Flow promotes insulin uptake and transcytosis in microvascular endothelial cells independently of nitric oxide – potential role of the actin cytoskeleton
Authors: Guan Sha (Lucy) , Azizi Paymon Michael , Wang Changsen , Heit Bryan , Klip Amira , Lee Warren Lester 
Published: November 2023

The muscle and adipose microvasculature forms a tight endothelial barrier separating tissues like muscle and adipose from the blood. Transport of circulating insulin out of the microvasculature to reach muscle is a rate‐limiting step to insulin action. Most evidence suggests that this occurs by transport of insulin through individual endothelial cells (transcytosis); however, little is known about how this occurs. We recently developed an assay to measure insulin transcytosis through primary microvascular endothelial cells from human adipose tissue (HAMEC). However, like most in vitro studies of endothelial cells, this was done under static conditions while, in vivo, endothelial cells are constantly exposed to shear stress, a frictional force derived from blood flow. The purpose of this study was to determine whether physiological shear stress induced by flow affects insulin transcytosis by endothelial cells and if so, to determine the mechanism(s).HAMEC were exposed to static conditions or shear stress (0.5 dynes/cm2) for 2h and measured uptake and transcytosis of fluorescently labelled insulin in HAMEC by confocal microscopy and Total Internal Fluorescence Microscopy, respectively. Flow increased insulin transcytosis and uptake in HAMEC by approximately two‐fold compared to static conditions. Since it has been reported that nitric oxide (NO) increases insulin uptake in aortic endothelial cells, and shear stress is known to activate nitric oxide synthase, we hypothesized that the effect of shear stress was mediated through NO. Surprisingly, exposure to the NO‐donor sodium nitroprusside had no effect on insulin uptake by HAMEC; furthermore, treatment with the nitric oxide synthase inhibitor L‐NAME before and during flow did not prevent the induction of insulin transcytosis by flow. To determine if modification to the actin cytoskeleton was involved instead, we treated HAMEC with cytochalasin D (CD), which induces actin depolymerization, or with Jasplakinolide (Jas), which promotes actin polymerization. While incubation with CD or Jas under static conditions had little effect on endothelial insulin uptake, Jas largely abrogated the effect of flow to induce insulin uptake. Conversely, CD significantly enhanced flow‐induced insulin uptake by HAMEC. Together, these findings suggest a likely role for actin dynamics in regulating flow‐induced insulin uptake by microvascular endothelial cells.In summary, flow increases insulin uptake and transcytosis in adipose microvascular endothelial cells in a NO‐independent fashion; this effect is likely mediated by changes in the actin cytoskeleton. These findings may have implications for the development of therapies to enhance insulin transcytosis by the endothelium in obesity/diabetes, in which defective insulin delivery has been recorded.Support or Funding InformationOntario Graduate Scholarship; Canadian Institute of Health Research; Canadian Diabetes Association; Banting and Best Diabetes Centre