The Study On UPR Induced By Dietary Cholesterol In The Enterocytes Of SD Rats And Its Regulation Of ACAT2

Hypercholesterolemia is a correlation factor resulting in atherosclerosis. The whole-body cholesterol homeostasis is a highly regulated balance of de novo synthesis, dietary cholesterol absorption and biliary clearance and excretion. While the cholesterol biosynthetic and clearance pathways are well defined, the mechanisms controlling the initial uptake of dietary cholesterol are less documented. As reported by some researchers, most of them enters the ER and is esterified by an ER enzyme ACAT2,then incorporated with apo B48 to produce de novo chylomicron which goes into the lymph and then to the circulation. In addition, the absorption of cholesterol in acat2 knockout mice are significantly reduced, which suggests ACAT2 is indispensable to cholesterol absorption. UPR (unfolded protein response) has played a significant role in stringent mechanism of cells. When cell suffers from various stress, such as glucose starvation, glycosylation inhibitor Tm(tunicamycin),or Ca2+-ATPase inhibitor Tg(thapsigargin), the unfolded protein will accumulate in the ER and activate UPR. In mammalian cells, there are three branches in UPR, initiated by membrane sensor IRE1, ATF6, PERK respectively, characterized by the upregulation of chaperons such as BIP and the activation of various transcription factors such as XBP1(S), ATF6, ATF4. Worth of notice, while IRE1 splices xbp1 mRNA, ATF6 upregulates the expression of xbp1 mRNA. The final effect of UPR is 1) gene expression 2) translational attenuation 3) ER-associated protein degradation(the ERAD system). It is reported that cholesterol overload in macrophages may lead to UPR, then will it be the same condition in enterocytes of high-cholesterol-fed SD rat's small intestinal mucosa? Besides, will UPR play a role in the absorption and metabolism of cholesterol, and which role?On the basis of information drew above, we designed following experiment: first, we fed SD rats on either 5%-cholesterol diet or non-cholesterol diet for 12h, 24h, 36h respectively. Then the small intestinal mucosa was scrapped and RT-PCR was used to detect the RNA level of acat2, bip and xbp1 in both groups. Our result was: compared with the control group, the level of xbp1, bip and acat2 mRNA in 5%-cholesterol group significantly increased, and the extent was dependent on the feeding time.Conclusion: in the enterocytes of cholesterol-fed SD rats'small intestinal mucosa, the expression of xbp1, bip and acat2 mRNA is upregulated, which strongly suggests that UPR is activated, and the ATF6 branch may play an important role in regulating the expression of acat2 via ERSE to control cholesterol absorption in enterocytes and metabolism in hepatocytes.