Impaired bile acid synthesis due to CYP7A1 and CYP7B1 suppression in malnutrition

点击翻译按钮获取中文摘要

Malnutrition contributes to half of all global child deaths. Liver function abnormalities, including decreased bile acid synthesis, are common in severe malnutrition. In the intestine, bile acid deficiency leads to nutrient malabsorption and impaired growth. In the liver, bile acids signal through nuclear receptors including farnesoid- X-receptor (FXR) to regulate a wide range of processes from energy homeostasis to coagulation. We discovered that decreased bile acid synthesis in malnutrition causes decreased FXR activation and decreased expression of FXR target genes, including coagulation factors. The resulting malnutrition-induced coagulopathy can be fatal. It is not known why bile acid synthesis is impaired in malnutrition. My ongoing K08 project uses a unique mouse model of early-life malnutrition, revealing loss of expression of peroxisome proliferator-activated receptor- α (PPARα) and subsequent loss of peroxisomes, which are required for an essential β-oxidation step to generate mature bile acids. Our published data and new preliminary data indicate that suppression of two key cytochrome P450 (CYP) enzymes upstream of peroxisomal β-oxidation also contributes to decreased bile acid synthesis in malnutrition, independently of PPARα. Hepatocytes synthesize bile acids from cholesterol in two parallel pathways. Our data suggest that the classic pathway of bile acid synthesis is suppressed through decreased activity of CYP7A1, the rate-determining enzyme. We predict that decreased CYP7A1 activity occurs because malnourished hepatocytes are depleted of heme, the essential prosthetic group for CYP7A1. Similarly, we have evidence that the alternative pathway of bile acid synthesis is suppressed through decreased expression of CYP7B1, the key enzyme in this pathway. We predict that decreased expression of CYP7B1 occurs because malnutrition upregulates its potent transcriptional repressor, the sterol regulatory element-binding protein-1 (SREBP-1). Thus, we hypothesize that, in addition to PPARα-dependent peroxisome loss, malnutrition impairs bile acid synthesis also by suppressing CYP7A1 and CYP7B1 to impair both the classic and the alternative pathways of synthesis. The Specific Aims are 1) to characterize altered heme metabolism in malnutrition and its role in decreased CYP7A1 activity by quantifying flux of labeled cholesterol, measuring expression levels of heme synthesizing enzymes, and treating hepatocytes and mice with heme; and 2) to determine the role of SREBP-1 in the transcriptional repression of CYP7B1 and its relative contribution to decreased bile acid synthesis in malnutrition by defining the corepressors recruited to the CYP7B1 promoter, and by inhibiting SREBP-1 with siRNA and pharmacologic antagonists in vitro and in vivo. Expected outcomes include a deeper understanding of how nutritional status influences bile acid homeostasis. This approach is innovative because it will explore CYP suppression as a novel link between malnutrition and impaired bile acid synthesis. The proposed research is significant because of the potential to develop new strategies to restore liver synthetic function in severe malnutrition and in other digestive diseases characterized by acquired bile acid deficiency.