No abstract

triglyceride (TG) metabolism ( 1 ). When it is overexpressed in transgenic mice, apoA-V reduces plasma TG levels by 65%, whereas inactivation of the apoA-V gene increases plasma TG by 4-fold ( 2 ). The preponderance of current literature suggests that apoA-V affects plasma TG turnover by stimulating LPL-mediated lipolysis of TG-rich lipoproteins, either directly or indirectly ( 3-7 ). ApoA-V has also been found to serve as a ligand for LDL receptor family members and other potential lipoprotein receptors and may thus contribute to the clearance of TG-rich lipoproteins and their remnants ( 8-11 ). However, recent studies have revealed that the effects of apoA-V on plasma TG concentration are complex and variable. In humans, several loss-of-function and null apoA-V alleles are associated with both reduced plasma apoA-V levels and elevated plasma TG ( 12, 13 ), yet other studies have found both positive and negative associations between plasma apoA-V and TG concentrations ( 7,14,15 ). Moreover, recent studies in mice have found a positive correlation between plasma apoA-V and TG concentrations ( 16,17 ).Despite its apparent impact on intravascular TG-rich lipoprotein lipolysis and clearance, a peculiar characteristic of apoA-V is that its plasma concentration is in the range of 100-200 µg/l, which is ‫ف‬ 10,000-fold lower than apoA-I and ‫ف‬ 1,000-fold lower than apoA-IV and corresponds to ‫ف‬ 1 molecule of apoA-V for every 1,000 VLDL particles ( 18,19 ). This presents a conundrum as to how an apolipoprotein circulating at such low levels could exert such a potent effect on plasma TG metabolism and concentration. Although it is certainly possible that apoA-V could function in plasma at extreme substoichiometric concentrations relative to that of TG-rich lipoproteins, it has also been suggested that apoA-V might function within the hepatocyte to directly modulate Apolipoprotein A-V (apoA-V), a member of the exchangeable apolipoprotein family synthesized predominantly in the liver, is a potent regulator of intravascular and This work was supported by National Institutes of Health Grants

Taxanes are first line drugs for treating prostate cancer recurrence after the failure of anti-androgen therapy. There is a need to make taxanes more effective since they only provide palliative benefit. Exploiting endoplasmic reticulum (ER) stress death signaling to enhance drug efficacy has not been delineated. Human PC-3 cells were used as a model of hormone refractory prostate cancer. Thapsigargin and methylseleninic acid (MSA) were examined as sensitizers. Thapsigargin is a classic ER stress inducer. The activity of MSA in inducing ER stress has recently been studied by our group. The efficacy of single drug and the various combinations was evaluated by measuring apoptosis with a cell death ELISA kit. Thapsigargin increased the cell killing potency of paclitaxel or docetaxel by 10-to 12-fold, while MSA caused a five to eight fold increase. Since thapsigargin is not used clinically because of its toxicity, the follow-up experiments were done with MSA. To test the hypothesis that a threshold level of ER stress is crucial to chemotherapeutic sensitization, three different approaches designed to dampen the severity of ER stress induced by MSA were examined. Lowering ER stress consistently attenuated the efficacy of MSA/taxane. GADD153 is a pro-apoptotic transcription factor which is upregulated during ER stress. Knocking down GADD153 by siRNA also reduced the cell killing effect of MSA/ taxane. Both the intrinsic and extrinsic apoptotic pathways were involved in the sensitization mechanism. Our study supports the idea that marshalling ER stress apoptotic response is conducive to chemotherapeutic sensitization.

