Genomic response of hypoxic Müller cells involves the very low density lipoprotein receptor as part of an angiogenic network

Abstract

Müller cells have recently been found to produce select angiogenic substances. In choosing a more comprehensive approach, we wanted to study the genomic response of Müller cells to hypoxia to identify novel angiogenic genes. An established Müller cell line (rMC-1) was exposed to standard or hypoxic conditions. We analyzed gene expression with three independent microarrays and determined differential expression levels compared to normoxia. Selected genes were confirmed by real-time PCR (RTPCR). Subcellular localization of proteins was examined by immunocytochemistry. A network-based pathway analysis was performed to investigate how those genes may contribute to angiogenesis. We found 19,004 of 28,000 known rat genes expressed in Müller cells. 211 genes were upregulated by hypoxia 1.5 to 14.9-fold (p<0.001, FDR<or=5%) and 220 genes were downregulated 1.5-4.6-fold (p<0.001, FDR<or=5%). Unexpectedly, expression patterns of cell proliferation, differentiation and organogenesis were increased besides predictable declines in cell function. Very low density lipoprotein receptor (VLDLR) and tribbles 3 (TRIB3) were further analyzed because of recent implication in retinal neovascularization and macular degeneration (VLDLR) and in ocular mesodermal development and differentiation (TRIB3), respectively. VLDLR was upregulated 3.1-fold (p=0.001, RTPCR 3.0-fold) and TRIB3 2.8-fold (p=0.025, RTPCR 5.1-fold). VEGF was increased 3.1-fold (p=0.003, RTPCR 8.3-fold) and apelin, a novel factor of retinal angiogenesis, 5.6-fold (p=0.006, RTPCR 8.7-fold). A network of interacting angiogenic genes was identified in silico that included VLDLR as a surface receptor. VLDLR protein localized to the perinucleus, cytoplasm and cell membrane, while TRIB3 was found in nucleoli, the nucleus and cytoplasm. We conclude that hypoxia triggers an angiogenic network response in Müller cells with VLDLR as a novel node and gene expression patterns of proliferation, differentiation and organogenesis.