Regulation of proliferation and migration in retinoic acid treated C3H10T1/2 cells by TGF‐β isoforms

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Abstract
Proliferation of mesenchymal precursors of osteogenic and chondrogenic cells and migration of these precursors to repair sites are important early steps in bone repair. Transforming growth factor-β (TGF-β) has been implicated in the promotion of bone repair and may have a role in these processes. Three isoforms of TGF-β, TGF-β1, -β2, and -β3, are expressed in fracture healing, however, their specific roles in the repair process are unknown. Differential actions of the TGF-β isoforms on early events of bone repair were explored in the multipotent mesenchymal precursor cell line, C3H10T1/2. Cell migration was determined using a modified Boyden chamber in response to concentrations of each isoform ranging from 10−12 to 10−9 g/ml. All three isoforms demonstrated a dose-dependent chemotactic stimulation of untreated C3H10T1/2 cells. Checkerboard assays indicated that all three isoforms also stimulated chemokinesis of the untreated cells. C3H10T1/2 cells treated with all-trans-retinoic acid (ATRA) and expressing relatively higher levels of osteoblastic gene markers such as alkaline phosphatase and collagen type I, lower levels of chondrocytic gene markers collagen type II and aggrecan, and unchanged levels of the adipose marker adipsin did not demonstrate significant chemokinesis or chemotaxis in response to TGF-β1 or -β3 at concentrations ranging from 10−12 to 10−9 g/ml. In the ATRA-treated cells, TGF-β2 stimulated a significant increase in chemotaxis only at the highest concentration tested. Cell proliferation was assessed by mitochondrial dehydrogenase activity and cell counts at TGF-β concentrations from 10−11 to 10−8 g/ml. None of the TGF-β isoforms stimulated cell proliferation in untreated or ATRA-treated C3H10T1/2 cells. Analysis of TGF-β receptors (TGF-βR1, -βR2, and -βR3) showed a 1.6- to 2.8-fold decrease in mRNA expression of these receptors in ATRA-treated cells. In conclusion: (1) while all three TGF-β isoforms stimulate chemotaxis/chemokinesis of multipotent C3H10T1/2 cells, TGF-β1 and -β3 do not stimulate chemotaxis in C3H10T1/2 cells treated with ATRA while TGF-β2 stimulated chemotaxis only at the highest concentration tested. (2) TGF-β isoforms do not appear to stimulate cell proliferation in C3H10T1/2 cells in either a multipotent state or after ATRA treatment when expressing higher levels of alkaline phosphatase and collagen type I gene markers. (3) Decrease in mRNA expression for TGF-βR1, -βR2, and -βR3 upon ATRA treatment could potentially explain the lack of chemotaxis/chemokinesis in these cells expressing higher levels of alkaline phosphatase and collagen type I.