Microsomal triglyceride transfer protein (MTP) is essential for the formation of apolipoprotein B (apoB)-containing lipoproteins. Previous results established that invertebrate forms of MTP (e.g., Drosophila ) are capable of phospholipid (PL) transfer, whereas vertebrate forms of MTP (e.g., human) engage in both PL and triglyceride (TG) transfer. To determine, in vivo, whether the PL and TG transfer activities of MTP play different roles in lipoprotein formation and protection from hepatic steatosis, we created B6D2F1 mice that express human MTP (hMTP) or Drosophila MTP (dMTP) under the control of the apoE 5’ proximal promoter and 3’ hepatic control region in the plasmid pLIV11. Immunoblot analysis confirmed that transgenic expression was limited to the liver and that expression of the transgenes did not impact the abundance of endogenous mouse MTP. To assess the ability of MTP to protect mice from hepatic steatosis, dMTP transgenic mice, hMTP transgenic, and wild type (WT) littermates were placed on a high fat (45% from lard) and cholesterol (0.2%) diet for eight weeks. Mice displayed no significant differences in plasma TG or total plasma cholesterol; however, hMTP mice displayed a trend toward reduced liver TG content, indicating that chronic MTP overexpression may protect the liver from hepatic lipid accumulation. Surprisingly, however, a 2-fold increase in liver TG was observed in two separate founder lines of dMTP transgenic mice. H&E staining confirmed an increase in neutral lipid accumulation in mice expressing the dMTP transgene, relative to both WT and hMTP mice. Quantitative PCR analysis did not reveal changes in expression of genes responsible for hepatic lipogenesis or lipid oxidation. These data complement earlier studies using adenovirus-mediated expression, indicating that acute hMTP and dMTP expression in liver-specific MTP knockout mice can reverse hepatic steatosis caused by MTP deficiency. In contrast, chronic hepatic overexpression of dMTP in a wild type background appears to interfere with endogenous lipoprotein formation and secretion. Understanding the basis for this phenotype may provide insights into how the specific lipid transfer and other activities of MTP contribute to apoB lipoprotein formation.

Apolipoprotein A-IV (apoAIV) is a lipid binding protein expressed primarily in mammalian intestine and also in rodent liver. While in vitro studies have suggested a role of apoAIV in lipoprotein formation and triglyceride (TG) transport, limited evidence supports such a function in vivo. To explore if hepatic apoAIV is uniformly regulated by increased hepatic TG levels, as is the case in the intestine, we studied three distinct mouse models of hepatic steatosis. These included high fat diet (HFD)-fed wild type mice, mice overexpressing a constitutive active form of SREBP-1a (SREBP-1aTg), and mice in which hepatic TG hydrolysis is blunted (mice treated with an anti-sense oligonucleotide (ASO) targeting knockdown of CGI-58/ABHD5). High fat diet-fed mice displayed a 4.5-fold increase in liver TG content and a 10-fold increase in apoAIV mRNA expression and protein mass. SREBP-1aTg mouse liver demonstrated an ∼35-fold increase in hepatic TG, apoAIV mRNA and apoAIV protein. A linear correlation was observed between hepatic TG content and apoAIV mRNA expression in the HFD fed wild type and SREBP-1aTg mice (r2 = 0.6852; p < 0.01). Interestingly, while the TG content of CGI-58/ABHD5 knockdown animals increased by 4-fold, no corresponding increase in apoAIV expression was observed. To determine if the increase in apoAIV expression in the steatotic livers of SREPB-1aTg mice plays a role in the secretion of TG-rich apoB-containing lipoproteins, we crossed these mice with apoAIV knock out mice (A4-KO). The SREBP1a-Tg/A4-KO mice did not demonstrate a change in liver to body weight ratio or a change in hepatic TG content. Despite the large induction of apoAIV in the livers of SREBP1a-Tg mice, no difference was observed in hepatic TG production rates or VLDL particle size when apoAIV was deficient (SREBP1a-Tg/A4-KO). These results indicate that hepatic apoAIV expression is upregulated under conditions of enhanced lipogenesis, but is not altered under condition of defective TG hydrolysis. Furthermore, even under conditions of a 35-fold induction, apoAIV does not exert a detectable impact on TG transport or VLDL particle characteristics in mouse liver.