الفهرس 
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Abstract
A promising treatment for obesity involves the use of therapeutic agents that increase the level of the glucagon-like peptide (GLP-1) which reduces appetite and food intake. Native GLP-1 is rapidly metabolized by the dipeptidyl peptidase-4 (DPP-4) enzyme and, as such, GLP-1 mimetics or DPP-4 inhibitors represent promising treatment approaches.
Interestingly, obese patient receiving such medications showed improved lipid profiles and cholesterol homeostasis, however the mechanism(s) involved are not known. Members of the ATP-binding cassette (ABC) transporters, including ABCA1 and ABCG1, play essential roles in reverse cholesterol transport and in high density lipoprotein (HDL) formation. These transporters are under the transcriptional regulation of liver X receptor alpha (LXR-α). Therefore, we hypothesize that GLP-1 mimetics and/or DPP-4 inhibitors modulate ABCA1/ABCG1 expression in adipocytes through an LXR-α mediated process and thus affecting cholesterol homeostasis (Aim 1).
Since some statins are almost ineffective in reducing TG, or enhancing HDL-C plasma levels. This provides a rationale to use combined therapy with other drugs to achieve either LDL-C- and TG-lowering or HDL-C-enhancing goals in the management of diabetic dyslipidemia, we made use of our results by adding the DPP-4 inhibitor vildagliptin to pravastatin treatment and investigated the effect of this combination on cholesterol efflux in adipocytes. Such effects of statins can be attributed to their effect on the LXR which regulates the expression of the cholesterol transporters ABCA1 and ABCG1. A decrease in expression of these transporters can eventually lead to decreased cholesterol efflux from peripheral tissues leading to low levels of HDL-C. While manipulating the LXR pathway may complement the effects of statins, LXR synthetic ligands as T091317 have showed significant hypertriglyceridemic action that limits their use. We showed that the anti-diabetic drug vildagliptin stimulates LXR expression leading to increased ABCA1/ABCG1 expression which improves cholesterol efflux from adipocytes. Therefore, a combination of vildagliptin and statin may provide a solution without having the hypertriglyceridemic action of LXR agonist. We hypothesize that a combination of vildagliptin and pravastatin will improve cholesterol efflux in adipocytes (Aim 2).
In order to fulfill our aims, 3T3-L1 adipocytes cell line was used and the study was conducted in 2 parts.
To fulfill aim 1, the study was divided into the following 3 groups:
a) Control group: untreated 3T3-L1 adipocytes
b) Vildagliptin group: 3T3-L1 adipocytes were treated with the DPP-4 inhibitor; vildagliptin (2nM).
c) Exendin-4 group: 3T3-L1 adipocytes were treated with the GLP-1 mimetic; exendin-4 (5nM).
Gene and protein expression of ABCA1, ABCG1 and LXR-α were determined and correlated with cholesterol efflux. Expression levels of interleukin-6 (IL-6), leptin and the glucose transporter-4 (GLUT-4) were also determined. In addition, transfection with LXR-α siRNA was performed and its effect on ABCA1, ABCG1 and LXR-α gene expression and cholesterol efflux was evaluated.
While for fulfilling aim 2, the study was divided into the following 6 groups:
a) Pravastatin group: 3T3-L1 adipocytes were treated with the HMG-CoA reductase inhibitor; pravastatin (10µM).
b) T091317 group: 3T3-L1 adipocytes were treated with the LXR synthetic ligand; T091317 (10µM).
c) Pravastatin with T091317 group: 3T3-L1 adipocytes were treated with pravastatin and T091317 combination (10µM each).
d) Pravastatin with vildagliptin group: 3T3-L1 adipocytes were treated with pravastatin and vildagliptin combination (10µM each).
e) Vildagliptin group: 3T3-L1 adipocytes were treated with the DPP-4 inhibitor vildagliptin (10 µM)
f) Control group: untreated 3T3-L1 adipocytes
Gene and protein expression of ABCA1, ABCG1 and LXR-α were determined and correlated with cholesterol efflux. In addition, transfection with LXR-α siRNA was performed and its effect on ABCA1, ABCG1 and LXR-α gene expression and cholesterol efflux was evaluated.
The results of the current study can be summarized as follows:
(1) Significantly elevated levels of ABCA1, ABCG1, LXR-α and GLUT-4 mRNA and protein expression were observed in vildagliptin (2nM) and exendin-4 (5nM) groups as compared to control group (P<0.05).
(2) Significantly decreased levels of IL-6 and leptin mRNA and protein expression were observed in vildagliptin (2nM) and exendin-4 (5nM) groups as compared to control group (P<0.05).
(3) BODIPY cholesterol efflux assay was highly significantly elevated in both vildagliptin (2nM) and exendin-4 (5nM) (P<0.05) as compared to control group. This suggests that GLP-1 based therapy improves cholesterol efflux from adipocytes.
(4) Following transfection with LXR-α siRNA, both the mRNA expression of ABCA1, ABCG1 and LXR-α as well as cholesterol efflux assay were significantly reduced (P<0.05) in both groups as compared to control group. Accordingly, it is reasonable to speculate that the pathway is LXR-ABCA1/ABCG1 mediated.
(5) Afterwards, our data showed significantly reduced levels of ABCA1, ABCG1 and LXR-α mRNA and protein expression in pravastatin group as compared to the control group (P<0.05). However, significantly elevated levels of ABCA1, ABCG1 and LXR-α mRNA and protein expression were observed in T091317 group, pravastatin with T091317 group, pravastatin with vildagliptin group and vildagliptin group as compared to control group (P<0.05).
(6) BODIPY cholesterol efflux assay was significantly reduced in pravastatin group (P<0.05) as compared to control group. While it was significantly elevated in T091317 group, pravastatin with T091317 group, pravastatin with vildagliptin group and vildagliptin group as compared to control group (P<0.05). This suggests that adding LXR synthetic ligand or vildagliptin to pravastatin improves cholesterol efflux from adipocytes.
(7) Following transfection with LXR-α siRNA, both the mRNA expression of LXR-α as well as cholesterol efflux assay were significantly reduced (P<0.05) in all groups as compared to control group. Accordingly, it is reasonable to speculate the important role of LXR-ABCA1/ABCG1 pathway in cholesterol efflux from adipocytes.
(8) Considering mRNA expression of ABCA1 and ABCG1 following transfection with LXR-α siRNA, both T091317 group and pravastatin with T091317 group showed unexpected significant elevation as compared to control group (P<0.05), which may be due to the non-specific nature of T091317 which also stimulates PXR with the same potency as it stimulates LXR.
(9) Significantly decreased levels of ABCA1 and ABCG1 mRNA expression were observed in pravastatin group, pravastatin with vildagliptin group with vildagliptin group as compared to control group (P<0.05) following transfection with LXR-α siRNA. Accordingly, it is reasonable to speculate that the pathway is LXR-ABCA1/ABCG1 mediated.
In conclusion,
• The current study provides evidence that GLP-1-based therapy modulate ABCA1/ABCG1 expression in adipocytes potentially through an LXR-α mediated process eventually improving cholesterol efflux.
• Such data may explain, at least in part, the improvement in HDL-C levels observed in patients receiving such medication.
• Also we showed that using a combination of vildagliptin and a statin induces improvement in cholesterol efflux through an LXR-ABCA1/ABCG1 cascade.
• Moreover, in patients with diabetic dyslipidemia, such combination will not only reduce that amount of drug administrated (as vildagliptin is an anti-diabetic drug), but may also complement the effect of statins on cardiovascular risk factor.