Genetic heterogeneity in the apolipoprotein C-III promoter and effects of insulin

In the present study, we have investigated the in vivo and in vitro role of two newly identified variants (G-(944)A and A-C-1180) located in the upstr eam promoter region of the apolipoprotein C-III (apoC-III) gene. These vari ants were studied in 30 probands diagnosed with FCHL, their relatives, and spouses. The allele frequencies of both variants were not different between the groups. No significant associations between plasma lipid traits and DN A variants were observed. We further analyzed the effect of the presence of these variants in the upstream enhancing region of the apoC-III gene, as f ive different in vivo occurring haplotypes, on the transcriptional activity of apoC-III in both HepG2 and Caco-2 cells. All five promoter constructs, including the wild type, showed similar enhancing activity of the apoC-III gene. The average transcription efficiency was enhanced 19-fold in HepG2 ce lls and 11-fold in Caco-2 cells. Previous studies have shown in vitro insul in-dependent downregulation of the apoC-III gene transcription in HepG2 cel ls by DNA variation in an insulin response element (IRE) in the apoC-III pr omoter. We observed a 30% insulin-dependent down-regulation of apoC-III exp ression that was, however, independent of the presence of the two IRE varia nts. In contrast, in Caco-2 cells, a more variable insulin-dependent up-reg ulation was found that was also independent of the presence of the ERE vari ants. In conclusion, our data suggested that the apoC-III gene transcriptio n in vitro is regulated by insulin but independent of the presence of the t wo IRE variants at -455 and -482. We were unable to detect associations bet ween these apoC-III variants and plasma lipids and insulin in our FCHL popu lation. This means that in vivo apoC-III transcription not only depends upo n insulin but appears to be mediated by other mechanisms.