HUMAN KERATINOCYTES EXPRESS TRANSCRIPTS FOR 3 ISOFORMS OF PARATHYROIDHORMONE-RELATED PROTEIN (PTHRP), BUT NOT FOR THE PARATHYROID HORMONE PTHRP RECEPTOR - EFFECTS OF 1,25(OH)(2), VITAMIN-D-3/

Parathyroid hormone-related protein (PTHrP) is strongly expressed in t he epidermis and has been implicated in the regulation of growth and d ifferentiation of keratinocytes. PTHrP has N-terminaI sequence homolog y with parathyroid hormone (PTH) and binds to the type I PTH/PTHrP rec eptor, but earlier reports suggest that keratinocytes do not possess t his cell surface receptor: In order to determine which PTHrP mRNA isof orms are expressed by keratinocytes and whether the type I receptor mR NA is present, we designed specific primers for reverse transcriptase- polymerase chain reaction, The interaction of PTHrP with other promote rs of keratinocyte differentiation is unclear In particular, 1,25(OH)( 2)D-3 is also fundamental in calcium homeostasis and induces changes i n intracellular calcium. We therefore investigated the effect of 1.25( OH)(2)D-3 on 10(-10)-10(-6) mol/L. expression and protein production i n cultured human keratinocytes. Cells were incubated for 3 days at con centrations of 1,25(OH)(2)D-3 of 10(-10)-10(-6) mol/L. PTHrP in cultur e supernatant, measured by two site immunoradiometric assay, was 915 /- 98 PTHrP fmol/mg of cell layer protein in untreated cultures decrea sing to 570 +/- 113 with 10(-8) mol/L and 402 +/- 24 with 10(-6) mol/L 1,25(OH)(2)D-3 (mean +/- SEM, P < 0.01, n = 6). Transcripts for all t hree PTHrP isoforms (139, 141 and 173 amino acids) were detectable in keratinocyte mRNA. Corresponding to the decrease in PTHrP protein we d emonstrated a reduction in all three PTHrP mRNA transcripts after 3 da ys' incubation with 1,25(OH)(2)D-3 over a concentration range 10(-10)- 10-(6) mol/L. Repeated studies failed to detect type I PTH/PTHrP recep tor mRNA in human keratinocytes, either in control cultures or in the presence of 1,25(OH)(2)D-3. We have shown that keratinocytes produce a bundant PTHrP and that this is modulated by 1,25(OH)(2)D-3, suggesting a physiological role. Further studies are required to investigate the relative expression of PTHrP isoforms, their role in keratinocyte sig nalling and the receptors involved.