Sterol regulatory element binding proteins (SREBP)-1a and SREBP-2 are linked to the MAP-kinase cascade

The classic sterol regulatory cis element (sre-1) in the LDL receptor promo ter mediates sterol regulatory element binding protein (SREBP)-binding and the effects of insulin and platelet derived growth factor (PDGF), To elucid ate whether SREBP-1a and SREBP-2 play a direct role in insulin and PDGF act ion, stable cell lines of HepG2 deficient in either SREBP-1 or SREBP-2 were used. Transfection of these cells with the wild-type promoter fragment of the low density Lipoprotein (LDL) receptor gene showed that the effects of insulin and PDGF were significantly reduced in both, SREBP-1- as well as SR EBP-2-deficient cells. Insulin and PDGF action could be reconstituted again in these deficient cell lines by reintroducing SREBP-1a or SREBP-2, Preinc ubation of cells with either the phosphatidylinositol (PI)-3 kinase inhibit or wortmannin or the mitogen-activated protein (MAP) kinase cascade inhibit or PD 98059 showed that the latter abolished the stimulatory effects of ins ulin and EDGE on LDL receptor promoter activity completely whereas wortmann in had no effect. Overexpression of upstream activators of the MAP kinases, like MEKK1 or MEK1, stimulated LDL receptor promoter activity several fold in an sre-1 related manner, These effects could be enhanced by coexpressio n of the transcriptional active N-terminal domains of SREBP-1a and SREBP-2, Using the heterologous Gal-4 system, we could show that intracellular acti vation of the MAP kinase cascade by ectopic expression of MEKK1 or MEK1 has a direct stimulatory effect on the transcriptional activity of SREBP-1a an d SREBP-2, Experimental evidence for a direct link between MAP kinases and SREBPs was obtained due to the MAP kinases ERK1 and ERK2 phosphorylating re combinant GST-fusion proteins of SREBP-1a and SREBP-2, in vitro.jlr We conc lude that SREBP-1a and SREBP-2 mediate different regulatory effects converg ing at sre-1 and that they appear to be linked to the MAP kinase cascade, p ossibly being direct substrates of ERK1 and ERK2.