IP3‐evoked Ca2+ release in Endothelial Cells is Modulated by the Immunosuppressant FK506‐binding Protein (FKBP12)

Abstract

The endoplasmic reticulum (ER) is integral to the regulation of cytoplasmic Ca2+ concentration in endothelial cells. Ca2+ release from the ER is controlled by two intracellular receptor/channel complexes, the inositol 1,4,5‐triphosphate receptor (IP3R) and the ryanodine receptor (RyR). These receptors have been shown to be regulated by the accessory FK506‐binding protein (FKBP). FK506 is a potent immunosuppressant, which is routinely used after organ transplant to lower the risk of rejection, and has also been used in drug‐eluting stents. FK506 has been shown to act on smooth muscle cells either directly by binding to the channel, or indirectly via FKBP modulation of two targets, calcineurin or the mammalian target of rapamycin (mTOR). However very little is known on the role FKBPs play in endothelial cell Ca2+ signaling.To investigate this, Ca2+ signals were measured in the endothelium of second order mesenteric arteries using Cal‐520 (5 μM). IP3Rs were activated by localised photo‐uncaging of IP3 (5 μM), and RyR by the application of Caffeine (10 mM). FK506 (10 μM), which displaces FKBP from each receptor to inhibit calceinurin, increased the IP3‐evoked Ca2+ rise. The phosphatase inhibitors cypermethrin and okadaic acid also increased the IP3‐mediated Ca2+ release. Cypermethrin and okadaic acid did not prevent FK506‐induced increase in Ca2+ release. These results suggest that while calceinurin may modulate IP3‐evoked Ca2+ release, the phosphatase is not necessary for the FK506‐mediated potentiation of IP3‐evoked Ca2+ increase.Rapamycin (10 μM), which disrupts the binding of FKBP12 to the receptor and inhibits mTOR, did not significantly alter IP3‐induced Ca2+ release in endothelial cells, indicating that mTOR does not exert a significant role in endothelial IP3‐induced Ca2+ release.Interestingly Caffeine, the activator of RyR‐mediated Ca2+ release, did not activate Ca2+ signals in the endothelium either in intact tissue or in freshly isolated endothelial patches, suggesting that Caffeine‐activated RyRs do not play a role in endothelial Ca2+ release.These results indicate that FKBPs play an integral role in regulating endothelial IP3‐mediated Ca2+ release, and may underlie the severe endothelial dysfunction associated with FK506 immunosuppressants.Support or Funding InformationWellcome TrustBritish Heart FoundationThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.