DIFFERENTIAL TRANSCRIPTIONAL REGULATION OF THE HUMAN SQUALENE SYNTHASE GENE BY STEROL REGULATORY ELEMENT-BINDING PROTEIN-1A AND PROTEIN-2 (SREBP) AND INVOLVEMENT OF 5' DNA-SEQUENCE ELEMENTS IN THE REGULATION

Transcription of the human squalene synthase (HSS) gene is regulated b y variations in the level of cellular cholesterol. Three regulatory el ements in the HSS promoter region are known to be involved in the regu lation: 1) a modified sterol regulatory element (SRE) 1 (HSS-SRE-1), 2 ) an inverted SRE-3 (Inv-SRE-3), 3) an inverted Y box (Inv-Y-Box). We report here the regulatory role of distinct cis-elements in the HSS pr omoter by using mutants of an HSS-luciferase promoter reporter. The ac tivity of a wild-type promoter reporter transiently transfected into H epG-2 cells is increased by sterol depletion of the cells or by coexpr ession of mature forms of the SRE-binding proteins (SREBP) 1a and SREB P-2. Differential activation by SREBP-1a and SREBP-2 of the reporter g ene mutated at various regions of the promoter is observed. Mutation o f either the HSS-SRE-1 or the Inv-SRE-3 sequence diminished the activa tion by SREBP-1a and by sterol depletion but did not affect the activa tion by SREBP-2. Simultaneous mutations of both of these sequences alm ost completely abolished activation of the promoter by SREBP-1a or by sterol depletion, but activation by SREBP-2 was retained at 70%. Mutat ion of the Inv-Y-Box sequence element decreased the activity of the pr omoter by 50% or more, and if mutated together with both SREs, the act ivation was almost completely abolished. Mutation of any single GC box of the two located at -40 to -57 did not affect activity, whereas sim ultaneous mutation of the two decreased activation by SREBP-2 by 60%, by lipid depletion by 20%, and had no effect on the activation by SREB P-1a. A Y box motif at -159 to -166 and an SRE-like sequence element ( SRE-1(8/10)) at position -101 to -108 are also involved in the sterol regulation. These results indicate that the complex sterol-mediated tr anscriptional of multiple copies of diverse cis elements in the HSS pr omoter. The differential activation of the HSS promoter may point to s pecific role of the SREBPs in cholesterogenesis.