This project delineates biochemical and pharmacological properties of sigma-1 receptors (s-1R). S-1R are one-transmembrane proteins at the endoplasmic reticulum (ER) that bind neurosteroids, dextrobenzomorphans, and certain psychostimulants such as cocaine. We previously demonstrated that overexpression of s-1R potentiated neurite sprouting caused by nerve growth factor in PC12 cells (Takebayashi et al., 2002). In this study we examined if s-1R may be involved in the action of epidermal growth factor (EGF). EGF is conventionally recognized as a mitogenic factor that stimulates only the proliferation of various types of cells including PC12 cells. We found here that in s-1 receptor-overexpressing PC12 cells (s-1R OE cells), EGF markedly stimulates neuritogenesis without affecting cellular proliferation. EGF receptors (EGFR) are largely reduced in lipid rafts and are enriched in non-raft regions in s-1R OE cells. The enrichment of EGFR in the non-raft region is correlated with enhanced downstream signaling of EGFR including the phosphorylation of both EGFR and extracellular signal-regulated kinases (ERKs). Destruction of cholesterol-containing rafts by treating cells with methyl-b-cyclodextrin also causes a reduction of EGFR in lipid rafts, a concomitant increase in the phosphorylation of both EGFR and ERK, and an increase in the EGF-induced neurite sprouting in wild type cells. Furthermore, while overexpression of s-1R increases the level of lipid raft-associated cholesterol, the overexpression alters the levels of gangliosides in lipid rafts: GM1 and GM2 are decreased, whereas GD1a is increased. We conclude that s-1R cause the remodeling of lipid rafts, at least by increasing the level of lipid raft-associated cholesterol and by altering the levels of certain critical lipid raft-forming gangliosides. s-1R may thus play an important role in directing EGF signaling towards neuritogenesis perhaps by shifting EGFR from the lipid raft into non-raft regions.
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