Sin3A-associated protein, 18 kDa, a novel binding partner of TRIB1, regulates MTTP expression.

Mammalian tribbles homolog 1 (TRIB1) is a human locus that has been shown to significantly impact plasma lipid levels across several ethnic groups. In addition, the gene has been associated with the occurrence of nonalcoholic fatty liver disease. In the present study, a yeast-two-hybrid system was used to screen for novel molecular targets of TRIB1 binding. Loci corresponding to clones that were positive for TRIB1 binding subsequently were assessed for roles in lipid metabolism in mice using adenoviral constructs to induce knockdown or overexpression. Sin3A-associated protein, 18 kDa (SAP18) was identified as a novel binding partner of TRIB1. Knockdown of the Sap18 in mouse liver decreased plasma lipid levels and increased hepatic lipid levels; SAP18 overexpression showed the opposite effects. Transcriptome analysis of the mouse liver revealed that Sap18 knockdown decreased and SAP18 overexpression increased microsomal TG transfer protein (MTTP) expression levels. Chromatin immunoprecipitation analysis showed that halo-tagged SAP18, halo-tagged TRIB1, and anti-mSin3A antibody enriched precipitates for regulatory sequences of the MTTP gene. Enforced expression of SAP18 enhanced and SAP18 knockdown conversely attenuated the enrichment of MTTP regulatory sequences seen with anti-mSin3A antibody. These studies indicated that SAP18 expression enhanced the recruitment of mSin3A in coordination with TRIB1 to MTTP regulatory elements and increased MTTP expression.