Proinflammatory cytokines (interleukin‐1β and tumor necrosis factor‐α) down regulate synthesis and secretion of thrombospondin by human endothelial cells

Abstract

We examined the effects of proinflammatory cytokines on the expression of two extracellular matrix proteins, e.g., thrombospondin (TSP) and fibronectin (FN) by cultured human umbilical vein endothelial cells (HUVECs). Treatment of HUVECs with human recombinant interleukin‐ 1β (IL‐1β) or human tumor necrosis factor‐α (TNF‐α) caused a time‐ and dose‐dependent decline in TSP production whereas FN production was not modified. At low concentrations, IL‐1β and TNF‐α in combination had a greater effect than either agent alone. Interferon‐γ (IFN‐γ) was without effect. The decline in TSP synthesis resulted in a decreased secretion of this glycoprotein into the extracellular matrix. Endothelial cell monolayers cultured on porous filters were used to study the polarity of TSP secretion. Approximately two thirds of the synthesized protein was secreted to the apical side medium and one third to the basal side medium and both types of secretion were inhibited to a similar extent by cytokine treatment. Immunoprecipitation experiments revealed no apparent degradation of secreted TSP, either in the apical or in the basal compartment. Treatment of HUVECs with IL‐1β, either alone or in combination with TNF‐α, had no significant effect on the steady‐state TSP mRNA levels, suggesting a posttranscriptional regulation. Our results indicate that IL‐1β and TNF‐α can selectively modulate the composition of the extracellular matrix by decreasing TSP deposition and suggest different regulatory mechanisms for the expression of various secreted proteins by endothelial cells.