Transcription of the human hepatic lipase gene is modulated by multiple negative elements in HepG2 cells

Abstract

The expression of the hepatic lipase ( HL) gene is highly tissue specific. In order to identify cis-acting elements which regulate the expression of this gene in the liver, multiple deletion mutants of the 5′-flanking region of the HL gene fused to the human growth hormone gene were transfected in HepG2 cells, which normally produce HL. Transient expression assays indicated the presence of negative (at nucleotides (nt) −1576 −1342 and −623 −407 ) and positive (at nt −1862 −1576 and −50 −9 ) regulatory elements. Transfection of HeLa cells, which do not produce HL, with the same deletion constructs resulted in a similar pattern of promoter activities. However, additional negative (nt −138 −50 ) and positive (nt −407 −138 ) elements were found. DNase I footprint analysis of the proximal and distal HLpromoter sequences with HepG2 and HeLa cell nuclear extracts identified seven protected regions: A, nt −1540 −1527 ; B, −1505 −1473 ; C, −1467 −1460 ; D, −592 −577 ; E, −565 −545 ; F, −234 −220 ; and G, −70 −48 . Sites A, B, C, D and E were located within regions containing negative regulatory elements. In order to determine which nuclear factor interacts with the negative elements, sites B, D and E were mutated and the effects of mutation on competition in a gel retardation assay and on promoter activity were studied. When the binding motif for API in sites B, D and E was mutated, the specific DNA-protein complexes were not competed with the mutant oligonucleotides and promoter activity increased twofold. The magnitude of the increase is less than expected from the deletion analysis, and simultaneous mutations did not cause further increase in promoter activity, which suggests that other sites are involved in this negative modulation. These results suggest that the transcription of the HLgene in HepG2 cells is negatively modulated by multiple cis-acting negative elements and AP1-like nuclear factor may play some role in this modulation