after treatment with 10%fetal bovine serum (100.2 ± 2.8 pmol/10⁶ cells vs.basal <15 pmol/10⁶ cells) in proliferating A375 cells butonly twofold in confluent cells. Treatment of A375 cells withincreasing concentrations of 1,25-dihydroxyvitamin D₃[1,25-(OH)₂D₃] or its low-calcemic analogEB-1089 revealed that EB-1089 was 10-fold more potent than 1,25-(OH)₂D₃ on inhibition of both cellproliferation and PTHrP expression. Furthermore, inoculation of A375cells into the mammary fat pad of female severe combinedimmunodeficiency mice resulted in the development ofhypercalcemia and elevated concentrations of plasma immunoreactivePTHrP in the absence of detectable skeletal metastases. Our study,therefore, demonstrates a stepwise increase in PTHrP expression whencells progress from normal to malignant phenotype and suggests thatEB-1089 should be further evaluated as a therapeutic agent in human melanoma.