B48 is preferentially translated over B100 in cells with increased endogenous apo B mRNA

Genetic polymorphism of APOB is associated with diabetes mellitus in sickle cell disease

Environmental variations have strong influences in the etiology of type 2 diabetes mellitus. In this study, we investigated the genetic basis of diabetes in patients with sickle cell disease (SCD), a Mendelian disorder accompanied by distinct physiological conditions of hypoxia and hyperactive erythropoiesis. Compared to the general African American population, the prevalence of diabetes as assessed in two SCD cohorts of 856 adults was low, but it markedly increased with older age and overweight. Meta-analyses of over 5 million single-nucleotide polymorphisms (SNPs) in the two SCD cohorts identified a SNP, rs59014890, the C allele of which associated with diabetes risk at P = 3.2 × 10(-8) and, surprisingly, associated with decreased APOB expression in peripheral blood mononuclear cells (PBMCs). The risk allele of the APOB polymorphism was associated with overweight in 181 SCD adolescents, with diabetes risk in 592 overweight, non-SCD African Americans ≥ 45 years of age, and with elevated plasma lipid concentrations in general populations. In addition, lower expression level of APOB in PBMCs was associated with higher values for percent hemoglobin A1C and serum total cholesterol and triglyceride concentrations in patients with Chuvash polycythemia, a congenital disease with elevated hypoxic responses and increased erythropoiesis at normoxia. Our study reveals a novel, environment-specific genetic polymorphism that may affect key metabolic pathways contributing to diabetes in SCD.

Mechanisms of protection by the betaine-homocysteine methyltransferase/betaine system in HepG2 cells and primary mouse hepatocytes

Betaine-homocysteine methyltransferase (BHMT) regulates homocysteine levels in the liver. We previously reported that the alteration of BHMT is associated with alcoholic liver steatosis and injury. In this study, we tested whether BHMT protects hepatocytes from homocysteine-induced injury and lipid accumulation. Both BHMT transfectants of HepG2 cells and primary mouse hepatocytes with suppressed BHMT were generated. Comparisons were made between the cell models with respect to their response to homocysteine treatments. Homocysteine metabolism was impaired in HepG2 cells, and the expression of BHMT in HepG2 cells ameliorated the impairment and stabilized the levels of intracellular homocysteine after the addition of exogenous homocysteine. BHMT expression inhibited homocysteine-induced glucose-regulated protein 78 (GRP78) and C/EBP-homologous protein (CHOP) and homocysteine-induced cell death. A betaine treatment protected primary mouse hepatocytes from a homocysteine-induced increase in GRP78 and cell death but not a tunicamycin-induced increase. Homocysteine induced greater CHOP expression (2.7-fold) in BHMT small interfering RNA (siRNA)-transfected cells than in a control (1.9-fold). Homocysteine-induced cell death was increased by 40% in the siRNA-treated cells in comparison with the control. Apolipoprotein B (apoB) expression was higher and triglycerides and cholesterol were lower in HepG2 expressing BHMT. In primary mouse hepatocytes, homocysteine induced the accumulation of triglycerides and cholesterol, which was reduced in the presence of betaine. Betaine partially reduced homocysteine-induced sterol regulatory element binding protein 1 expression in HepG2 cells and increased S-adenosylmethionine in primary mouse hepatocytes.

CONCLUSION

The BHMT/betaine system directly protects hepatocytes from homocysteine-induced injury but not tunicamycin-induced injury, including an endoplasmic reticulum stress response, lipid accumulation, and cell death. This system also exhibits a more generalized effect on liver lipids by inducing ApoB expression and increasing S-adenosylmethionine/S-adenosylhomocysteine.

