Mechanism of increased parathyroid hormone mRNA in experimental uremia: Roles of protein RNA binding and RNA degradation

Patients with chronic renal failure develop secondary hyperparathyroidism w ith increased synthesis and secretion of parathyroid hormone (PTH) resultin g in severe skeletal complications. In rats with secondary hyperparathyroid ism due to 5/6 nephrectomy, there are increased PTH mRNA levels, and this m echanism was studied. Parathyroid glands were microdissected from control a nd 5/6 nephrectomy rats and analyzed for PTH mRNA and control genes, and th e nuclei were used for nuclear run-on experiments. The cytosolic proteins o f the parathyroids were used to study PTH mRNA protein binding by ultraviol et cross-linking and the degradation of the PTH transcript in vitro. Nuclea r run-ons showed that the increase in PTH mRNA levels was posttranscription al. Protein binding to the PTH mRNA 3'-UTR determines PTH mRNA stability an d levels. Parathyroid proteins from uremic rats bound PTH mRNA similar to c ontrol rats by ultraviolet cross-linking. To determine the effect of uremia on PTH mRNA stability, an in vitro RNA degradation assay was performed wit h parathyroid proteins from uremic rats. When parathyroid proteins from con trol rats were incubated with PTH mRNA, there was transcript degradation al ready at 30 min, reaching 50% at 60 min and 90% at 180 min. With uremic par athyroid proteins, the PTH mRNA was not degraded at all at 120 min and was moderately decreased at 180 min. This decrease in degradation by uremic par athyroid proteins suggests a decrease in parathyroid cytosolic endonuclease activity in uremia resulting in a more stable PTH transcript. The increase d PTH mRNA levels would translate into increased PTH synthesis and serum PT H levels, which would lead to metabolic bone disease in many patients with chronic renal failure.