Abstract
Asporin (ASPN) is a small leucine-rich proteoglycan that is involved in pathological processes of osteoarthritis. Previously, we showed that asporin can inhibit transforming growth factor-beta1 (TGF-beta1)-mediated expression of cartilage matrix genes and chondrogenesis in vitro (Kizawa, H., Kou, I., Iida, A., Sudo, A., Miyamoto, Y., Fukuda, A., Mabuchi, A., Kotani, A., Kawakami, A., Yamamoto, S., Uchida, A., Nakamura, K., Notoya, K., Nakamura, Y., and Ikegawa, S. (2005) Nat. Genet. 37, 138-144). However, details about regulation of asporin itself are not yet known. Here, we examined ASPN expression in skeletal tissue and potential regulation of ASPN by TGF-beta. In situ hybridization revealed the presence of ASPN mRNA in the perichondrium/periosteum of long bones, but its absence in articular cartilage and growth plates. Immunohistochemical analysis also showed ASPN protein expression predominantly in the perichondrium/periosteum. TGF-beta1 induced endogenous ASPN mRNA expression over time in vitro, and this induction was suppressed by the TGF-beta type I receptor kinase inhibitor SB431542. Inhibition of Smad3 significantly reduced TGF-beta1-induced ASPN expression, whereas overexpression of Smad3 augmented the induction. Characterization of the human ASPN promoter region revealed a region from -126 to -82 that is sufficient for full promoter activity; however, TGF-beta1 failed to increase activity through the ASPN promoter. Our findings indicate that TGF-beta1 induces ASPN through Smad3 but that this induction is indirect.
Highlights
TGF- is a multifunctional cytokine involved in many growth processes, including cartilage formation (5)
We demonstrate that ASPN expression is up-regulated by transforming growth factor-1 (TGF-1) and that the Smad pathway is crucial for this induction
Expression of Asporin in Adult Mouse—In previous work, we showed that ASPN expression is increased in articular cartilage from osteoarthritis patients using real-time PCR (3) and immunohistochemistry
Summary
TGF- is a multifunctional cytokine involved in many growth processes, including cartilage formation (5). We demonstrate that ASPN expression is up-regulated by TGF-1 and that the Smad pathway is crucial for this induction. Induction of ASPN by TGF-—NHAC cells were cultured in 12-well plates at a density of 1 ϫ 105 cells/well in DMEM/ nutrient mixture F-12 containing 10% FBS.
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