Growth factor-induced phosphorylation of sterol regulatory element-binding proteins inhibits sumoylation, thereby stimulating the expression of their target genes, low density lipoprotein uptake, and lipid synthesis

Abstract

The destiny and activity of sterol regulatory element-binding proteins (SREBPs) in the nucleus are regulated by modification with ubiquitin, small ubiquitin-like modifier (SUMO), or phosphorus. ERK-dependent phosphorylation causes an increase in their transcriptional activity, whereas SUMO modification halts it. We hypothesized a causal linkage between phosphorylation and sumoylation because their sites are very closely located in SREBP-1 and -2 molecules. When Ser(455), a phosphorylation site in SREBP-2, was substituted with Ala, this SREBP-2 mutant was more efficiently modified by SUMO-1. On the other hand, substitution of Asp inhibited SUMO conjugation, mimicking phosphoserine. When cells were cultured with insulin-like growth factor-1, sumoylation of SREBP-2 was decreased with an increase in its phosphorylation, but SREBP-2(S455A) was continuously sumoylated. An ERK cascade inhibitor, U0126, inversely augmented SUMO modification of SREBP-2. Insulin-like growth factor-1 treatment stimulated the expression of SREBP target genes such as the low density lipoprotein (LDL) receptor, squalene synthase, and hydroxymethylglutaryl-CoA synthase genes. These results indicate that growth factor-induced phosphorylation of SREBP-2 inhibits sumoylation, thereby facilitating SREBP transcriptional activity. Glutathione S-transferase pulldown assays revealed that wild-type SREBP-2, but not a mutant lacking Lys(464), interacts with HDAC3 preferentially among the histone deacetylase family members. HDAC3 small interfering RNA induced gene expression of the LDL receptor and thereby augmented fluorescently labeled LDL uptake in HepG2 cells. In summary, growth factors inhibit sumoylation of SREBPs through their phosphorylation, thus avoiding the recruitment of an HDAC3 corepressor complex and stimulating the lipid uptake and synthesis required for cell growth.