PTH receptors and apoptosis in osteocytes

Osteocytes comprise a heterogenous population of terminally differentiated osteoblasts that direct bone remodeling in response to applied mechanical loading of bone. Increased osteocyte density accompanies the anabolic effect of PTH in vivo, whereas accelerated osteocyte death may be precipitated by estrogen deficiency or excess glucocorticoid exposure (conditions benefitted by intermittent PTH therapy) and by renal failure (where circulating intact PTH and, especially, PTH carboxylfragments are elevated). Osteocytes express type-1 PTH/ PTHrP receptors (PTH1Rs), which are fully activated by aminoterminal PTH fragments and couple to multiple signal transducers, including adenylyl cyclase and phospholipase C. Activation of PTH1Rs in osteocytes promotes gap junction-mediated intercellular coupling, increases expression of MMP-9, potentiates calcium influx via stretch-activated cation channels, amplifies the osteogenic response to mechanical loading in vivo, and regulates apoptosis. Control of osteocyte apoptosis by PTH1Rs is complex, in that intermittent PTH(1-34) administration reduces the fraction of vertebral apoptotic osteocytes at 1 month in adult mice but increases femoral metaphyseal osteocyte apoptosis at 1-2 weeks in young rats. In MLO-Y4 cells, PTH(1-34) prevents apoptosis otherwise induced within 6 hr by dexamethasone. In older studies, large doses of intact PTH(1-84) caused rapid "degenerative" morphologic changes in osteocytes, similar to those described in renal osteodystrophy. We isolated clonal conditionally immortalized osteocytic (OC) cell lines from mice homozygous for targeted ablation of the PTH1R gene. OC cells express abundant (2-3 x 10(6) per cell) receptors specific for the carboxyl(C)-terminus of intact PTH(1-84) ("CPTHRs") but, as expected, do not express PTH1Rs or respond to PTH(1-34). CPTHRs are expressed at much lower levels by other skeletally-derived cell lines. Several highly conserved ligand determinants of CPTHR binding have been identified, including PTH(24-27), PTH(53-54) and the sequence PTH(55-84), loss of which reduces binding affinity by over 100-fold. Human PTH(53-84), like PTH(1-84), PTH(24-84), and PTH(39-84), increases OC cell apoptosis. Ala-scanning mutagenesis to define sequences within PTH(55-84) important for binding and bioactivity is underway. We conclude that osteocytes may be important targets for CPTH fragments that are secreted by the parathyroid glands or generated by peripheral metabolism of intact PTH and that accumulate in blood, especially in renal failure. Studies of functional interplay between responses to CPTHRs and (transfected) PTH1Rs, using receptor-specific ligands in OC cells, should provide new insight into PTH regulation of osteocyte function and survival.