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Role of thrombospondin in mesangial cell growth: possible existence of an autocrine feedback growth circuit
G N Marinides1, S J Suchard, B K Mookerjee
1Department of Medicine, Dept. of Veterans' Affairs Medical Center, Buffalo, New York.
Abstract:
Thrombospondin (TSP) is an extracellular matrix glycoprotein involved in mesangial cell (MC) adhesive and migratory function. We have studied the role of TSP in activation and proliferation of rat MC in serum-free media. TSP, in a concentration dependent manner (5 to 20 micrograms/ml), caused an increase in thymidine uptake, first detectable at 28 hours and more prominent at 48 hours. This effect was inhibited by heparin and heparan sulfate. TSP induced epidermal growth factor (EGF) secretion and significantly augmented constitutive platelet-derived growth factor-AB (PDGF-AB) secretion by MC in a concentration dependent fashion. It did not, however, induce TGF-beta, IL-1, IL-6, IL-8, or TNF-alpha production. TSP had an additive effect with exogenous EGF and PDGF on thymidine uptake. Anti-PDGF neutralizing antibody eliminated the effect of TSP on MC growth. MC displayed a single class of heparin-inhibitable TSP binding sites (Bmax 3.8 +/- 1.8 x 10(6)/cell, Kd = 80 +/- 29 nM). Based on these observations, we propose the existence of an autocrine positive feedback loop of MC proliferation involving TSP and growth factors, and regulated by heparan sulfate.
Insights
Thrombospondin (TSP) promotes rat mesangial cell (MC) proliferation by stimulating growth factor secretion. Heparan sulfate inhibits this TSP-induced cell growth, suggesting a feedback loop.
Area of Science:
- Cell Biology
- Extracellular Matrix Biology
- Renal Cell Biology
Background:
- Thrombospondin (TSP) is an extracellular matrix glycoprotein.
- TSP influences mesangial cell (MC) adhesion and migration.
Purpose of the Study:
- Investigate TSP's role in rat MC activation and proliferation in serum-free conditions.
- Elucidate the mechanisms underlying TSP-mediated MC growth.
Main Methods:
- Assessed thymidine uptake to measure MC proliferation.
- Measured growth factor secretion (EGF, PDGF-AB).
- Utilized heparin, heparan sulfate, and anti-PDGF antibodies for inhibition studies.
- Characterized TSP binding sites on MCs.
Main Results:
- TSP (5-20 µg/ml) increased thymidine uptake in MCs, peaking at 48 hours.
- TSP induced epidermal growth factor (EGF) and augmented platelet-derived growth factor-AB (PDGF-AB) secretion.
- Heparin and heparan sulfate inhibited TSP's proliferative effect.
- Anti-PDGF antibody abolished TSP-induced MC growth.
- MCs possess heparin-inhibitable TSP binding sites (Bmax=3.8x10^6/cell, Kd=80nM).
Conclusions:
- TSP stimulates rat MC proliferation via an autocrine loop involving EGF and PDGF-AB.
- Heparan sulfate negatively regulates TSP-mediated MC proliferation.
- A positive feedback loop for MC proliferation involving TSP and growth factors, modulated by heparan sulfate, is proposed.