Apo B exists as B48 and B100 produced through a novel posttranscriptive mechanism of apo B mRNA editing producing protein products of differing size with differing metabolisms. The goal of our research is to understand apo B triglyceride-rich lipoprotein (TRL) assembly contrasting B100 and B48 synthetic pathways. McArdle RH-7777 cells (McA) have provided many useful insights into inherent differences in B100 and B48 in TRL biogenesis. Recent studies indicate that B100-TRL assembly occurs by lipidation of conformationally "acceptable" B100. B48 undergoes initial lipidation similar to B100 followed by neutral lipidation of the precursor high density lipoprotein particle (B48-HDL) to form B48-TRL. More than 70% of the triglyceride (TG) fatty acids (FA) arise from cytosolic, stored TG through a process involving lipolysis/reesterification. Lipolyzed FAs, possibly through acylation reactions with PL intermediates, traverse the ER membrane by unknown means prior to fusion with B48-HDL. Unlike B100, that is synthesized and rapidly secreted by rat hepatocytes (RH), there is a kinetically distinct pool of B48 that has a long cellular retention time. We hypothesize that this pool may provide a ready supply of B48-HDL as a precursor for the rapid assembly of TRL via a later assembly step. In cells that make predominantly B48, such as intestine, the cellular pool of B48 allows TRL secretion to occur without a requirement for de novo apo B synthesis. We recently identified a novel protein factor, an apo B secretion enhancer (BSE), whose expression correlates with apo B mRNA abundance in McA cells transfected with BSE. Apo B mRNA abundance, however, does not correspond with a proportional increase in apo B secretion. Studies explore the role of BSE in the observed changes in apo B mRNA stability/transcription. Apo B degradation studies address the non-proportional secretory rate. A phospholipid (PL)-dependent pathway for the stimulation of apo B secretion by BSE is suggested as BSE is homologous with betaine homocysteine methyltransferase (BHMT), a methylation enzyme. An hypothesis explored is that, apart from its ability to stabilize apo B mRNA, an ER form of BSE may provide newly synthesized phosphatidylcholine (PC) via phosphatidylethanolamine (PE) methylation of phosphatidylserine (PS) and increased supply of PL may be a mechanism for the enhancing secretion of apo B.

Importance of the research is related to the role of apo B in lipid transport and as a factor associated with risk of arterial disease. Additional research focuses on the role of apo B as a risk factor in developing heart attacks, strokes, and in recurrent heart attacks in human populations.

SPARKS JD and SPARKS CE: Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion. Biochim. Biophys. Acta 1215:9-32 (1994).(REVIEW). (PubMed)

SPARKS JD, COLLINS HL, SABIO I, SOWDEN MP, SMITH HC, CIANCI J, and SPARKS CE: Effects of fatty acids on apolipoprotein B secretion by McArdle RH-7777 rat hepatoma cells. Biochim.Biophys. Acta. 1347: 51-61 (1997). (PubMed)

PHUNG TL (fellow), RONCONE A, DE MESY JENSEN KL, SPARKS CE and SPARKS JD: Phosphoinositide 3-kinase activity is necessary for insulin-dependent inhibition of apolipoprotein B secretion by rat hepatocytes and localizes to the endoplasmic reticulum. J. Biol. Chem. 272: 30693-30702 (1997). (PubMed)

SPARKS JD, PHUNG TL, BOLOGNINO M, CIANCI J, KHURANA R, PETERSON RG, SOWDEN MP, CORSETTI JC and SPARKS CE. Lipoprotein alterations in 10- and 20- week-old Zucker diabetic fatty rats: hyperinsulinemic versus insulinopenic hyperglycemia. Metabolism 47: 1315-1324(1998). (PubMed)

SOWDEN MP, COLLINS HL, SMITH HC, GARROW TA, SPARKS JD and SPARKS CE. Apolipoprotein B mRNA and lipoprotein secretion are increased in McArdle RH-7777 cells by expression of betaine-homocysteine methyltransferase. Biochem. J. 341: 639-645 (1999). (PubMed)

CORSETTI JP, SPARKS JD, PETERSON RG, SMITH RL and SPARKS CE. Effect of dietary fat on the development of non-insulin dependent Diabetes Mellitus in Zucker Diabetic fatty male and female rats. Atherosclerosis 148:231-241 (2000). (PubMed)

SPARKS JD, SHAW WN, BOLOGNINO M, CORSETTI JP, PESEK JF and SPARKS CE. Insulin-treated Zucker diabetic fatty rats retain the hypertriglyceridemia associated with obesity. Metabolism 49:1424-1430 (2000). (PubMed)

CHIRIEAC DV, CHIRIEAC LR, CORSETTI JP, CIANCI J, SPARKS CE and SPARKS JD. Glucose-stimulated insulin secretion suppresses hepatic triglyceride-rich lipoprotein and apolipoprotein B production in vivo. Am. J. Physiol. 279:E1003-E1011 (2000). (PubMed)

ZAREBA W, PANCIO G, MOSS AJ, KALARIA VG, MARDER VJ, WEISS HJ, MILLER WATELET LF, SPARKS CE. Increased level of von Willebrand factor is significantly and independently associated with diabetes in postinfarction patients. Thromb Haemost 86: 791-9 (2001). (PubMed)

CHIRIEAC DV, CIANCI J, COLLINS HC, SPARKS JD, and SPARKS CE: Insulin suppression of VLDL apo B secretion is not mediated by the LDL receptor. Biochem. Biophys. Res. Commun. 297:134-137 (2002). (PubMed)

CORSETTI JP, ZAREBA W, MOSS AJ, RIDKER PM, MARDER VJ, RAINWATER DL, and SPARKS CE: Metabolic syndrome best defines the multivariate distribution of blood variables in postinfarction patients. Atherosclerosis 171(2):351-8 (2003). (PubMed)

CHIRIEAC DV, COLLINS HL, CIANCI J, SPARKS JD, and SPARKS CE. Altered triglyceride-rich lipoprotein production in Zucker diabetic fatty rats. Am J Physiol Endocrinol Metab. 287(1):E42-9 (2004). (PubMed)

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