A Deficiency of Microsomal Triglyceride Transfer Protein Reduces Apolipoprotein B Secretion

Abstract

Microsomal triglyceride transfer protein (MTP) transfers lipids to apolipoprotein B (apoB) within the endoplasmic reticulum, a process that involves direct interactions between apoB and the large subunit of MTP. Recent studies with heterozygous MTP knockout mice have suggested that half-normal levels of MTP in the liver reduce apoB secretion. We hypothesized that reduced apoB secretion in the setting of half-normal MTP levels might be caused by a reduced MTP:apoB ratio in the endoplasmic reticulum, which would reduce the number of apoB-MTP interactions. If this hypothesis were true, half-normal levels of MTP might have little impact on lipoprotein secretion in the setting of halfnormal levels of apoB synthesis (since the ratio of MTP to apoB would not be abnormally low) and might cause an exaggerated reduction in lipoprotein secretion in the setting of apoB overexpression (since the MTP:apoB ratio would be even lower). To test this hypothesis, we examined the effects of heterozygous MTP deficiency on apoB metabolism in the setting of normal levels of apoB synthesis, half-normal levels of apoB synthesis (heterozygous Apob deficiency), and increased levels of apoB synthesis (transgenic overexpression of human apoB). Contrary to our expectations, half-normal levels of MTP reduced the plasma apoB100 levels to the same extent (ϳ25-35%) at each level of apoB synthesis. In addition, apoB secretion from primary hepatocytes was reduced to a comparable extent at each level of apoB synthesis. Thus, these results indicate that the concentration of MTP within the endoplasmic reticulum rather than the MTP:apoB ratio is the critical determinant of lipoprotein secretion. Finally, we found that heterozygosity for an apoB knockout mutation lowered plasma apoB100 levels more than heterozygosity for an MTP knockout allele. Consistent with that result, hepatic triglyceride accumulation was greater in heterozygous apoB knockout mice than in heterozygous MTP knockout mice. Microsomal triglyceride transfer protein (MTP)1 is expressed at high levels in the absorptive enterocytes of the intestine and hepatocytes, at moderate levels in the visceral endoderm of the yolk sac during embryonic development, and at relatively low levels in kidney and cardiac muscle (1-4). In each of these tissues, MTP plays a key role in the assembly and secretion of apoB-containing lipoproteins (1, 3, 5). MTP is located within the lumen of the endoplasmic reticulum (ER) and is thought to transfer lipids to apoB as that molecule is translated, allowing apoB to assume a proper conformation for forming a spherical lipoprotein with a neutral lipid core (6). In the absence of MTP, triglyceride-rich, apoB-containing lipoproteins cannot be assembled or secreted (3, 6, 7). The mechanism of MTP action has attracted considerable scrutiny. Of note, several lines of evidence have suggested that the assembly of lipoproteins involves a direct interaction between MTP and apoB. First, Wu et al. (8) demonstrated that apoB can be immunoprecipitated from the microsomes of cult...

ApoB100 is required for increased VLDL-triglyceride secretion by microsomal triglyceride transfer protein in ob/ob mice

“…This apoB-isoform dependence upon Mttp was dramatically illustrated in studies using isolated hepatocytes from mice with liver-specific deletion of Mttp, where the secretion of apoB100 was virtually eliminated while the effects on apoB48 secretion were modest (12). Furthermore, similar studies in hetero-zygous Mttp knockout mice demonstrated that apoB100 secretion was also selectively reduced, suggesting that the secretion of apoB100 from mouse liver may be limited by as little as a 50% reduction in Mttp availability (7,13).…”

Comparison of the pharmacological profiles of murine antisense oligonucleotides targeting apolipoprotein B and microsomal triglyceride transfer protein

“…In DIO mice ( Table 1 ), the reductions in TPC were due to reductions in nonHDL-C and HDL-C, a plasma lipid profi le that is similar to that observed in ApoB +/ Ϫ or MTP +/ Ϫ mice (20,21). In addition, neither apoB nor MTP ASO had any effect on plasma TG, which was not surprising given the relatively low plasma TG levels of this model.…”

Hepatic Expression of Microsomal Triglyceride Transfer Protein and In Vivo Secretion of Triglyceride-Rich Lipoproteins Are Increased in Obese Diabetic Mice

“…Studies of rabbits and hamsters treated with MTP inhibitor drugs have shown that an ϳ50% reduction in MTP activity effectively lowered plasma lipoprotein levels and had only minor (but definitive) effects on triglyceride deposition in liver cells (7,9). Even though simple mild hepatic steatosis often has a benign course in nondiabetic individuals, ϳ20% of individuals with type 2 diabetes develop nonalcoholic steatohepatitis that ultimately results in cirrhosis in 20 -30% of the cases (42).…”