Avian and mammalian receptors for 1,25-dihydroxyvitamin D3: in vitro translation to characterize size and hormone-dependent regulation.

In vitro translation of cellular poly(A)+ RNA coupled with immunoprecipitation was developed as a technique for characterizing 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors and assessing receptor mRNA activity. Cell-free translation of poly(A)+ RNA isolated from chicken intestine revealed two immunoprecipitable forms of avian receptor at 60 kDa and 58 kDa. These two species were identical in electrophoretic mobility to those detected directly in intestinal cytosol by immunoblot analysis. Liver, a tissue devoid of 1,25-(OH)2D3 binding activity, contained no apparent translatable receptor mRNA. 1,25-(OH)2D3 receptors were also synthesized in vitro employing poly(A)+ RNA obtained from several cultured mammalian cell lines. Selective immunoprecipitation revealed a single form of receptor at 54 kDa in mouse fibroblasts (3T6) and pig kidney cells (LLC-PK1) and a 52-kDa species in human breast carcinoma (T47D). Each of these in vitro translated mammalian 1,25-(OH)2D3 receptors migrated identically with its cellular counterpart that was synthesized in vivo employing metabolic labeling of cell protein with [35S]methionine. In vitro translation of poly(A)+ RNA derived from mouse 3T6 cells treated with 1,25-(OH)2D3 for 24-48 hr disclosed a 5-fold increase in receptor mRNA activity over untreated control cells. These results are consistent with the conclusions that 1,25-(OH)2D3 receptors are protein species ranging from 52 to 60 kDa and that, though their functional and immunological domains have been evolutionarily conserved, an inverse relationship apparently exists between phylogenetic status and receptor mass. The data also support the hypothesis that the presence of 1,25-(OH)2D3 leads to a significant increase in receptor mRNA activity in 3T6 cells, indicative of receptor autoregulation.