Using a human keratinocyte model of tumor progression, we have examined the regulation of gene expression and secretion of a parathyroid hormone-like peptide (PLP) that has been implicated in the pathogenesis of hypercalcemia in cancer. A rapid and transient induction of PLP mRNA in response to serum stimulation was demonstrated in both established (HPK1A) and malignant (HPK1A-ras) cells; however the dose dependent increases were greater in HPK1A than in HPK1A-ras. Significant inhibition of this induction was noted with the addition of 1,25-dihydroxyvitamin D₃ at a lower concentration in HPK1A than in HPK1A-ras. Amino-terminal PLP immunoreactivity and bioactivity correlated well (r = 0.98) when measured in conditioned medium. In the absence of mitogenic stimuli, malignant keratinocytes (HPK1A-ras) secreted significantly more PLP than established (HPK1A) keratinocytes. However, in response to increasing concentrations of epidermal growth factor and fetal bovine serum, PLP release was far greater from HPK1A (maximum 13 × basal) than from HPK1A-ras (maximum 3 × basal) cells. In addition, 1,25-dihydroxyvitamin D₃ was more effective in inhibiting both basal and stimulated PLP secretion in HPK1A than in HPK1A-ras cultures. Reduction of extracellular Ca²⁺ from 2.0 mm to 0.5 mm appeared to be more effective at an early time point in reducing PLP secretion from the established cells compared with the malignant cells. These studies therefore demonstrate a progressive dysregulation of PLP expression and secretion in human keratinocytes in the transformation from established to malignant phenotype and may have important implications for understanding the pathogenetic mechanisms involved in vivo in the development of hypercalcemia in cancer.