Abstract-The endothelium interacts extensively with lipids and lipoproteins, but there are very few data regarding the ability of endothelial cells to secrete lipases. In this study, we investigated the ability of endothelial cells to secrete the triglyceride lipase and phospholipase activities characteristic of endothelial lipase (EL), a recently described member of the triglyceride lipase gene family. No lipase activities were detected under basal conditions, but treatment with cytokines significantly stimulated the expression of both activities. Using antibodies to EL, we determined that both activities were primarily a result of this enzyme. and is also transported to the endothelial surface of the sinusoids, where it interacts with lipoproteins. In addition, HL is found on endothelium in the adrenals and gonads. 4 Neither LPL nor HL is synthesized by endothelial cells. Indeed, there has been relatively little evidence that endothelial cells themselves synthesize any enzymes with lipolytic activity. A recent report indicated that human umbilical vein endothelial cells (HUVECs) secreted an enzyme that had the ability to degrade lysolecithin but expressed relatively little phospholipase and no triglyceride lipase activity under the conditions tested. 5 We and others recently reported the discovery of a new member of the triglyceride lipase gene family, 6,7 and in contrast to the case of LPL or HL, its mRNA was demonstrated in endothelial cells. The enzyme was named endothelial lipase (EL) because of this unique characteristic, although its expression is not restricted to endothelial cells. 8 Previous work showed that transient expression of the EL cDNA led to secretion of phospholipase activity, 6,7 and we subsequently demonstrated that EL also has significant triglyceride lipase activity. 9 Overexpression of human EL in mice significantly reduced HDL cholesterol levels, 6 and our recent data show that acute inhibition of EL results in increased HDL cholesterol levels in vivo. 10 Therefore, EL expression appears to be an important modulator of HDL metabolism.EL mRNA has also been shown to be upregulated by inflammatory cytokines in HUVECs. 11 Despite these findings, expression of native lipolytic activity by endothelial cells because of secretion of EL has never been reported. In this report, we demonstrate that although endothelial cells do not secrete triglyceride lipase and phospholipase activity

Cytokinesis requires a complex network of equatorial and global proteins to regulate cell shape changes. Here, using interaction genetics, we report the first characterization of a novel protein, enlazin. Enlazin is a natural fusion of two canonical classes of actin-associated proteins, the ezrin-radixin-moesin family and fimbrin, and it is localized to actin-rich structures. A fragment of enlazin, enl-tr, was isolated as a genetic suppressor of the cytokinesis defect of cortexillin-I mutants. Expression of enl-tr disrupts expression of endogenous enlazin, indicating that enl-tr functions as a dominantnegative lesion. Enlazin is distributed globally during cytokinesis and is required for cortical tension and cell adhesion. Consistent with a role in cell mechanics, inhibition of enlazin in a cortexillin-I background restores cytokinesis furrowing dynamics and suppresses the growth-in-suspension defect. However, as expected for a role in cell adhesion, inhibiting enlazin in a myosin-II background induces a synthetic cytokinesis phenotype, frequently arresting furrow ingression at the dumbbell shape and/or causing recession of the furrow. Thus, enlazin has roles in cell mechanics and adhesion, and these roles seem to be differentially significant for cytokinesis, depending on the genetic background.

Cytokinesis is the mechanical process that allows the simplest unit of life, the cell, to divide, propagating itself. To divide, the cell converts chemical energy into mechanical energy to produce force. This process is thought to be active, due in large part to the mechanochemistry of the myosin-II ATPase. The cell's viscoelasticity defines the context and perhaps the magnitude of the forces that are required for cytokinesis. The viscoelasticity may also guide the force-generating apparatus, specifying the cell shape change that results. Genetic, biochemical, and mechanical measurements are providing a quantitative view of how real proteins control this essential life process.