In recent years, nanoparticles have been largely produced and extensively applied in various fields, such as medicine, animal husbandry, battery technologies and sewage treatment, due to their unique physical properties at the nano scale [[1], [2], [3]]. Metal oxide nanoparticles are regarded as highly versatile materials among all the nanomaterials currently in use, owing to their diverse range of properties and functionalities. Particularly, Mn dioxide (MnO2) nanoparticles (NPs) are among the metal oxide nanoparticles widely utilized in biosensors, biomedical devices, fertilizers, drug-delivery, contrast agents for magnetic resonance imaging, wastewater treatment and feed additives [1,[4], [5], [6]]. Global MnO2 NPs production can reach up to 1000 tons per year and is considered to be “mass production” [7]. Significantly, along with the extensive application of MnO2 NPs, their potential effects on animals and human health have attracted increasing attention. Among various exposure routes to NPs, dietary exposure is recognized as one of the most crucial in humans [8,9], and aquatic organisms [10]. Experimental data suggest that MnO2 NPs induced neuronal oxidative stress in PC12 cells and rats [11,12]. Exposure to high-concentrations of MnO2 NPs also induced abnormal neurobehavior, damaged memory function in rats, caused significant histological alterations and Mn accumulation in rats [13,14]. Additionally, MnO2 nanosheets induced mitochondrial toxicity in fish gill epithelial cells [15]. However, although many studies have shown that MnO2 NPs cause harmful biological responses in various organisms, the underlying mechanisms are largely unclear.

The liver is the major solid organ for the NPs accumulation [16,17]. It is also the central organ for lipid metabolism, which is crucial to maintain physiological functions of the organisms [17,18]. There is increasing evidence that exposure to metal nanoparticles can cause lipid metabolism disorders such as non-alcoholic fatty liver disease (NAFLD), which is characterized by excessive accumulation of lipid droplets (LDs) in hepatocytes [17,19]. To date, only few studies have demonstrated MnO2 NPs-induced liver damage in rats and mice [14,20]. However, the underlying mechanisms remain largely unexplored. Studies demonstrated that oxidative stress disrupts hepatic lipid metabolism and induces lipotoxic disease, thereby initiating the development of NAFLD [17,18,21,22]. Furthermore, oxidative stress has been identified as the primary factor contributing to the cytotoxicity induced by MnO2 NPs [23,24]. Our recent publication pointed out that MnO2 NPs promote lipid uptake and lipogenesis and trigger oxidative stress in the intestine of yellow catfish [25], but the effects and exact mechanism is unclear in the liver tissues. Importantly, HSF1 has been demonstrated to be a sensor of redox homeostasis [22,26,27]. Under oxidative stress, Hsf1 translocates to the nucleus and binds to conserved heat shock-responsive DNA elements (HSEs) to upregulate transcription of heat shock proteins (HSPs); these serve as molecular chaperones to protect cells from stress [26]. Hsf1 can also regulate lipid metabolism [22,28,29]. During cellular stress, Hsf1 undergoes phosphorylation at serine 326 (S326), which is a critical posttranslational modification (PTM) for its transcriptional activation [30]. However, it is still unclear whether these responses are related to the regulation of MnO2 NPs-induced hepatic lipid metabolism.

At elevated concentrations, MnO2 NPs exhibit cytotoxicity mainly due to their reactivity with biological systems and their enhanced potentials for cellular uptake [24]. Their main target are mitochondria, which are highly dynamic and multifunctional organelles that play a critical role in keeping cellular functions [23,31]. Several studies have indicated that MnO2 NPs induce oxidative stress and damage the mitochondrial structure and function [15,23,24,31]. Mitochondrial balance is maintained by two interconnected processes, mitochondrial dynamics and mitophagy [32]. The removal of damaged mitochondria, known as mitophagy, is an evolutionarily conserved process that plays a crucial role in maintaining cellular homeostasis [33]. Mitophagy is primarily governed by two molecular pathways: a) the PINK1 (PTEN induced kinase 1)/PRKN (parkin RBR E3 ubiquitin protein ligase)-dependent mitophagy mediated by the ubiquitin proteasome system [33], and b) mitophagy receptors‐mediated mitophagy, such as BCL2 and adenovirus E1B 19-kDa-interacting protein 3 (Bnip3), BNIP3-like (Bnip3l), FUN14 domain-containing protein 1 (Fundc1), FK506-binding protein 8 (Fkbp8), and Bcl2-like 13 (Bcl2l13) [34]. Bnip3 is highly expressed in the liver and directly interacts with LC3B to initiate the process of mitophagy [34]. Several studies have indicated that Bnip3-mediated mitophagy contributes to protection against liver injury and relief of hepatic lipid deposits [35,36]. Excessive ROS generation by metals or metal nanoparticles exposure leads to mitochondrial dysfunction and activation of Bnip3-dependent mitophagy [37,38]. However, the effects and underlying mechanisms of MnO2 NPs on mitophagy have not been elucidated.

Fish have almost 30,000 species and are the biggest group of vertebrates in the world. Their metabolic reaction pathways and nutrient-sensing systems are evolutionarily conserved with mammals [18,39]. Yellow catfish Pelteobagrus fulvidraco, a freshwater economic fish widely farmed in China and other countries [40], is an excellent model for studying lipid metabolism since it has a lipid metabolism pattern similar to that of mammals and its complete genome sequence were published and available internationally [[39], [40], [41]]. Moreover, many studies have used yellow catfish as a model to analyze the mechanism and treatment of metabolic disorders [18,[41], [42], [43], [44]], and have attracted wide attention by international researchers [[45], [46], [47]]. Therefore, P. fulvidraco provides an excellent experimental model to identify regulatory mechanisms of lipid metabolism. Therefore, the aim of this study was to explore the mechanisms of MnO2 NPs-induced changes of lipid metabolism in the liver of yellow catfish. Our study reveals an unprecedented significant regulatory role of mtROS-triggered Hsf1S326 phosphorylation in MnO2 NPs-induced hepatic lipotoxicity and mitophagy. Importantly, we identified novel targets of Hsf1 mediating lipogenesis and mitophagy, and correspondingly strengthened the role of Hsf1 as a regulator of hepatic lipid metabolism. Our findings provide new insights into the effects of metal oxides nanoparticles on hepatotoxicity in vertebrates.