Hepatic steatosis is characterized by excessive hepatic lipid accumulation, and it has become a widespread issue worldwide [1]. Hepatic lipid accumulation is regulated by a complex network involving lipogenesis, dietary fatty acids (FAs) uptake, FAs β-oxidation, and lipid transport/secretion [2]. Since the mitochondrial matrix is the main site for FAs β-oxidation [3], maintaining mitochondrial integrity and homeostasis is significant for FAs β-oxidation. Mitochondria (MT), being double-membrane organelles, form a highly dynamic network and continually undergo cycles of fusion and fission [4], which are essential for mitochondrial integrity, homeostasis and function [5,6]. As such, mitochondrial dynamics are closely linked to hepatic lipid metabolism and homeostasis [7], but in-depth mechanistic investigations are required for further verification.

Mitochondrial fusion facilitates the transfer of gene products between mitochondria for optimal functioning [8]. Recent studies have highlighted the role of mitochondrial fusion in promoting mitochondrial mixing and increasing cellular energy production via oxidative phosphorylation (OXPHOS) [9]. Considering that in most cases, FAs β-oxidation represents the primary pathway for generating substrates for OXPHOS, albeit influenced by various factors such as diet conditions, feeding status, or tissue type [10,11], and taking into account that mitochondrial fusion plays a role in the restructuring of mitochondrial cristae [12], it can be inferred that mitochondrial fusion plays a pivotal and positive role in FAs β-oxidation. However, the present understanding of whether and how mitochondrial fusion facilitates FAs β-oxidation, particularly the underlying molecular mechanism, remains limited.

The mitochondrial outer membrane (MOM) fusion is facilitated by two large membrane GTPases, known as Mitofusin 1 (Mfn1) and Mitofusin 2 (Mfn2) [13]. During MOM fusion, the primary function of Mitofusins (Mfns) is the promotion of docking and fusion of the MT [14,15]. The tethering of two MOMs occurs through the oligomerization of the GTPase domains of Mitofusins (Mfns), and this oligomerization process necessitates GTP hydrolysis [16,17]. Additionally, despite the high similarities, Mfn1 and Mfn2 have also been reported to play distinct roles in mediating mitochondrial fusion via GTPase activity in an in vitro study [16]. Specifically, recent research has identified that Mfn2 appears to be more versatile than Mfn1 due to its higher efficiency in tethering membranes. This characteristic has led to its frequent association with human diseases [18]. Although, emerging studies have shed light on the critical role of Mfns in regulating lipid metabolism [19], such as the synthesis of phospholipids and FAs, the role of Mfns in FAs β-oxidation has yet to be defined.

The rate of mitochondrial FAs β-oxidation is mainly limited by the process of delivering FAs into MT [20]. This transportation process is catalyzed by carnitine palmitoyltransferase I (Cpt1) [3]. Notably, Rambold et al. [21] proposed that Mfns-mediated mitochondrial fusion facilitates mitochondrial import of FAs, but the underlying molecular mechanism is unclear. Given both Mfns and Cpt1α are located on the outer mitochondrial membrane [3,22], there exists an integral metabolic connection between Mfns and Cpt1α, particularly during the process of mitochondrial fusion. However, the molecular details of whether and how Mfn2 affects Cpt1α and their role in subsequent FAs β-oxidation remain unclear.

Fish constitute the most extensive group of vertebrates globally. The yellow catfish Pelteobagrus fulvidraco is a freshwater teleost fish found in various regions of China with other countries [23], [24], [25]. Notably, this species is commonly used as an effective model for studying lipid metabolism due to the similarities between its genome, metabolic processes, and regulatory pathways, and those found in mammals [2]. Although it has been extensively established that the influence of dietary FAs on hepatic lipid balance depends on the FAs ratio [26], [27], [28], the present study introduces a novel finding that the dietary ratio of palmitic acid (PA) to fish oil (FO) can indeed play a role in reducing hepatic lipid accumulation. Further findings reveal how substituting some FO with PA enhances Mfn2-mediated mitochondrial fusion by reducing Mfn2 ubiquitination. This promotes mitochondrial FAs β-oxidation through the interaction of Mfn2 with Cpt1α via its GTPase domains. The present data confirm that these partial mechanisms are conserved across species, extending from fish to mammals. These discoveries establish a novel and essential role for Mfn2-mediated mitochondrial fusion in the regulation of FAs β-oxidation in metabolic processes.