RNA-PROTEIN BINDING AND POSTTRANSCRIPTIONAL REGULATION OF PARATHYROID-HORMONE GENE-EXPRESSION BY CALCIUM AND PHOSPHATE

Parathyroid hormone (PTH) regulates serum calcium and phosphate levels , which, in turn, regulate PTH secretion and mRNA levels, PTH mRNA lev els are markedly increased in rats fed low calcium diets and decreased after low phosphate diets, and this effect is posttranscriptional. Pr otein-PTH mRNA binding studies, with parathyroid cytosolic proteins, s howed three protein-RNA bands, This binding was to the 3'-untranslated region (UTR) of the PTH mRNA and was dependent upon the terminal 60 n ucleotides. Parathyroid proteins from hypocalcemic rats showed increas ed binding, and proteins from hypophosphatemic rats decreased binding, correlating with PTH mRNA levels, There is no parathyroid cell line; however, a functional role was provided by an in vitro degradation ass ay. Parathyroid proteins from control rats incubated with a PTH mRNA p robe led to an intact transcript for 40 min; the transcript was intact with hypocalcemic proteins for 180 min and with hypophosphatemic prot eins only for 5 min, A PTH mRNA probe without the 3'-UTR, or just the terminal 60 nucleotides, incubated with hypophosphatemic proteins, sho wed no degradation at all, indicating that the sequences in the 3'-UTR determine PTH mRNA degradation, Hypocalcemia and hypophosphatemia reg ulate PTH gene expression post-transcriptionally, This correlates with binding of proteins to the PTR mRNA 3'-UTR, which determines its stab ility.