Role of autophagy in PPAR-induced changes in lipid metabolism in the liver
Creators: name: Jo, Eunjung
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 320299
subject: Clinical sciences not elsewhere classified
classificationCode: 320507
subject: Metabolic medicine
classificationCode: 420899
subject: Traditional, complementary and integrative medicine not elsewhere classified

description: Excessive energy stored in our body results in an accumulation of lipid which can impair action of insulin in metabolic tissues such as liver and muscle in both animals and humans by interfering with insulin signal transduction, molecular mechanisms and pathways. Insulin as a major hormone regulates blood glucose levels by promoting glucose utilization in muscle while blocking glucose production in the liver. These tissues become less responsive to insulin’s action due to a defect in insulin signal transduction, becoming insulin resistant. Liver is a major tissue for the metabolism of glucose, lipids and proteins. It is susceptible to developing insulin resistance with triglyceride fat/lipid accumulation at an early stage.<br><br>This thesis investigated whether alterations to a process for degrading cellular debris for recycling (so called autophagy) can impact on liver lipid metabolism and if so, the possible mechanism involved. The thesis focused on the molecular metabolic pathway in relation to lipid metabolism, leading to the pathogenesis of hepatic steatosis and insulin resistance, especially in the context of over consumption of a lipogenic fat, and stimulation of protein receptors.<br><br>Peroxisome proliferator activated receptors (PPARs) are nuclear protein receptors that function as transcriptional factors to regulate lipid metabolism. PPARα and PPARγ are two isoforms of the subfamily of PPARs. Both PPARα and PPARγ play important roles directly or indirectly in the process of lipid metabolism in the liver. PPARα, highly expressed in liver, directly regulates genes involved in fatty acids uptake, β-oxidation and ω-oxidation during fasting or pharmacological stimulation with a fibrate ligand. In comparison, PPARγ is predominantly expressed in adipose tissue where it potentiates adipogenesis to maintain lipid storage to modulate the distribution of fatty acids to various organs. Pharmacological activation of the PPARγ receptor with the glitazone ligand improves insulin sensitivity and influences lipid metabolism in liver.<br><br>Autophagy is a defensive catabolic mechanism that promotes normal physiological function by regulation or degradation of proteins/lipids. This process is essential in the removal of damaged cellular components, misfolded proteins due to endoplasmic reticulum (ER) stress of unfolded protein resonse (UPR) and accumulated lipid droplets to maintain protein and energy homeostasis in the cell under physiological conditions. Destruction of autophagy function promotes lipid accumulation as lipid droplets resulting in liver steatosis.<br><br>Therefore, this thesis aimed to investigate 1) the effect/role of PPARγ and/or PPARα in pharmacological regulation on autophagy in coordination with lipid metabolism in the liver after consumption of a high fat diet; 2) whether the autophagy pathway is altered along with nuclear protein receptors, PPARγ and/or PPARα, in relation to lipid metabolism; 3) the possible molecular mechanism associated with alteration of autophagy which can impact on the above metabolic disorders.<br><br>Firstly, in Chapter 3, I investigated the effect/role of PPARγ in a rat model of insulin resistance (IR) accompanied with hepatic steatosis produced by a high fat diet (HF), to mimic the over-consumption of fat commonly occurring in humans, on the autophagy pathway. Stimulation of PPARγ activation with the pharmacological activator, rosiglitazone (RG; 15mg/kg/day), restored insulin sensitivity and lowered lipid content in the liver. PPARγ activation by RG showed moderate effects on certain lipogenic enzymes and autophagy proteins in the liver, however, there was no clear indication of an increase or decrease in lipogenesis and autophagy activity. I further examined ER stress in the UPR pathway, which is a protective mechanism to inhibit protein synthesis, to promote protein folding and degradation, and to maintain protein homeostasis in the cell under physiological conditions. Findings from this study revealed that there was no involvement of ER stress in PPARγ activation by RG in liver. Therefore, the results led to the conclusion that PPARγ activation by RG has no overall effect or role on autophagy in relation to lipid metabolism and ER stress in the liver despite increased insulin sensitivity and lowered lipid accumulation in the liver via fatty acids re-distribution.<br><br>In Chapter 4, I examined the effect/role of PPARα on autophagy in the liver. In this study, stimulated PPARα activation by fenofibrate (FB; 100mg/kg/day) showed a significant decrease in multiple autophagy proteins irrespective of diets. PPARα activation by FB also revealed a significant ER stress inhibition along with upregulation of lipogenic enzymes, indicating an intrinsic causal relationship among autophagy, ER stress and lipid metabolism. My results highlight a crucial role of autophagy in the context of PPARα activation by FB and provide a proof-of-concept to target the autophagy pathway, possibly for the treatment of metabolic disorders.<br><br>In order to obtain the evidence for the definitive role of PPARα, Chapter 5 examined these effects of PPARα action in PPARα knockout (PPARα-/-) mice treated with or without FB (50mg/kg/day). My results clearly indicated that PPARα activation is required for FB to up-regulate peroxisomal fatty acid oxidation, lipogenesis and to suppress the multiple autophagy proteins in the liver. For PPARα-induced suppression of autophagy, I further investigated the reported possible upstream mechanism. These included mammalian target of rapamycin (mTOR), AMP-activated protein kinase (AMPK), Akt, ER stress and fibroblast growth factor 21 (FGF21) hepatokine induced by PPARα activation. Interestingly, PPARα-induced suppression of autophagy was found to occur independent of mTOR inhibition, activation of AMPK, Akt, ER stress, and FGF21 stimulation in the liver.<br><br>PPARα activation by pharmacological or physiological effect, such as starvation, has been suggested to alleviate autophagy induction through cytosolic forkhead box protein O1 (FoxO1) function. My results revealed a reduction in the level of FoxO1, which is a key transcriptional factor controlling the expression of multiple autophagy proteins in the presence of suppressed autophagy by prolonged activation of PPARα with FB. Collectively, these findings suggest that PPARα induced suppression of autophagy may play an important role in regulating liver metabolism which may impact on the future treatment of metabolic disorders.<br><br>In summary, my findings in PPARγ activation by RG revealed no correction of dysregulated multiple autophagy proteins in HF fed rats, despite improved insulin sensitivity and lowered triglyceride/lipid content in the liver. More importantly, the results from studies in this thesis provided strong evidence that autophagy is likely suppressed in prolonged PPARα activation with FB, which is independent of FGF21 and accompanied with enhancement of lipogenic enzymes for upregulation of de novo lipogenesis (DNL). PPARα induced suppression of autophagy is via FoxO1 inhibition in the presence of disarrayed signalling pathways of the mTOR, AMPK, Akt and ER stress. The discoveries from this thesis have identified the interactions of these pathways with autophagy beyond their previously recognised roles in liver metabolism. My studies are also believed to provide a potential rationale for further investigation of the possible role of autophagy in PPARα-induced adapted response in the liver.