CONDITIONALLY IMMORTALIZED MURINE BONE-MARROW STROMAL CELLS MEDIATE PARATHYROID HORMONE-DEPENDENT OSTEOCLASTOGENESIS IN-VITRO

PTH recruits and activates osteoclasts to cause bone resorption. These actions of PTH are thought to be mediated indirectly via type 1 PTH/P TH-related peptide receptors (PTH1Rs) expressed by adjacent marrow str omal or osteoblastic cells, although some evidence suggests that PTH m ay act directly on early hematopoietic osteoclast progenitors. We have established clonal, conditionally immortalized, PTH-responsive, bone marrow stromal cell lines from mice that harbor both a transgene encod ing a temperature-sensitive mutant of the simian virus 40 large T anti gen and deletion of a single allele of the PTH1R gene. Of 60 stromal c ell lines isolated, 45 expressed functional PTH1Rs. During coculture w ith normal murine spleen cells, 5 of 42 such cell Lines could support formation of tartrate-resistant acid phosphatase-positive, multinuclea ted cells (TRAP(+) MNCs) in response to 1,25-dihydroxyvitamin D-3, but only 2 of these did so in response to PTH. One of these, MS1 cells, e xpressed numerous cytokines and proteins characteristic of the osteoge nic lineage and showed increased production of interleukin-6 in respon se to PTH. MS1 cells supported dose-dependent induction by rat (r) PTH -(1-34) (0.1-100 nM) of TRAP(+) MNCs that expressed calcitonin recepto rs and formed resorption lacunae on dentine slices. This effect of PTH , which required cell to cell contact between MS1 and spleen cells, wa s mimicked by coadministration of cAMP analog and phorbol ester but on ly partially by either agent alone. The carboxyl-terminal fragment rPT H-(53-84) also induced osteoclast-like cell formation, but the maximal effect was only 30% as great as that of rPTH-(1-34). Importantly, rPT H-(1-34) induced TRAP(+) MNC formation even when PTH1R(-/-) osteoclast progenitors (from fetal liver of mice homozygous for ablation of the PTH1R gene) were cocultured with MS1 cells. We conclude that activatio n of PTH1Rs on cells of the osteoclast lineage is not required for PTH -(1-34)-induced osteoclast formation in the presence of appropriate PT H-responsive marrow stromal cells. MS1 cells provide a useful model fo r further study of PTH regulation of osteoclastogenesis.