The novel function of HINFP as a co-activator in sterol-regulated transcription of PCSK9 in HepG2 cells

PCSK9 (proprotein convertase subtilisin/kexin type 9) plays an important role in control of plasma LDL (low-density lipoprotein) cholesterol metabolism by modulating the degradation of hepatic LDL receptor. Previous studies demonstrated that PCSK9 is a target gene of the SREBP2 [SRE (sterol-regulatory element)-binding protein 2] that activates PCSK9 gene transcription through an SRE motif of the promoter. In addition to SREBP2, HNF1 alpha (hepatic nuclear factor 1 alpha) positively regulates PCSK9 gene transcription in hepatic cells through a binding site located 28 bp upstream from SRE. In the present study, we have identified a novel HINFP (histone nuclear factor P) recognition motif residing between the HNF1 motif and SRE that is essential for basal and sterol-regulated transcriptions of the PCSK9 promoter. Mutation of this motif lowers the basal promoter activity and abolishes the sterol-mediated repression as well as the SREBP2-induced activation of the PCSK9 promoter. We show further that the activity of SREBP2 in stimulating PCSK9 promoter activity is greatly enhanced by HINFP. Additional experiments suggest that HINFP and its cofactor NPAT (nuclear protein of the ataxia telangectasia mutated locus) form a functional complex, and NPAT may subsequently recruit the HAT (historic acetyltransferase) cofactor TRRAP (transformation/transactivation domain-associated protein) to facilitate the histone H4 acetylation of the PCSK9 promoter. Knockdown of HINFP, NPAT or TRRAP each markedly reduces the amount of acetylated histone H4 on the PCSK9 promoter region and lowers PCSK9 protein levels. Importantly, by utilizing co-immunoprecipitation assays, we have demonstrated a direct interaction between SREBP2 and HINFP and its cofactors NPAT/TRRAP. Taken together, these new findings identify HINFP as a co-activator in SREBP-mediated transactivation of PCSK9 gene expression.