Epidermal growth factor attenuates cell proliferation by down-regulating the transforming growth factor-beta receptor in the osteoblastic cell line MC3T3-E1

Activation of FGF receptors is a new mechanism by which strontium ranelate induces osteoblastic cell growth

BACKGROUND/AIMS

Strontium ranelate (SrRan) is an anti-osteoporotic treatment that reduces the risk of vertebral and hip fractures. Recent in vitro studies suggest that the effect of strontium ranelate on osteoblastic cell growth likely involves two processes including activation of the calcium sensing receptor (CaSR) and a yet undefined mechanism. In the present study, we investigated the CaSR-independent molecular mechanism by which SrRan stimulates osteoblast growth.

METHODS

MC3T3-E1 and primary osteoblastic cells, specific inhibitors of receptor tyrosine kinases (RTK) and western blot analysis were used to characterize the CaSR-independent mechanism in osteoblastic cells.

RESULTS

A selective inhibitor of FGF receptor but not other RTK inhibitors markedly blunted cell growth induced by SrRan in osteoblastic cells. Associated with this observation, SrRan induced rapid activation of FGFR signaling pathways such as PLCγ, FRS2, Akt, ERK1,2 and p38. FGFR-dependent stimulation of osteogenic cell growth was also observed with other cations but not with neomycin, a selective CaSR agonist. Also, in cultured conditions used in this study, MC3T3-E1 cells and primary osteoblasts did not express the CaSR.

CONCLUSION

data presented in this study suggest that activation of FGFRs is a new potential mechanism by which strontium can stimulate osteoblastic cell growth. Activation of FGFR-dependent cell growth is also observed in response to other cations suggesting that activation of FGF receptors is a new cation sensing mechanism in osteoblasts.

Expression of TGF-beta type II receptors in the olfactory epithelium and their regulation in TGF-alpha transgenic mice

Numerous in vitro studies of neurogenesis of olfactory receptor neurons (ORNs) suggest that transforming growth factor (TGF)-beta promotes the maturation/differentiation of olfactory progenitors. We demonstrate that in vivo both mature and immature ORNs, and possibly a basal neuronal progenitor cell, express the TGF-beta type II receptor (TGF-betaRII), suggesting that these cells are targets for TGF-beta signaling. In a previous study of neurogenesis in the OE of TGF-alpha overexpressing transgenic (T) mice, we observed an apparent reduction in the expression of olfactory marker protein (OMP), a marker of terminal differentiation in ORNs in T mice compared to nontransgenic (NT) littermate controls; this was confirmed by Western blotting and immunohistochemistry. In contrast, there was no apparent difference between T and NT mice in the intensity of immunoreactivity for a neuronal marker, protein gene product 9.5. Because TGF-alpha overexpression has been reported to affect TGF-beta signaling in other epithelia, we compared the expression of the TGF-beta type II receptor (TGF-betaRII) in T and NT mice. The intensity of TGF-betaRII immunoreactivity on ORNs was substantially reduced in T compared to NT mice. Similar reductions in TGF-betaRII expression in vomeronasal receptor neurons and in other epithelia in the nasal cavity of T mice were also observed. Taken together, these results indicate that TGF-beta signaling regulates terminal differentiation of ORNs in vivo and suggest ways in which interactions between TGF-alpha and TGF-beta signaling pathways may interact in the OE.

Autocrine growth mechanism by transforming growth factor (TGF)-beta 1 and TGF-beta 1-receptor regulation by epidermal growth factor in a human endometrial cancer cell line IK-90

Transforming growth factor-beta 1 (TGF-beta 1) enhanced cell proliferation in a concentration-dependent manner in a human endometrial cancer cell line, IK-90. Scatchard analysis of TGF-beta 1 receptor in IK-90 cells, using 125I-TGF-beta 1 as a ligand, revealed the presence of a class of high-affinity TGF-beta 1 receptors (2,000 sites per cell, KD = 74pM). Moreover, IK-90 cells produced and secreted TGF-beta 1: TGF-beta 1 messenger RNA was detected at 2.5 and 4.0 kb by Northern-blot analysis using 32P-labeled TGF-beta 1 cDNA as a probe, and TGF-beta 1 activity in conditioned medium by the inhibition of 3H-thymidine uptake into CCl 64 mink lung epithelial cells. We investigated the regulation of TGF-beta 1 receptor by 4 kinds of growth factor: epidermal growth factor (EGF) but not TGF-beta 1, insulin or insulin-like growth factor-1 increased the level of TGF-beta 1 binding sites in a concentration- and time-dependent manner. These findings suggest that TGF-beta 1 may be a potential autocrine growth factor in a human endometrial cancer cell line IK-90 and that this autocrine mechanism may be affected by EGF.