Background/Purpose:

Sequential pathologic observations in the early stages of SSc demonstrated evidence for platelet aggregation and binding to blood vessels that is generally followed by vascular effacement and the development of SSc vasculopathy. In this study we thought to investigate the effects of platelet constituents on microvascular endothelial cells (MVEC) apoptosis. We particularly investigated the role of thrombospondin 1 (TSP1) as a possible platelet derived signal since it is a potent angiogenic inhibiter, it can mediate endothelial apoptosis and activate TGF-β and is reported to be overexpressed in SSc. Furthermore, we investigated the role of CD36 in MVEC apoptosis since TSP1 vascular effects are known to results from its binding to CD 36 receptors.

Methods:

MVEC were isolated from involved SSc skin and from match healthy control subjects. Platelets were collected from healthy subjects, sonicated, ultracentrifuged and the resulting supernatant (PLSN) was used in MVEC cultures. MVEC apoptosis was evaluated by TUNEL and active caspases-3 staining.  CD 36 expression in MVEC was measured by quantitative RT-PCR and CD36 expression was knocked down using CD36 specific siRNA.

Results:

The following results were observed in this study:

1. Addition of PLSN to MVEC cultured in 0.5% serum concentration resulted in a dose dependent apoptosis of MVEC. SSc-MVEC were more susceptible to apoptosis than control MVEC, thus at 20% PLSN concentration, MVEC apoptosis was 20% ± 5 and 48% ± 8 in control vs SSc MVEC respectively (mean ± SD of three cell lines).
2. Addition of TSP1 neutralizing antibody to PLSN significantly reduced PLSN induced MVEC apoptosis, suggesting that TSP1 in PLSN mediate MVEC apoptosis.
3. CD 36 expression levels were significantly higher in SSc MVEC than in control MVEC (7.4 ± 2.3 folds, mean ± SD of 3 cell lines)
4. CD36 knock down using CD36 specific siRNA but not ir-siRNA resulted in > 90% decrease in CD36 expression and the inhibition of TSP1 and PLSN induced MVEC apoptosis.  

Conclusion: Platelets interaction with MVEC results in MVEC apoptosis. This effect is largely mediated by interaction of TSP1 with CD36.  SSc-MVEC are more susceptible to this effect possibly because of an upregulated CD36 expression in SSc-MVEC. The data propose the platelet as a possible source for the initial MVEC apoptotic signal in the early stages of SSc vasculopathy and suggest TSP1 as a crucial player in SSc pathogenesis in view of its abundance in SSc and its documented role in the genesis of vasculopathy and tissue fibrosis

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/platelet-release-products-mediate-endothelial-apoptosis-a-possible-role-for-thrombospondin-1-cd36-pathway-in-ssc-endothelial-apoptosis/