Role of chemerin, a novel adipochemokine, in the human microvascular endothelial cell (HMEC)-1 line

Chemerin is a newly identified adipokine and exerts its functional effects by binding to its natural GPCR, known as CMKLR1. Chemerin is highly expressed in the adipose tissue and in lower levels in other body tissues; and is known to play an important role in adipocyte differentiation and metabolism. Chemerin circulates at the normal physiological concentrations of approximately 3-4nM in humans, and circulating chemerin levels positively correlate with various facets of metabolic abnormalities; such as insulin resistance, type 2 diabetes, high triglycerides, hypertension, and associated risks of development of diseases of cardiovascular system. Endothelial Cells (ECs) line the vasculature of the entire circulatory system and form a direct contact with the bloodstream. In this project, the role of chemerin in EC biology was proposed, and was studied in terms of activation of important signalling Mitogen-activated Protein Kinases (MAPKs) including Extracellular signal-regulated Kinase (ERK) 1/2, ERK5, p38, Stress-activated Protein Kinase/c- Jun NH2-terminal Kinase (SAPK/JNK); and Akt/Protein Kinase B (PKB) and Adenosine Monophosphate Protein Kinase (AMPK)-α in a time- and concentrationdependent manners. These signalling kinases regulate the activity of different transcription factors which then regulate the expression of different genes. Chemerin increased the expression of Hypoxia-inducible Factor (HIF)-1α, a hypoxia-inducible transcription factor which is known to regulate the Vascular Endothelial Growth Factor (VEGF) gene expression. Interestingly, VEGF165, the most potent angiogenic isoform of VEGF protein expression was down-regulated by chemerin in a concentration-dependent manner; whereas, chemerin upregulated the protein expression of VEGF165b, an opposite anti-angiogenic counterpart of VEGF165. Chemerin mediated EC proliferation, migration and capillary tube formation; which are the key processes implicated in the process of normal and pathological angiogenesis. Chemerin altered the protein expression levels of Cell Adhesion Molecules (CAMs) including E-selectin, ICAM-1 and VCAM-1 – increased the activity of Nuclear Factor (NF)–kappa (κ) B pathway – and encouraged Endothelial- Monocyte cell adhesion in a concentration-dependent manner. Nitric Oxide (NO), not only keeps the vascular health in check by downregulating the expression levels of adhesion molecules, but also acts as a potent vasodilator. Endothelial Nitric Oxide Synthase (eNOS), an enzyme constitutively expressed in the endothelial cells regulates the production of NO in the endothelium. Chemerin increased eNOS activity by causing eNOS phosphorylation at Ser1177, and dephosphorylating at Thr495 phosphorylation sites. Chemerin increased the protein expression of nonconstitutively expressed enzyme, inducible Nitric Oxide Synthase (iNOS), which is mainly induced during injury or inflammation and is known to produce 100- to 1000-times more NO compared to that of eNOS. However, interestingly, chemerin failed to show any significant changes in the amounts of combined nitrite and nitrate (NOx) levels in HMEC-1 cells; whereas, nitrite (NO2–) levels were decreased in a concentration-dependent manner.