Insulin does not regulate the promoter of Cholesteryl Ester Transfer Protein (CETP) in HIRc/pCETP-CAT cells

Cholesteryl ester transfer protein (CETP) is a plasma enzyme involved in ch olesterol metabolism. As a potential target in the treatment of atheroscler osis, a number of studies have focused how this enzyme is regulated. It has been postulated that insulin may regulate CETP gene expression, and these effects may be mediated through CCAAT/enhancer binding protein alpha (C/EBP alpha). The present study examines the effects of insulin on the activity of the CETP promoter in rat fibroblasts expressing the human insulin recept or (HIRc). HIRc cells were stably transfected with a chimeric construct con taining 3.2 kb of the CETP promoter attached to the bacterial chloramphenic ol acyltransferase gene (pCETP-CAT) without significantly affecting the exp ression of the insulin receptor. CAT activity was 8-fold higher in cultured HIRc/pCETP-CAT in the presence of 100 mg/dL LDL cholesterol, than those cu ltured without cholesterol (p < 0.05). However, culturing these cells in th e presence of 100 nM insulin did not result in any change in CAT activity w hen compared to control cells. In HIRc/pCETP-CAT cells transiently transfec ted with a construct that constitutively expressed C/EBP alpha protein, a 3 -fold increase in CAT activity was observed when compared to cells transien tly transfected with non-specific DNA (p < 0.05). However, no observable ef fect on the CETP promoter was observed in the presence of insulin. Thus, in HIRc/pCETP-CAT cells, we were unable to substantiate the hypothesis that i nsulin regulates CETP gene transcription. These results suggest that the ef fects of insulin on CETP expression regulation may be downstream of transcr iption.