5'-flanking Region of the Parathyroid Hormone Gene Mediates Negative Regulation by 1,25-(OH)2 Vitamin D3

To study the effects of 1,25-(OH)2 vitamin D3 on the transcription of the human parathyroid hormone (PTH) gene, 684 base pairs of the 5'-flanking portion of the human PTH gene were fused to the bacterial neo gene. The fusion gene was transfected into rat pituitary cells, and mixed populations of colonies were selected using the neomycin analog, G418. The level of RNA initiated from the human PTH gene promoter region in these cells was suppressed by 1,25-(OH)2 vitamin D3. Synthesis of the same transcript under control of a viral promoter was not regulated by 1,25-(OH)2 vitamin D3. The effect of 1,25-(OH)2 vitamin D3 was detected within 24 h at physiologic doses of 1,25-(OH)2 vitamin D3, and was not influenced by addition of cycloheximide. Thus 1,25-(OH)2 vitamin D3 acts on the 5'-flanking portion of the PTH gene to decrease the rate of transcription by a mechanism that requires no new protein synthesis.