Mice Expressing the Human CYP7A1 Gene in the Mouse CYP7A1 Knock-out Background Lack Induction of CYP7A1 Expression by Cholesterol Feeding and Have Increased Hypercholesterolemia When Fed a High Fat Diet

Abstract

Cholesterol 7␣-hydroxylase (CYP7A1) catalyzes the rate-limiting step in the pathway responsible for the formation of the majority of bile acids. Transcription of the gene is regulated by the size of the bile acid pool and dietary and hormonal factors. The farnesoid X receptor and the liver X receptor (LXR) are responsible for regulation by bile acids and cholesterol, respectively. To study the effects of dietary cholesterol and fat upon expression of the human CYP7A1 gene, mice were generated by crossing transgenic mice carrying the human CYP7A1 gene with mice that were homozygous knockouts (CYP7A1 ؊/؊ ). The mice (mCYP7A1 ؊/؊ /hCYP7A1) expressed the human gene at much higher levels than did the transgenics bred in the wild-type background. A diet containing 1% cholic acid reduced the expression of the human gene in mCYP7A1 ؊/؊ /hCYP7A1 mice to undetectable levels. Cholestyramine (5%) increased the level of expression of the human gene and the mouse gene. Thus, farnesoid X receptor-mediated regulation was preserved. A diet containing 2% cholesterol increased expression of the mouse gene in wild-type mice, but it did not affect expression of the human gene in mCYP7A1 ؊/؊ /hCYP7A1 mice. None of the diets altered the serum cholesterol or triglyceride levels in these mice; 1% cholic acid caused a redistribution of cholesterol from the high density lipoprotein to the low density lipoprotein density in the humanized mice but not in wild-type mice. A diet containing 30% saturated fat and 2% cholesterol caused a decrease in CYP7A1 levels in mCYP7A1 ؊/؊ /hCYP7A1 mice. The serum cholesterol levels rose in all mice fed this diet. The increase was greater in the mCYP7A1 ؊/؊ /hCYP7A1 mice. Together, these data suggest that the lack of an LXR element in the region from ؊56 to ؊49 of the human CYP7A1 promoter may account for some of the differences in response to diets between humans and rodents.Cholesterol is a lipid molecule that plays unique and specific roles in cellular membrane function and embryonic development. It also serves as a precursor for the synthesis of steroid hormones and bile acids. Accordingly, a sophisticated system has evolved to ensure its availability to cells. The liver plays a central role in coordinating many of the components of this process. The accumulation of excess cholesterol in blood vessels leads to atherosclerosis and its accumulation in the bile leads to gallstones. Humans, in contrast to rodents, are highly susceptible to these conditions. Hepatic cholesterol homeostasis is controlled by the rate of cholesterol entry to the liver from dietary and endogenous lipoproteins, as well as by the rate of synthesis of new cholesterol relative to the rate of hepatic lipoprotein secretion, the rate of cholesterol degradation to bile acids, and the rate of excretion of the intact molecule into the bile. These various processes are modulated by the levels of the enzymes catalyzing the rate-limiting steps of each process. An important determinant of the level of these proteins is the rate of transcripti...

Cholesterol feeding of mice expressing cholesterol 7α-hydroxylase increases bile acid pool size despite decreased enzyme activity

“…In both studies, wild-type mice responded to dietary cholesterol by up-regulating Cyp7a1 (5,8). In one study, dietary cholesterol did not up-regulate the human transgene (8); whereas, in the other study, the transgene was down-regulated (5). In both studies the authors suggested that humans are more responsive to dietary cholesterol than mice, because they cannot up-regulate Cyp7a1 activity in response to dietary cholesterol.…”

“…However, if this were true, then one would expect dietary cholesterol to increase plasma cholesterol levels in human Cyp7a1 transgenic mice on the mouse Cyp7a1 KO background. Yet, in the one study that made this measurement, 2% dietary cholesterol failed to increase plasma cholesterol in these animals or in wild-type mice (8). This brings into question the relevance of dietary cholesterol regulation of Cyp7a1 in plasma cholesterol responsiveness and suggests the existence of other important regulatory processes.…”

“…To examine this question, two recent studies were performed that fed dietary cholesterol to wild-type and human Cyp7a1 transgenic mice on the mouse Cyp7a1 knockout (KO) background. In both studies, wild-type mice responded to dietary cholesterol by up-regulating Cyp7a1 (5,8). In one study, dietary cholesterol did not up-regulate the human transgene (8); whereas, in the other study, the transgene was down-regulated (5).…”

Regulatory network of lipid-sensing nuclear receptors: roles for CAR, PXR, LXR, and FXR

“…The ability of LXR to induce Cyp7a1 in mice and rats makes these animals extremely resistant to a high cholesterol diet whereas other species, including man, rapidly develop hypercholesterolemia under comparable conditions. Accordingly, high cholesterol-fed mice that transgenically express human CYP7A1 in a mouse Cyp7a1 knockout background lack induction of CYP7A1 and become hypercholesterolemic [66,67].…”

“…The ability of LXR to induce Cyp7a1 in mice and rats makes these animals extremely resistant to a high cholesterol diet whereas other species, including man, rapidly develop hypercholesterolemia under comparable conditions. Accordingly, high cholesterol-fed mice that transgenically express human CYP7A1 in a mouse Cyp7a1 knockout background lack induction of CYP7A1 and become hypercholesterolemic [66,67] Similar to FXR, CAR and PXR promote metabolism and excretion of bile acids.They partly do so by inducing the same target genes including the canalicular bile acid transporter multidrug-resistance-associated protein 2 and 3 (MRP2/ABCC2 and MRP3/ABCC3) [80][81][82]. However, the xenosensors also increase alternate, compensatory pathways for lowering hepatic bile acid levels by inducing their hydroxylation, conjugation and subsequent excretion via blood and urine [50,51,54,55,83].…”