Interactions between retinoic acid and 1,25(OH)2D in mouse immortalized osteoblastic C1 cells

The interactions between 1,25-dihydroxyvitamin D3 [1,25(OH)2D] and retinoic acid (RA) on alkaline phosphatase (ALP) activity and vitamin D receptors (VDR) were determined in the SV40 immortalized osteoblastic cell line C1. Biochemical and cytochemical analysis showed that ALP activity increased with C1 cell density and that 1,25(OH)2D inhibited ALP activity at high density, whereas RA increased ALP activity at low density. The interactions of the two hormones were also cell density dependent, with a predominant stimulatory effect of RA at low density and a predominant inhibitory effect of 1,25(OH)2D at high density. In contrast, the two hormones inhibited C1 cell proliferation independently of cell density. Scatchard analysis and immunocytochemical staining showed that nuclear VDR levels increased with cell density and that RA, 1,25(OH)2D, and their combination upregulated VDR levels mainly at high cell density. Although RA increased VDR levels, RA did not potentiate the effect of 1,25(OH)2D on ALP activity. The results indicate that the effects and interactions of 1,25(OH)2D and RA on ALP activity in osteoblastic C1 cells depend on the state of phenotypic maturation.