PDGF Receptor β Is a Potent Regulator of Mesenchymal Stromal Cell Function

Abstract

Mesenchymal stromal cells (MSCs) in bone marrow are important for bone homeostasis. Although platelet-derived growth factor (PDGF) has been reported to be involved in osteogenic differentiation of MSCs, the role remains controversial and the network of PDGF signaling for MSCs has not been clarified. To clarify the underlying regulatory mechanism of MSC functions mediated by PDGF, we deleted the PDGF receptor (PDGFR)␤ gene by Cre-loxP strategy and examined the role of PDGF in osteogenic differentiation of MSCs and fracture repair. In cultured MSCs, the mRNA expression of PDGF-A, -B, -C, and -D as well as PDGFR␣ and ␤ was detected. Depletion of PDGFR␤ in MSCs decreased the mitogenic and migratory responses and enhanced osteogenic differentiation as evaluated by increased alkaline phosphatase (ALP) activity and mRNA levels of ALP, osteocalcin (OCN), bone morphogenetic protein (BMP) 2, Runx2, and osterix in quantitative RT-PCR. PDGF-BB, but not PDGF-AA, inhibited osteogenic differentiation accompanied by decreased ALP activity and mRNA levels, except for BMP2. These effects of PDGF-BB were eliminated by depletion of PDGFR␤ in MSCs except that PDGF-BB still suppressed osterix expression in PDGFR␤-depleted MSCs. Depletion of PDGFR␤ significantly increased the ratio of woven bone to callus after fracture. From the combined analyses of PDGF stimulation and specific PDGFR␤ gene deletion, we showed that PDGFR␤ signaling distinctively induces proliferative and migratory responses but strongly inhibits osteogenic differentiation of MSCs. The effects of PDGFR␣ on the osteogenic differentiation were very subtle. PDGFR␤ could represent an important target for guided tissue regeneration or tissue engineering of bone.

Predominant role of PDGF receptor transactivation in Wnt3a-induced osteoblastic cell proliferation

“…Genetic depletion of PDGF-Rb in these cells decreased mitogenic and migratory responses and enhanced osteogenic differentiation. (46) The use of a selective PDGF-R inhibitor further supported the hypothesis that PDGF-R activation is involved in Wnt3a-induced osteoblastic cell proliferation. This inhibitor dosedependently reduced Wnt3a-induced cell proliferation in MC3T3-E1 cells, and a low concentration of this molecule completely blunted this response in hMSCs.…”

“…These findings indicate that Wnt3a-induced osteoblastic cell proliferation implicates a change in phosphorylation/activation of PDGFRs, an observation that is consistent with previous observations reported on PDGF/PDGF-R effects in bone-forming cells. (45,46) Indeed, PDGF-Ra is required for the growth and differentiation of cranial and cardiac neural crest cells and normal craniofacial bone development. (47) A role of PDGF-Rb in bone development has not yet been reported.…”

The tyrosine kinase inhibitor dasatinib dysregulates bone remodeling through inhibition of osteoclasts in vivo

“…PDGF is an important regulator of OB formation because it enhances OB proliferation while inhibiting OB differentiation in vitro. (13)(14)(15)(16) Treatment with imatinib relieved the inhibitory effects of PDGF on mineralization in human bone explant cultures and MC3T3-E1 cells, (17,19) suggesting that inhibition of PDGFR signaling may be one mechanism whereby imatinib activates differentiation in vitro. It is likely that dasatinib also inhibits OB proliferation and stimulates differentiation owing to its specificity for the PDGFR, although this remains to be demonstrated.…”

“…PDGF is a potent mitogen for OB precursors that also inhibits OB differentiation in vitro. (13)(14)(15)(16) Studies from our laboratory (17) and those of others (18)(19)(20)(21) have shown that imatinib activates OB activity while inhibiting cell proliferation at least in part through the inhibition of PDGFR. In cultures of human and murine stromal cells and cell lines, treatment with therapeutically achievable concentrations of imatinib significantly increased mineral deposition (17,19,21) and decreased cell proliferation.…”

In situ cell manipulation through enzymatic hydrogel photopatterning

“…27), each engineered to comprise a Q-peptide. These biomolecules were selected because they play a role during MSC invasion [28][29][30] and because our MSCs express high levels of receptors for these signals (not shown). Multicellular clusters (microtissues) assembled from MSCs were incorporated into MMP-sensitive PEG hydrogels 23 bearing tethered caged K-peptide (Fig.…”