Endothelial Cells and the Circulatory System

Vascular endothelial cells are multi-functional cells that line the vessels of the circulatory system. Endothelial cell monolayers form the innermost layer of vessels, serving protective and barrier functions, and interacting with factors in the blood to mediate wound healing and the inflammatory response. In particular, microvascular endothelial cells are located in the smallest vessels of the circulatory system, such as the capillaries. Vascular endothelial cells have an important role in vessel homeostasis and endothelial cell dysfunction can result in multiple diseases, including atherosclerosis. In addition, the endothelium is the last barrier for metastatic tumor cells to cross before seeding a new tumor in a secondary organ site, an obstacle that is investigated in one study described below.

Recent Studies Featuring Lifeline® Microvascular Endothelial Cells

Metastasis of a primary tumor to a secondary organ site is a main cause of cancer-associated death. Importantly, for circulating tumor cells to colonize a new site and form a new tumor, they must exit the bloodstream through the endothelial cell monolayer in the secondary organ. In a 2013 study, Evani and colleagues investigated the hypothesis that tumor cell metastasis is aided by white blood cells in the blood. To test this hypothesis, the authors used a system in which human aortic endothelial cells and Lifeline® human dermal microvascular endothelial cells were grown as confluent monolayers in culture. MDA-MB-231 breast cancer cells were then added to monolayers under different conditions to test their ability to adhere, mimicking the extravasation of circulating tumor cells.

First, the authors established that MDA-MB-231 cells could not adhere to the endothelial monolayer under shear flow conditions, suggesting these cells must use other mechanisms to adhere and escape blood flow. Next, using THP1 monocytic cells, the authors found that under inflammatory conditions induced by addition of TNF-α, THP1 cells formed aggregates with MDA-MB-231 cells and these aggregates were then able to adhere to the endothelial monolayer.

To uncover the molecular mechanisms behind monocyte-tumor cell aggregate binding to endothelial cells, the authors examined the requirements of adhesion molecules on this process. They found that ICAM-1 expression was upregulated by TNF-α-induced activation of NF-κB signaling. In turn, blocking ICAM-1 inhibited aggregate formation and endothelial adhesion. These results were also repeated using primary human monocytes.

Finally, using simulations, the authors determined that monocytes likely facilitate tumor cell adhesion to endothelial cells by forming a bridge between the two. While these observations need to be confirmed in vivo, they demonstrate that monocyte-tumor cell aggregates are a mechanism by which circulating tumor cells can access the endothelial layer to escape the bloodstream.

To mediate their repair functions, endothelial cells secrete various factors into the bloodstream, including Von Willebrand factor (VWF), which is a critical factor for blood clotting. Secretion occurs through the process of exocytosis, during which intracellular vesicles fuse with the plasma membrane and release their contents into the extracellular space. The process of exocytosis is mediated by a number of cellular components that comprise a SNARE complex, which mediate vesicle docking and fusion to the plasma membrane. In a study from 2015, Zhu et al. identified and characterized the role of SNAP23 as a major constituent of the endothelial SNARE complex, which regulates endothelial exocytosis.

Using Lifeline® human umbilical vein endothelial cells (HUVECs), Lifeline® human aortic endothelial cells, human brain microvascular endothelial cells, and a number of murine tissues, the authors established that of the SNAP isoforms examined, SNAP23 was the most broadly expressed. In particular, they found that SNAP23 was also the only isoform examined that was localized to the plasma membrane. Interestingly, they observed that this localization was somewhat dependent on cell confluency, such that sub-confluent cells had more cytosolic SNAP23.

To establish the importance of SNAP23 function in endothelial exocytosis, the authors knocked down SNAP23 and found that histamine-induced VWF exocytosis was significantly decreased compared to knockdown of other SNAP isoforms. The authors next used Lifeline® dermal microvascular endothelial cells to explore the role of SNAP23 in exocytosis.

Again, SNAP23 knockdown significantly decreased the exocytosis of VWF in response to histamine, thrombin, and calcium, suggesting that SNAP23 is required for VWF exocytosis in endothelial cells. Finally, to determine the particular SNARE proteins with which SNAP23 interacts, the authors fractionated HUVEC lysates and analyzed the SNARE proteins that co-sedimented with SNAP23, confirming by co-immunoprecipitation and co-localization. They found that SNAP23 interacted with STX4, VAMP3, and VAMP8. Together, their results demonstrate that SNAP23 regulates exocytosis through the SNARE complex in endothelial cells.

Lifeline® Microvascular Endothelial Cells

Lifeline® offers multiple microvascular endothelial cell types for your research needs, including:

Microvascular endothelial cells are optimized for growth in VascuLife® Medium. Visit our website to view our human microvascular endothelial cells catalog.