Increased Cell Surface Expression of Receptors for Transforming Growth Factor-β on Osteoblasts from Patients with Osteogenesis imperfecta

Abstract

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder usually characterized by either a reduction in the production of normal collagen I or the synthesis of abnormal collagen. The variability in the clinical phenotype is not in each case sufficiently explained by the underlying mutation in the collagen I genes. Also, biochemical differences between mutant collagen from different tissues suggest additional regulatory mechanisms possibly involved in matrix deposition and maturation, two processes in which transforming growth factor-β (TGF-β) plays an important role. We, therefore, studied the cell surface expression and functional properties of TGF-β receptors I, II and III on osteoblasts from a group of OI patients compared to healthy controls. Receptor number and affinity were determined by Scatchard analysis of binding data and TGF-β receptor II gene expression was assessed by RT-PCR. Ligand-induced downregulation of TGF-β receptors was analyzed to demonstrate the dynamic response to exogenous stimuli. All experiments were performed in parallel in human osteoblastic cells from OI patients and from age-matched controls. TGF-β receptors I, II and III (betaglycan) were present on osteoblasts from both healthy donors and OI patients. The receptor numbers were significantly higher (29,000 per cell) on OI osteoblasts than on age-matched control osteoblasts (12,000 per cell) in spite of similar steady state levels for TGF-β receptor II mRNA in OI and control cells. Furthermore, receptor affinity was not significantly different in OI osteoblasts (181 vs. 177 nM^–1), and the receptor number did not depend on the culture substrate. With respect to dynamic adaption, ligand-induced downregulation of TGF-β receptors was reduced in OI osteoblasts. In conclusion, the human osteoblastic cells from patients with OI investigated all have an elevated number of cell surface receptors for TGF-β, without any evidence for a transcriptional regulation of TGF-β receptor II. On the functional level, there is some evidence for an impaired adaptive behavior of receptor presentation, whereas receptor affinity is unchanged.

Effect of Anti-TGF-β Treatment in a Mouse Model of Severe Osteogenesis Imperfecta

“…In the present study, we found higher gene expression levels of TGF‐β receptor I and II, suggesting elevated TGF‐β signaling due to a higher amount of TGF‐β receptors. This is in line with in vitro TGF‐β binding studies showing that osteoblasts from individuals with OI type I and IV have a higher number of TGF‐β receptors on the cell surface than age‐matched controls . It is, therefore, possible that in OI increased TGF‐β signaling is not only due to disturbed bone matrix but also due to increased TGF‐β receptor levels in osteoblasts.…”

Age-related Changes in Human Bone Proteoglycan Structure

“…OI bone cells do not exhibit a TGF-␤-dependent increase in collagen synthesis and alkaline phosphatase activity seen with normal bone cells (48). OI osteoblasts had a greater reduction in TGF-␤ receptor I-III number than agematched controls, and receptor affinity was unchanged, whereas ligand-induced modulation of receptor number was found to be impaired in the OI cells (49). Thus, the association of elevated thrombospondin levels with altered post-translational modifications in OI osteoblasts is unlikely to involve TGF-␤.…”

Role of TGF-β in a Mouse Model of High Turnover Renal Osteodystrophy

“…Additionally, TGF‐β1 circulating levels and bone expression are upregulated during CKD progression along with increased sclerostin and FGF23 . Elevated TGF‐β activity is also associated with enhanced osteoblast and/or osteoclast activity in genetic bone diseases such as Camurati‐Engelmann disease (CED) and osteogenesis imperfecta (OI) . Finally, TGF‐β and Wnt/β‐catenin cross‐talk has been well described …”