Peroxisome proliferator-activated receptor gamma regulates interleukin-6 induced lipoprotein(a) gene expression in human HepG2 cells

Abstract

Abstract Background Lipoprotein(a)[Lp(a)] is an independent risk factor for atherosclerotic cardiovascular disease (ASCVD) and outcomes. The LPA gene promoter contains an interleukin 6 (IL-6) responsive binding site. IL-6 is a pro-inflammatory cytokine and data from patients with inflammatory diseases indicate an impact of the inflammatory milieu on LPA gene regulation and Lp(a) production. In a clinical study, the peroxisome proliferator-activated receptor gamma (PPARγ) agonist pioglitazone was associated with decreased Lp(a) levels. Publicly accessible ChIP-sequencing of human hepatocytes and measures of Lp(a) production by HepG2 cells provide an opportunity to explore PPARγ regulation of hepatic Lp(a) production. Purpose To investigate the role of PPARγ as a regulator of hepatic Lp(a) production. Methods Public ChIP-sequencing of human hepatocytes was analyzed. Commercially available HepG2 cells were incubated with one of the following: (1) scrambled small interfering RNA (siRNA) alone (control); (2) IL-6 (20ng/mL) and scrambled siRNA ; (3) IL-6 and PPARγ siRNA ; and (4) IL-6 and pioglitazone (25uM). LPA transcription was measured using the quantitative polymerase chain reaction. Results Analysis of the ChIP-sequencing data demonstrated that the LPA locus variant rs56393506, which is associated with increased Lp(a) levels and ASCVD risk in the general population, was within the location of the PPARγ binding site in the LPA promoter (Figure 1). IL-6 significantly increased LPA transcription in the HepG2 cells, compared to control (1.65±0.44 vs. 0.66±0.45, p=0.0027). In the presence of siRNA-mediated knock-down of PPARγ (58±17% PPARγ reduction, p<0.0001 vs. scrambled control), the effect of IL-6 on LPA transcription was significantly amplified (6.78±2.4 vs. 1.65±0.44, p=0.0009). IL-6 did not increase LPA transcription in the presence of the PPARγ agonist pioglitazone compared to control (0.16±0.03 vs. 0.66+0.45, p=0.11, Figure 2). Conclusion PPARγ is a negative regulator of IL-6 induced hepatic Lp(a) production and may represent a new therapeutic target in patients with inflammatory conditions.Figure 1Figure 2