Vitamin D inhibits growth of human airway smooth muscle cells through growth factor‐induced phosphorylation of retinoblastoma protein and checkpoint kinase 1

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Abstract
Background and purpose: Airway remodelling in asthma is manifested, in part, as increased airway smooth muscle (ASM) mass, reflecting myocyte proliferation. We hypothesized that calcitriol, a secosteroidal vitamin D receptor (VDR) modulator, would inhibit growth factor‐induced myocyte proliferation. Experimental approach: Human ASM cell cultures were derived from bronchial samples taken during surgery. ASM cells were treated with platelet‐derived growth factor (PDGF) (10 ng·mL−1) for 24 h in the presence of calcitriol, dexamethasone or a checkpoint kinase 1 (Chk1) inhibitor (SB218078). The effects of calcitriol on PDGF‐mediated cell proliferation were assessed by thymidine incorporation assay, propidium iodide‐based cell cycle analysis, caspase‐3 assay and immunoblotting for specific cell cycle modulators. Key results: Calcitriol, but not dexamethasone, inhibited PDGF‐induced ASM DNA synthesis concentration dependently (IC50= 520 ± 52 nM). These effects were associated with VDR‐mediated expression of cytochrome CYP24A1 with no effects on ASM apoptosis. Calcitriol substantially inhibited (P < 0.01) PDGF‐stimulated cell growth in ASM derived from both normal (59 ± 8%) and asthmatic subjects (57 ± 9%). Calcitriol inhibited PDGF‐induced phosphorylation of retinoblastoma protein (Rb) and Chk1, with no effects on PDGF‐mediated activation of extracellular signal‐regulated kinases 1/2, PI3‐kinase and S6 kinase, or expression of p21Waf/Cip‐1, p27Kip1, cyclin D and E2F‐1. Consistent with these observations, SB218078 also inhibited (IC50= 450 ± 100 pM) PDGF‐induced cell cycle progression. Conclusions and implications: Calcitriol decreased PDGF‐induced ASM cell growth by inhibiting Rb and Chk1 phosphorylation. This Research Paper is the subject of a Commentary in this issue by Clifford and Knox (pp. 1426–1428). To view this article visit http://www3.interscience.wiley.com/journal/121548564/issueyear?year=2009