Biochemical and Biophysical Research Communications
Activation of PPARδ up-regulates fatty acid oxidation and energy uncoupling genes of mitochondria and reduces palmitate-induced apoptosis in pancreatic β-cells
Introduction
The peroxisome proliferator-activated receptor δ (PPARδ) is a member of the nuclear hormone receptor superfamily and is activated by fatty acids (FAs) and FA derivatives in non-β-cells. All members of the PPAR family are expressed in pancreatic β-cells [1], but investigations of PPARδ in β-cells are rare. Previous studies have suggested that PPARα and PPARγ may play an important role in FA metabolism in β-cells by regulating carnitine palmitoyltransferase 1 (CPT1) and uncoupling protein 2 (UCP2), which are critical genes in fatty acid oxidation (FAO) and energy uncoupling of mitochondria [2], [3]. Riserus et al. [4] found that activated PPARδ could increase the relative proportion of exhaled CO2 that originated from the fat content of a meal. PPARδ can ameliorate diet-induced obesity as well [5]. However, in β-cells we do not know whether activated PPARδ can up-regulate mRNA expression of pyruvate dehydrogenase kinase 4 (PDK4), which is a gene important for preferential lipid utilization. In addition, the relationship between activation of PPARδ and the changes of some important genes, such as CPT1 and UCP2, remains unknown.
PPARδ is a transcriptional regulator of FA homeostasis. Free fatty acids (FFAs) can lead to insulin resistance and pancreatic β-cells impairment. Moreover, high levels of FFAs are known to induce mitochondrial aberrations [6]. After long-term exposure to high concentration of FFAs, β-cells secrete more basal insulin but less glucose-stimulated insulin [7]. As a subtype-selective ligand for PPARδ, GW501516 (GW) has 1000-fold higher affinity towards PPARδ (with a Kd value of 1 nM) than towards the other subtypes of the PPAR family [8].
We hypothesized that activating PPARδ via a PPARδ-specific agonist (GW) could enhance the mitochondrial function of lipid oxidation, thus protecting against lipotoxicity-induced β-cell damage. We cultured pancreatic HIT-T15 cells (a β-cell line) in the presence of various concentrations of palmitate and GW, and observed the changes of mitochondrial morphology, cellular apoptosis, insulin secretion, and expression of lipid-regulatory genes induced by activated PPARδ. We found that activation of PPARδ could improve mitochondrial function and decrease the apoptotic rate of β-cells. These effects of PPARδ may be associated with increases in the activation of FA β-oxidation enzyme genes and energy uncoupling protein expression, including CPT1, long-chain acyl-CoA dehydrogenase (LCAD), PDK4 and UCP2. However, we did not investigate the effect of inactivated PPARδ in β-cells due to lack of availability of specific inhibitors of PPARδ.
Section snippets
Materials and methods
Cell culture. HIT-T15 cells from a hamster clonal pancreatic β-cell line were provided by the State Key Laboratory of Oral Diseases of Sichuan University. The cells were cultured in DMEM medium (Gibco) with 20% calf serum (Hali, China), 5.6 mM glucose, 100 units/ml penicillin, 100 μg/ml streptomycin, at 5% CO2 and 95% O2 at 37 °C. Palmitate (Sigma) solutions were prepared as previously reported [9], and the final concentrations were 0.25, 0.5, and 1.0 mM in serum-free media. GW (Alexis,
PPARδ activation rescued mitochondrial impairment and GW decreased apoptosis caused by palmitate
Mitochondrial structures and cellular membranes were intact in the control group, whereas mitochondria were swollen in palmitate-exposed cells (Fig. 1A and B). Activation of PPARδ produced morphologically normal mitochondria (Fig. 1C). The rate of apoptosis rose with increasing palmitate concentration (P < 0.05, Fig. 1D). However, addition of GW to the palmitate media decreased apoptotic rates (P < 0.05, Fig. 1D), although the apoptotic rate in the GW alone group was slightly higher than that in
Discussion
PPARδ is expressed ubiquitously in almost all tissues. While PPARα enhances FAO and PPARγ promotes fat synthesis and storage, activation of PPARδ increases lipid utilization and FAO, improves energy expenditure, and protects against diet-induced obesity. These functions are important under some stressed physiological states, such as exposure to cold, starvation, or long-term exercise [5]. In fact, Jucker et al. reported that PPARδ can shift metabolic substrate utilization from carbohydrate to
Acknowledgments
This work was supported by West China Hospital of Sichuan University. The authors thank the State Key Laboratory of Oral Diseases of Sichuan University and Xiaoyu Li for technical guidance.
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