Control of the extracellular matrix by hypoxic lymphatic endothelial cells: Impact on the progression of lymphedema?

Background: Initial lymphatic vessels do not have a continuous basement membrane. Therefore, the ability of lymphatic endothelial cells (LECs) to produce extracellular matrix (ECM) has received little attention. Untreated lymphedema is a chronic disease that progresses to massive fibrosclerosis in advanced stages. Expansion of the intercellular space and fibrosclerosis cause hypoxia, which also affects the LECs. Results: We studied the expression of genes in human LECs in vitro by RNA sequencing, analyzed the effects of hypoxia (1% O2) vs. normoxia (21% O2), and focused on ECM genes. LECs express fibrillin‐1 and many typical components of a basement membrane such as type IV, VIII, and XVIII collagen, laminin β1, β2, and α4, perlecan, and fibronectin. Under hypoxia, we found significant upregulation of expression of genes controlling hydroxylation of procollagen (PLOD2, P4HA1), and also cross‐linking, bundling, and stabilization of collagen fibrils and fibers. Also striking was the highly significant downregulation of elastin expression, whereas fibulin‐5, which controls the assembly of tropoelastin monomers, was upregulated under hypoxia. In the dermis from genital lymphedema, we observed significant PLOD2 expression in initial lymphatics. Conclusions: Overall, hypoxia results in the picture of a dysregulated ECM production of LECs, which might be partly responsible for the progression of fibrosclerosis in lymphedema. Key Findings: Initial lymphatics do not produce a continuous basal lamina, nevertheless, lymphatic endothelial cells (LECs) express numerous ECM genes including collagen IV, V, VIII, XVIII, laminin, perlecan, fibronectin and fibrillin.RNASeq analyses show that LECs express approx. 16.800 genes. We observed significant regulation of 162 genes after 4 days of hypoxia (1% pO2). 13% (21 genes) of the hypoxia‐regulated genes are connected to the ECM.It is very likely that hypoxia is a crucial factor in the development of fibrosis in chronic lymphedema (LE), and our data suggest that hypoxic LECs can contribute to fibrosclerosis during LE progression. [ABSTRACT FROM AUTHOR]