Parathyroid hormone-related protein production by normal human keratinocytes in vitro

The role of parathyroid hormone-related protein (PTHrP) and its regulation in normal epidermal physiology is not currently known. Recent evidence suggests that PTHrP production may be related to the degree of differentiation of keratinocytes in vitro. This investigation characterized the production of PTHrP by normal human foreskin keratinocytes (NHFK) during both spontaneously occurring and induced differentiation in vitro. PTHrP production in keratinocyte serum-free conditioned medium was determined using an N-terminal radioimmunoassay for human PTHrP (1-36). Agents known to stimulate (calcium, 1,25-dihydroxyvitamin D3) or inhibit (transforming growth factor-β) keratinocyte differentiation were examined for their ability to alter production of PTHrP. Measurements of cell number and involucrin content of the cultures were made to confirm the effects of these agents on keratinocyte growth and differentiation. The production of PTHrP in control cultures (under low calcium conditions, 0.08 mM) was decreased and involucrin content increased (P < 0.01) after the cells became confluent. The addition of 1 mM calcium to keratinocyte medium increased cell number and involucrin content of the cultures (P < 0.05) but inhibited production of PTHrP (P < 0.01). The 1,25-dihydroxyvitamin D3 (10 nM) had no significant effect on cell number or PTHrP production, but increased involucrin content (P < 0.05). Transforming growth factor-β (5 ng/ml) decreased both cell number (P < 0.05) and involucrin content (P < 0.01), but significantly stimulated PTHrP production (P < 0.01). These data indicate that the production of PTHrP by NHFK is inhibited with the onset of both spontaneously occurring and calcium-induced differentiation in vitro, while transforming growth factor-β-inhibited differentiation and upregulated PTHrP production in normal human keratinocytes.