PLATELET-DERIVED GROWTH-FACTOR ENHANCES SP1 BINDING TO THE LDL RECEPTOR GENE

We have previously demonstrated that growth activation of quiescent ce lls enhances LDL receptor gene transcription and that the proximal 5' flanking region of the LDL receptor gene could transduce a platelet-de rived growth factor (PDGF) response. This portion of the LDL receptor gene encompasses a previously characterized sterol response element an d an adjacent Spl binding site. By use of mobility shift analyses we s how that PDGF activation of quiescent cells enhances binding of Sp1 to the LDL receptor gene. Transfection analyses indicated that the Sp1 s ite, but not the sterol response element binding protein site, could c onfer PDGF responsiveness to a heterologous promoter in quiescent cell s. Furthermore, cotransfection of an LDL receptor reporter gene (conta ining -141 to +35 bp of the LDL receptor gene promoter) along with an expression construct coding for high-level constitutive expression of an Spl cDNA led to marked enhancement in expression of the LDL recepto r reporter gene in quiescent cells. Increased Sp1 binding due to PDGF could be due to enhanced production of Sp1; alternatively, posttransla tional activation of binding could be involved. Western blot analysis showed no difference in Sp1 abundance in quiescent cells versus PDGF-s timulated cells, suggesting a posttranslational mechanism for activati on of Sp1 binding by growth induction. Our data demonstrate that PDGF stimulation of quiescent cells leads to enhanced Spl binding to the LD L receptor gene. This enhanced binding could participate in PDGF induc tion of LDL receptor gene transcription.