Regulation of apoA(1) gene expression with acidosis: requirement for a transcriptional repressor

Serum apolipoprotein A(1) (apoA(1)) concentration is inversely correlated w ith the risk of premature atherosclerosis. Serum apoA(1) concentrations are regulated, in part, at the transcriptional level. ApoA(1) mRNA is synthesi zed primarily in the liver and small intestine, under the direction of a nu mber of signaling molecules and tissue-specific regulatory elements. Previo usly, we demonstrated that extracellular acidosis suppresses apoA(1) mRNA l evels at the level of transcription. Here we demonstrate that intracellular acidosis, in the absence of extracellular pH changes, represses apoA(1) pr omoter activity. Repression occurs through a pH responsive element (pH-RE) located within the apoA(1) gene promoter. Acidosis increases the specific D NA binding activity of a putative repressor protein within the immediate 5 ' -flanking region of the apoA(1) gene. The cis-element that binds the puta tive repressor protein contains a negative thyroid hormone response element (nTRE) located 3 ' and adjacent to the apoA(1) TATA box. Mutation of the n TRE/pH-RE abrogates protein binding and alters the activity of reporter gen es controlled by this element. Repression by acidosis did not require de no vo mRNA and protein synthesis. Inhibition of tyrosine kinase activity and d iacylglycerol-stimulated protein kinase C (PKC) signaling pathways with tyr ophostin A47 and phorbol myristate acetate, respectively, did not affect th e repression of apoA(1) promoter activity with acidosis. These results sugg est that transcriptional repression of the apoA(1) gene by alterations in a mbient pH is associated with enhanced DNA binding activity of a repressor p rotein, through a mechanism which appears to be independent of de novo mRNA and protein synthesis, tyrosine kinase activity, or PKC activation.