Primary cilia mediate parathyroid hormone receptor type 1 osteogenic actions in osteocytes and osteoblasts via Gli activation.

Mechanical stimulation of primary cilia in osteocytes and osteoblasts has been proposed as a mechanism that participates in bone cell survival and skeletal remodeling. Among different signaling pathways stimulated by primary cilia, the hedgehog signaling pathway has been associated with the regulation of bone development. Parathyroid hormone (PTH)-related protein (PTHrP) signaling through PTH 1 receptor (PTH1R) also regulates bone cell survival and remodeling and has been associated with the hedgehog pathway during skeletal development. We hypothesize that primary cilia and PTH1R concomitantly regulate bone remodeling and cell survival and aim to describe the mechanisms that mediate these effects in osteocytes and osteoblasts. Colocalization of PTH1R with primary cilia was observed in control and PTHrP-stimulated MLO-Y4 osteocytic and MC3T3-E1 osteoblastic cells. Activation of PTH1R by PTHrP increased cell survival, osteoblast gene expression (osteocalcin, runt-related transcription factor 2, and bone alkaline phosphatase) and the expression of the hedgehog transcription factor Gli-1 in osteocytes and osteoblasts. These effects were abrogated by small interfering RNAs for the primary cilia protein IFT88 or by a primary cilia specific inhibitor (chloral hydrate). Preincubation of MLO-Y4 osteocytic and MC3T3-E1 osteoblastic cells with the Gli-1 antagonist GANT61 inhibited PTHrP prosurvival actions but did not affect PTHrP-induced overexpression of osteogenic genes. Mechanical stimulation by fluid flow increased the number of primary cilia-presenting cells in osteocytes and osteoblasts. We propose that PTH1R activation induces prosurvival actions via primary cilia- and Gli-1-dependent mechanism and modulates osteogenic responses via a primary cilia-dependent and Gli-1-independent pathway in osteocytes and osteoblasts.