Calreticulin Inhibits Vitamin D's Action on the PTH Gene in Vitro and May Prevent Vitamin D's Effect in Vivo in Hypocalcemic Rats

A. Sela-Brown
1998 Molecular Endocrinology  
1,25-dihydroxyvitaminD 3 [1,25-(OH) 2 D 3 ] and PTH both act to increase serum calcium. In addition, 1,25-(OH) 2 D 3 decreases PTH gene transcription, which is relevant both to the physiology of calcium homeostasis and to the management of the secondary hyperparathyroidism of patients with chronic renal failure. In chronic hypocalcemia there is secondary hyperparathyroidism with increased levels of PTH mRNA and serum PTH despite markedly increased levels of 1,25-(OH) 2 D 3 . We have studied the
more » ... We have studied the role of calreticulin in this resistance to 1,25-(OH) 2 D 3 . Weanling rats fed a lowcalcium diet were hypocalcemic and had increased PTH mRNA levels despite high serum 1,25-(OH) 2 D 3 levels. 1,25-(OH) 2 D 3 given by continuous minipump infusion to normal rats led to the expected decrease in PTH mRNA. The hypocalcemic rats had an increased concentration of calreticulin in the nuclear fraction of their parathyroids, but not in other tissues. Gel shift assays showed that a purified vitamin D receptor and retinoid X receptor-␤ bound to the PTH promoter's chicken and rat vitamin D response element (VDRE), and this binding was inhibited by added pure calreticulin. Transfection studies with a PTH VDRE-chloramphenicol acetyltransferase (CAT) construct showed that 1,25-(OH) 2 D 3 decreased CAT tran-scription. Cotransfection of PTH VDRE-CAT with a calreticulin expression vector in the sense orientation prevented the transcriptional effect of 1,25-(OH) 2 D 3 , but a calreticulin vector in the antisense orientation had no effect. These results show that calreticulin prevents the binding of vitamin D receptor-retinoid X receptor-␤ to the PTH VDRE in gel retardation assays and prevents the transcriptional effect of 1,25-(OH) 2 D 3 on the PTH gene. This is the first report of calreticulin inhibiting a downregulatory function of a sterol hormone and may help explain the refractoriness of the secondary hyperparathyroidism of many chronic renal failure patients to 1,25-(OH) 2
doi:10.1210/me.12.8.1193 pmid:9717845 fatcat:pfsyphsdhbhppljsdru3l4vqii