VARIATION IN 1,25-DIHYDROXYVITAMIN-D3 REGULATION OF PROLIFERATION ANDALKALINE-PHOSPHATASE ACTIVITY IN LATE-PASSAGE RAT OSTEOBLASTIC CELL-LINES

The effects of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], 1,25-dihydroxy- 16ene-23yne-vitamin D3[1,25(OH)2-16ene-23yne-D3] (a synthetic analog) and retinoic acid on proliferation, protein synthesis, and alkaline ph osphatase activity and mRNA were compared in two late-passage (P > 70) clonal rat osteoblastic cell lines (G2 and C12) in order to character ize variations in the basal and hormonally-regulated phenotypes. All a gents inhibited proliferation (measured as cell number after 3 days of treatment) in late-passage (P > 70) G2 and C12 cells without inhibiti ng the rate of protein synthesis ([H-3]leucine incorporation into TCA- precipitable protein) during the last 18 h of incubation. Basal and ho rmone-treated alkaline phosphatase activities were lower in late-passa ge G2 and C12 cells than those previously reported for early-passage G 2 and C12 cells. 1,25(OH)2D3 and 1,25(OH)2-16ene-23yne-D3 up-regulated alkaline phosphatase activity in late-passage C12 cells and down-regu lated it in late-passage G2 cells. The direction of these regulatory c hanges in late-passage cells was opposite to that reported for early p assages of these clones, and changes were related to the levels of tis sue-unspecific alkaline phosphatase mRNA normalized for actin mRNA. Ef fects of 1,25(OH)2D3 or 1,25(OH)2-16ene-23yne-D3 and retinoic acid wer e not additive, suggesting a competitive mechanism of action. It appea rs that increased sensitivity to the antiproliferative effects of regu latory hormones and defects in proliferation and specialization of the osteoblast are observed with increasing passage number in vitro in tw o model osteoblastic cell lines (G2 and C12).