Mengmeng Xia

Metabolic dysfunction-associated steatotic liver disease (MASLD) has emerged as the most prevalent chronic liver disease globally, characterized by excessive lipid accumulation in hepatocytes. This condition represents a spectrum of liver disorders, ranging from simple steatosis to more severe forms such as metabolic dysfunction-associated steatohepatitis (MASH), fibrosis, cirrhosis, and ultimately hepatocellular carcinoma (HCC). The mechanisms underlying the progression from simple steatosis to advanced inflammatory stages remain poorly understood, however, lipotoxicity, particularly from saturated fatty acids (SFAs), is believed to play a pivotal role. This thesis explores various aspects of MASLD, focusing on the roles of lipotoxicity, hepatic stellate cell (HSC) activation, and intercellular communication mediated by extracellular vesicles (EVs). Additionally, it examines potential therapeutic agents that may mitigate the progression of this disease. The findings are organized into six chapters, each addressing specific aspects of MASLD.

Chapter 1 provides a comprehensive overview of MASLD, detailing its epidemiology, pathophysiology, and the critical role of lipid metabolism in liver health. It highlights the increasing global incidence of MASLD, emphasizing its association with rising rates of obesity and metabolic syndrome. The chapter discusses the pathological features of MASLD, including the accumulation of lipids in hepatocytes, inflammation, and fibrosis, which can ultimately lead to severe liver damage. It sets the stage for the subsequent chapters by outlining the need for a deeper understanding of the underlying mechanisms and potential therapeutic interventions.

Chapters 2, 3, and 4 focus on revealing the pathophysiological mechanisms involved in the different stages of MASLD using experimental models and natural or synthetic compounds. Chapter 2 identifies saturated fatty acids (SFAs), particularly palmitate (PA), as key contributors to lipotoxicity, which leads to hepatocyte necrosis. The chapter presents experimental data demonstrating that exposure to PA induces significant cell death in hepatocytes, while a mixture of oleate and palmitate (OA/PA) does not; hence, neutral lipid droplet accumulation may serve as a protective mechanism by sequestering harmful lipids, thereby preventing cellular damage, although it was historically viewed as detrimental. Furthermore, Chapter 2 discusses the mechanisms of lipid accumulation, emphasizing the imbalance between lipid acquisition and disposal in the liver. The findings suggest that targeting lipotoxicity may be a viable therapeutic strategy for MASLD. Importantly, Chapter 2 also notes that coumarin derivatives such as esculetin, serve as a protective compound against lipotoxicity as well as lipid accumulation, due to its strong anti-oxidative effect and the activation of the metabolic sensor AMPK, respectively. Chapter 3 focuses on the mechanism of activation of hepatic stellate cells (HSCs) and its modulation by the natural compound arctigenin. The chapter reveals that ER stress inducers enhance the expression of activation markers in HSCs, while inhibitors of ER stress can reverse these effects. Chapter 3 introduces arctigenin, a natural compound derived from Fructus Arctii, which can reduce HSC activation under ER stress stimuli and discusses the dynamics of lipid droplets (LDs) during HSC activation, demonstrating that ER stress reduces the size of neutral lipid-containing LDs, which can be rescued by arctigenin. Therefore, targeting ER stress may represent a promising therapeutic strategy to reverse HSC activation and liver fibrosis. Chapter 4 investigates the role of glycogen synthase kinase-3β (GSK-3β) in regulating collagen type I synthesis in HSCs. The data in Chapter 4 show that inhibition of GSK-3β leads to increased collagen production, suggesting that GSK-3β functions as a negative regulator of HSC activation and collagen synthesis. Moreover, Chapter 4 explores the interplay between autophagy and collagen synthesis, revealing that autophagy may facilitate collagen degradation rather than synthesis in HSCs. This finding is significant, as it challenges previous assumptions about the role of autophagy in collagen metabolism. Chapter 4 highlights the complex signaling pathways involved in HSC activation, including mTORC1 signaling and its negative correlation with autophagy after rapamycin treatment. The findings suggest that GSK-3β activation may represent a potential therapeutic target for reducing collagen type I accumulation in liver fibrosis. Chapter 4 concludes by emphasizing the need for further research to elucidate the precise mechanisms by which GSK-3β and autophagy interact in the context of HSC activation and liver fibrosis.

Chapter 5 reviews the emerging role of EVs in MASLD, highlighting their function in intercellular communication and EVs as the novel perspective on the pathogenesis of MASLD. Chapter 6 discusses how EVs released from steatotic hepatocytes can influence HSC behavior, promoting senescence and modulating their activation state. Chapter 6 emphasizes the importance of understanding the cargo of EVs, including microRNAs (miRNAs), and their potential to activate signaling pathways such as AKT/mTOR in target cells. Moreover, the chapter discusses the implications of EV-mediated communication for the development of potential therapeutic strategies aimed at modulating HSC activation and liver fibrosis. By elucidating the role of EVs in liver pathology, Chapter 6 contributes to a growing body of evidence supporting the importance of intercellular communication in the progression of MASLD.

In conclusion, this thesis identifies novel strategies for managing MASLD, emphasizing the need for further research to clarify the mechanisms underlying lipid metabolism, HSC activation, and intercellular communication. Addressing the limitations of current studies, such as reliance on in vitro models and the need for rigorous standardization of natural products, will be crucial for translating these findings into clinical practice. In addition, future studies should also explore the long-term effects of cellular senescence and the role of EVs in liver disease to develop effective therapeutic interventions for MASLD.

In summary, this thesis offers a comprehensive exploration of MASLD, highlighting critical mechanisms involved in its pathogenesis and presenting potential therapeutic avenues for intervention. The findings contribute to the growing body of knowledge surrounding liver diseases and underscore the importance of continued research in this field to ultimately improve patient outcomes and therapeutic options for MASLD.