Hepatic very-low-density lipoprotein and apolipoprotein B production are increased following in vivo induction of betaine-homocysteine S-methyltransferase

We have previously reported a positive correlation between the expression of BHMT (betaine-homocysteine S-methyltransferase) and ApoB (apolipoprotein B) in rat hepatoma McA (McArdle RH-7777) cells [Sowden, Collins, Smith, Garrow, Sparks and Sparks (1999) Biochem. J. 341, 639-645]. To examine whether a similar relationship occurs in vivo, hepatic BHMT expression was induced by feeding rats a Met (L-methionine)-restricted betaine-containing diet, and parameters of ApoB metabolism were evaluated. There were no generalized metabolic abnormalities associated with Met restriction for 7 days, as evidenced by control levels of serum glucose, ketones, alanine aminotransferase and L-homocysteine levels. Betaine plus the Met restriction resulted in lower serum insulin and non-esterified fatty acid levels. Betaine plus Met restriction induced hepatic BHMT 4-fold and ApoB mRNA 3-fold compared with Met restriction alone. No changes in percentage of edited ApoB mRNA were observed on the test diets. An increase in liver ApoB mRNA correlated with an 82% and 46% increase in ApoB and triacylglycerol production respectively using in vivo Triton WR 1339. Increased secretion of VLDL (very-low-density lipoprotein) with Met restriction plus betaine was associated with a 45% reduction in liver triacylglycerol compared with control. Nuclear run-off assays established that transcription of both bhmt and apob genes was also increased in Met-restricted plus betaine diets. No change in ApoB mRNA stability was detected in BHMT-transfected McA cells. Hepatic ApoB and BHMT mRNA levels were also increased by 1.8- and 3-fold respectively by betaine supplementation of Met-replete diets. Since dietary betaine increased ApoB mRNA, VLDL ApoB and triacylglycerol production and decreased hepatic triacylglycerol, results suggest that induction of apob transcription may provide a potential mechanism for mobilizing hepatic triacylglycerol by increasing ApoB available for VLDL assembly and secretion.

Inhibition of the synthesis of apolipoprotein B-containing lipoproteins

Increased serum concentrations of low density lipoproteins represent a major cardiovascular risk factor. Low-density lipoproteins are derived from very low density lipoproteins secreted by the liver. Apolipoprotein (apo)B that constitutes the essential structural protein of these lipoproteins exists in two forms, the full length form apoB-100 and the carboxy-terminal truncated apoB-48. The generation of apoB-48 is due to editing of the apoB mRNA which generates a premature stop translation codon. The editing of apoB mRNA is an important regulatory event because apoB-48-containing lipoproteins cannot be converted into the atherogenic low density lipoproteins. The apoB gene is constitutively expressed in liver and intestine, and the rate of apoB secretion is regulated post-transcriptionally. The translocation of apoB into the endoplasmic reticulum is complicated by the hydrophobicity of the nascent polypeptide. The assembly and secretion of apoB-containing lipoproteins within the endoplasmic reticulum is strictly dependent on the microsomal tricylceride transfer protein which shuttles triglycerides onto the nascent lipoprotein particle. The overall synthesis of apoB lipoproteins is regulated by proteosomal and nonproteosomal degradation and is dependent on triglyceride availability. Noninsulin dependent diabetes mellitus, obesity and the metabolic syndrome are characterized by an increased hepatic synthesis of apoB-containing lipoproteins. Interventions aimed to reduce the hepatic secretion of apoB-containing lipoproteins are therefore of great clinical importance. Lead targets in these pathways are discussed.

Regulation of human apolipoprotein B gene expression at multiple levels

Apolipoprotein B is a large, amphipathic protein that plays a central role in lipoprotein metabolism. Because its overproduction and deficiency leads to metabolic and pathologic disorders, much effort has been paid to investigate the mechanisms of how its homeostasis is achieved. Earlier and recent studies have showed that apoB gene locus might reside in different chromatin domains in the hepatic and intestinal cells, and two sets of very distinct regulatory elements operate to control its transcription. Posttranscriptional modification of apoB mRNA is performed by a multicomponent enzyme complex, several possible pathways regulate the editing efficiency. Understanding of the mechanism responsible for apoB mRNA editing will provide the basis for C-to-U editing in gene therapy. In addition to apoB mRNA abundance and stability, its translation can be also regulated at the steps of elongation. The translocation of apoB into the ER is an important and complicated process that is less understood. Successful transport and correct folding of apoB may lead to its final secretion, otherwise subject to intracellular degradation, which is accomplished by proteasomal and nonproteasomal pathways at multiple levels and may differ among cell types.