Lower Squalene Epoxidase and Higher Scavenger Receptor Class B Type 1 Protein Levels Are Involved in Reduced Serum Cholesterol Levels in Stroke-Prone Spontaneously Hypertensive Rats

Abstract

A lower serum cholesterol level was recently shown to be one of the causes of stroke in an epidemiological study. Spontaneously hypertensive rats stroke-prone (SHRSP) have lower serum cholesterol levels than normotensive Wistar-Kyoto rats (WKY). To elucidate the mechanisms responsible for the lower serum cholesterol levels in SHRSP, we determined whether the amounts of cholesterol biosynthetic enzymes or the receptor and transporter involved in cholesterol uptake and efflux in the liver were altered in SHRSP. When the mRNA levels of seven cholesterol biosynthetic enzymes were measured using real-time polymerase chain reaction (PCR), farnesyl pyrophosphate synthase and squalene epoxidase (SQE) levels in the liver of SHRSP were significantly lower than those in WKY. SQE protein levels were significantly reduced in tissues other than the brain of SHRSP. No significant differences were observed in low-density lipoprotein (LDL) receptor (uptake of serum LDL-cholesterol) or ATP-binding cassette transporter A1 (efflux of cholesterol from the liver/formation of high-density lipoprotein (HDL)) protein levels in the liver and testis between SHRSP and WKY, whereas scavenger receptor class B type 1 (SRB1: uptake of serum HDL-cholesterol) protein levels were higher in the livers of SHRSP. These results indicated that the lower protein levels of SQE and higher protein levels of SRB1 in the liver were involved in the reduced serum cholesterol levels in SHRSP.Key words squalene epoxidase; scavenger receptor class B type 1; stroke; cholesterol; liver An epidemiological study identified lower serum cholesterol levels as one of the causes of cerebral hemorrhage because the risk of death from cerebral hemorrhage was found to be threefold higher in men with lower serum cholesterol levels than in those with higher cholesterol levels. 1,2)Cholesterol is a major constituent of cellular membranes; therefore, reductions in the cholesterol content in the cell have been shown to reduce the proliferation of cells, enhance cell membrane fluidity, and lead to fragile plasma membranes. 3-5)Therefore, lower serum cholesterol is regarded as an important cause of cerebral hemorrhage.Spontaneously hypertensive rats (stroke-prone) (SHRSP) are rats with severe hypertension and stroke.6,7) Serum cholesterol levels in these rats are lower than those in normotensive Wistar Kyoto rats (WKY).8) Shiota et al. reported that the pathogenesis of stroke in SHRSP was inhibited by the feeding of high fat and cholesterol diets. 9) These findings showed that reductions in the content of cholesterol in vascular endothelial cells (VECs) in the SHRSP brain due to lower serum cholesterol levels may lead to cerebral hemorrhage following a disruption in the blood-brain barrier (BBB). Disruptions in the BBB have previously been reported in the hypothalamus and hippocampus of SHRSP.10,11) Furthermore, damage to endothelial cells has been implicated in disruptions in the BBB. Therefore, lower serum cholesterol levels in SHRSP may play a role in the development of cerebral...

Stroke‐prone spontaneously hypertensive rats have reduced hydroxysteroid 17‐β dehydrogenase 7 levels for low cholesterol biosynthesis

“…We had previously reported that low levels of total cholesterol, including high-density lipoprotein (HDL)-cholesterol and low-density lipoprotein (LDL)-cholesterol, were caused by decreased levels of SQE and MPD (cholesterol biosynthetic enzymes) and increased levels of SR-B1 (scavenger receptor class B type 1, which participates in the uptake of HDL-cholesterol from the serum to the liver) in the liver of SHRSP. 17 The results of this study suggest that, apart from MPD and SQE, HSD17B7 is also involved in the decrease in cholesterol synthesis and the release of very low-density lipoprotein (VLDL)-cholesterol in the liver of SHRSP. It must also be considered that the increase in the levels of SR-B1, which is involved in the uptake of serum HDLcholesterol, may supplement the low cholesterol content caused by the low levels of the three enzymes (SQE, MPD, and HSD17B7) in the liver of SHRSP.…”

“…Although relative mRNA levels of PMVK were significantly lower in the liver of SHRSP than in WKY, its activity was similar in the two types of rats . We have also previously shown that relative mRNA levels of IPI, SQS, LSS, sterol‐C5‐desaturase (SC5D), and 7‐dehydrocholesterol reductase (DHCR7) in the liver of SHRSP were the same as those in WKY . Further, the relative mRNA and protein levels of MPD and SQE were lower in the liver of SHRSP than in WKY .…”

“…15,16 We have also previously shown that relative mRNA levels of IPI, SQS, LSS, sterol-C5-desaturase (SC5D), and 7-dehydrocholesterol reductase (DHCR7) in the liver of SHRSP were the same as those in WKY. 17 Further, the relative mRNA and protein levels of MPD and SQE were lower in the liver of SHRSP than in WKY. [18][19][20][21] Anderson et al 22 reported a correlation coefficiency of 0.48 between the abundance of mRNA and protein as determined by transcriptome and proteome analysis, suggesting that post-transcriptional regulation of gene expression is a frequent phenomenon in higher organisms.…”

Brain Regions with Reduced Amounts of Mevalonate Pyrophosphate Decarboxylase Correspond to Sites of Strokes in Stroke-Prone Spontaneously Hypertensive Rats

“…The stability of cell membranes is ascribed to free cholesterol. The decline in cholesterol content in the SHRSP brain may be due to the following factors: 1) Decrease in cholesterol biosynthesis by low levels of MPD in the brain, 2) Reduction in uptake due to a lower serum cholesterol caused by decreased expression of MPD, SQE, and HSD17b7 in the liver, 8,10,11) and 3) Effect of cholesterol esterification due to increased acyl-CoA cholesterol acyltransferase (ACAT) in the brain. We previously reported that MPD activity in testicles, brains, and livers was proportional to its protein level.…”

“…6,7) Sawamura et al reported that in the livers of SHRSP, the activities of 3-hydroxy-3-methylglutaryl (HMG) CoA reductase, meva-lonate kinase, and phosphomevalonate kinase, which play roles in the cholesterol biosynthetic pathway, were not significantly different from those in WKY. 6) We previously reported that in the livers of SHRSP, reductions in the activity and expression (protein and mRNA levels) of mevalonate pyrophosphate decarboxylase (MPD), 6,8,9) and hydroxysteroid 17-β dehydrogenase 7 (HSD17B7), 10) in expression of squalene epoxidase (SQE), 11) which is an enzyme involved in cholesterol biosynthesis, may be attributed to low serum cholesterol levels. Furthermore, when the tissue distribution of MPD was examined in WKY and SHRSP, we found that MPD levels were significantly lower in the brains of SHRSP.…”