Vitamin D3 and Bilirubin Synergize to Lower Lipid Accumulation and Cholesterol Levels

Zachary A. Kipp¹, Mei Xu¹, and Terry D. Hinds, Jr.^1,2,3

¹Department of Pharmacology and Nutritional Sciences, University of Kentucky, Lexington, KY.
²Barnstable Brown Diabetes Center, University of Kentucky, Lexington, KY.
³Markey Cancer Center, University of Kentucky, Lexington, KY.

Abstract: According to the National Health and Nutrition Examination Survey, over 42% of all Americans are considered obese. Obesity is often coupled with high plasma cholesterol levels (hypercholesterolemia), increasing the risk for cardiovascular disease (CVD). For unknown reasons, overweight individuals tend to have lower levels of both bilirubin and vitamin D3 (VitD3). Within the body, bilirubin and VitD3 bind to nuclear receptors, PPARa and vitamin D receptor (VDR), respectively. We have previously shown that bilirubin lowers adiposity by activating PPARa. This work aims to determine if there is a synergism between bilirubin and VitD3 that reduces adiposity and cholesterol levels. Sterol regulatory-element binding proteins (SREBPs), are important transcription factors that regulate of cholesterol and lipid synthesis. For our studies, we used 3T3-L1 adipocytes and AML12 mouse hepatocytes treated with bilirubin and VitD3 to determine how they affect the SREBP pathways. We found that bilirubin and VitD3 work synergistically to reduce lipid accumulation compared to either alone. In AML12 hepatocytes, the cotreatment significantly increased the expression of target genes further than either alone, such as Fgf21 for PPARa (p= <0.0001) and Cyp24a1 for VDR (p= <0.0001). To understand the role of bilirubin and VitD3 in adiposity and cholesterol regulation, we further studied their effects on the SREBPs in AML12 cells. There were no significant differences in Srebf1 or Srebf2 mRNA expression when cotreated with bilirubin and VitD3. Within the SREBPs protein maturation pathway, only the expression of Insig1 was significantly increased (p= <0.0001). The expression of SREBPs target genes Ldlr (p= <0.0001), Scd1 (p=0.027), and Hmgcr (p= <0.0001) were significantly increased with the cotreatment. The increase in Ldlr and Hmgcr occurred within the liver cells using statins, the most common cholesterol-lowering medication. These findings indicate that the synergism between bilirubin and vitamin D3 may be beneficial for treating obesity-induced hypercholesterolemia and CVD.

Funding: This work was supported by the National Institutes of Health 1R01DK121797-01A1 (T.D.H.J.).