RGC-32 regulates expression of insulin-like growth factor binding proteins in astrocytes

Abstract

Abstract Response gene to complement (RGC)-32 plays an important role in the mediation of TGF-β downstream effects and activation of cell cycle. During experimental autoimmune encephalomyelitis (EAE), RGC-32 knock-out (KO) mouse astrocytes displayed an elongated, bipolar phenotype, resembling immature astrocytes and glial progenitors whereas those from wild-type (WT) mice had a reactive, hypertrophied phenotype, suggesting a role for RGC-32 in astrocyte differentiation. To gain more insight into the role played by RGC-32 in astrocytes, we investigated the transcriptional profile induced by TGF-β in astrocytes using qPCR arrays. The majority of genes differentially expressed were related to cell growth, extracellular matrix remodeling and signal transduction. Among them, a group of genes belonging to the family of insulin-like growth factor binding proteins (IGFBPs) and insulin-like growth factors (IGFs) were found to be most significantly regulated by RGC-32. The IGFBP category includes IGFBP2, IGFBP3, IGFBP6 and connective tissue growth factor (CTGF). Using Real-time PCR and Western Blotting, we confirmed the differential expression of these genes in RGC-32 KO astrocytes after TGF-β stimulation. When glial fibrillary acidic protein (GFAP) and CTGF levels were compared between WT and KO astrocytes after TGF-β stimulation, significantly lower levels of mRNA and protein expression were found in RGC-32 KO astrocytes. These data suggest that TGF-β stimulation of CTGF is RGC-32 dependent and that RGC-32 plays a role in the regulation of CTGF downstream effects. The differential regulation of IGFBPs and GFAP suggests that they might be responsible for the lack of astrocyte differentiation seen in RGC-32 KO mice during EAE.