FGF8 promotes lipid droplet accumulation via the FGFR1/p-p38 axis in chondrocytes

Abstract

Chondrocytes store lipids in the form of lipid droplets (LDs) and maintain cartilage lipid metabolic homeostasis by consuming or regenerating LDs. This modulation is largely mediated by a series of biochemical factors. Fibroblast growth factor 8 (FGF8) is one of the most important factors involved in the proliferation, differentiation, and migration of chondrocytes and has attracted increasing attention in the physiology and pathology of cartilage. However, the effect of FGF8 on LD accumulation in chondrocytes remains unclear. This study aims to elucidate the role of FGF8 in LDs and explore the underlying biomechanism involved. The results reveal that FGF8 promotes LD accumulation in chondrocytes by upregulating perilipin1 (Plin1) expression. FGF8 activates the cytoplasmic p-p38 signaling pathway via fibroblast growth factor receptor 1 (FGFR1) to increase LD accumulation in chondrocytes. Subsequent experiments with siRNAs and specific inhibitors further confirm the importance of the FGFR1/p38 axis for LD accumulation in chondrocytes exposed to FGF8. The results increase our understanding of the role of FGF8 in the lipid metabolic homeostasis of chondrocytes and provide insights into the physiology and pathology of cartilage.

1. Introduction

The lipid droplet (LD), a principal hub organelle for intracellular lipid storage, consists of a neutral lipid core enveloped by a phospholipid monolayer membrane [1–3]. LD biogenesis begins mostly in the endoplasmic reticulum (ER), the main organelle involved in neutral lipid synthesis [4]. The first step in LD biosynthesis is the synthesis of neutral lipids such as triacylglycerols (TGs) and cholesterol esters (CEs) in the ER [4,5]. These neutral lipids are dispersed in the ER at low concentrations. With continuous neutral lipid synthesis, biophysical processes result in the formation of a neutral lipid lens (also called the oil lens) [5]. A widely recognized model states that neutral lipids nucleate an oil phase that minimizes the entropy costs involved in disrupting the ER bilayer membrane [1,4,5]. Subsequently, the oil phase transitions to the oil lens. The oil lens accommodates more neutral lipids and promotes the aggregation of neutral lipids to form LDs [5]. After being released into the cytoplasm, these nascent LDs gradually develop into larger mature LDs by storing more lipids or merging with each other [3–5]. In addition, LDs interact with other organelles, such as mitochondria and lysosomes, to regulate their metabolism and function. For example, Meng et al. [6] reported that phosphofructokinase, a glycolytic enzyme, promotes LD-mitochondrion tethering to increase β-oxidation. Menon et al. [7] reported that ARL8B, a GTPase, promotes LD-lysosome contact and induces lysosomal lipolysis of LDs. More importantly, lipophagy, an autophagy process that specifically targets LDs, plays an important role in maintaining the cellular energy supply and alleviating metabolic diseases such as atherosclerosis [8]. Lin et al. [9] reported that the recovery of lipophagy via the inhibition of PPAR/PI3K/AKT signaling relieves atherosclerosis-related symptoms, including lipid accumulation, apoptosis, and inflammation.

The primary function of intracellular LDs is to serve as storage reservoirs for neutral lipids and supply essential lipid precursors whenever the cellular lipid level decreases [10]. These neutral lipids can be used for cell energy supply through mitochondrial β-oxidation and for structural composition, such as lipid membrane expansion [10]. In addition, LDs can assist cells in preventing lipid toxicity [11]. Free lipids such as fatty acids can act as detergents to disrupt the cell membrane structure. Synthesizing fatty acids into triglycerides and storing them in LDs effectively prevents intracellular lipid toxicity. However, despite the numerous positive effects of LDs within cells, excessive LD accumulation can result in the development of various diseases, such as hepatic steatosis [10,11]. As organelles with important functions in cells, LDs are also regulated by a variety of proteins [12]. These proteins, including a series of enzymes that regulate the dynamic process of LD biogenesis and linker proteins that connect LDs to other organelles, regulate LD activities [12]. These proteins are roughly divided into two types: integral and peripheral proteins. Integral proteins are inserted into the monolayer membrane of LDs and can recruit peripheral proteins to LDs [12]. Integral proteins play vital roles in the generation, maintenance, and lipolysis of LDs and are composed of two functional subclasses according to their trafficking pathways. One is a Class I LD protein that originates in the ER [12]. These proteins are then transferred to