MOLECULAR BIOLOGY AND ACTIONS OF THE VITAMIN-D HORMONE RECEPTOR.

Abstract

The active form of vitamin D is the steroid hormone 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃]. Central to the mechanism of action of 1,25(OH)₂D₃ is its specific, high affinity intracellular receptor. This research focused on the participation of this receptor in the biology, biochemistry, and molecular biology of the vitamin D regulatory system. The effects of 1,25(OH)₂D₃ on the differentiation of hematopoietic cells were investigated using the cultured human promyelocytic leukemia cell line, HL-60, as a model. It was observed that 1,25(OH)₂D₃ induced macrophage differentiation in HL-60 cells and that a direct biochemical correlation existed between 1,25(OH)₂D₃ receptor saturation and a 1,25(OH)₂D₃-stimulated bioresponse. These data implicate 1,25(OH)₂D₃ as a natural cell differentiating agent and the 1,25(OH)₂D₃ receptor as the mediator of this hormone's action. Since the most fundamental level of control occurs by the regulation of gene expression, studies were undertaken to define the transcriptional control by 1,25(OH)₂D₃ over a known vitamin D-regulated endpoint protein. This work resulted in the molecular cloning of cDNAs to two avian intestinal calcium binding proteins, vitamin D-dependent calcium binding protein and a novel calmodulin-like protein. To gain further insight into the role of the 1,25(OH)₂D₃ receptor as a transcriptional regulator, avian and mammalian 1,25(OH)₂D₃ receptor mRNAs were characterized extensively by the techniques of in vitro translation and immunoprecipitation. These mRNAs were then utilized to construct cDNA libraries from which avian and human intestinal 1,25(OH)₂D₃ receptor cDNAs were isolated and their identity verified by hybrid-selected translation, sequencing, and Northern analysis. It was concluded that demonstrated 1,25(OH)₂D₃ receptors are polypeptides of 52-60 kDa whose activity is regulated by 1,25(OH)₂D₃ at both an mRNA and posttranslational level. Furthermore, the deduced amino acid sequence of receptor mRNA included a highly conserved cysteine, lysine, and arginine rich region that is homologous to other steroid receptors and the oncogene product v- erbA. Thus, the vitamin D receptor to be a specific trans -acting factor, modulating the pleiotropic effects of vitamin D including calcium homeostasis, and cellular differentiation.