Marchf6: A guardian against cytosol-spilled POMC-induced ferroptosis

Graphical abstract

Approximately one-third of newly synthesized eukaryotic proteins undergo co-/post-translational translocation into the endoplasmic reticulum (ER) on their way to final destinations, including secretory vesicles, membranes, and specific organelles (Hegde and Zavodszky, 2019). However, ER stress or inherited mutations in signal peptides disrupt precise targeting and translocation into the ER, resulting in the production of mislocalized proteins. These proteins often adopt incorrect folds and exhibit promiscuous interactions with their proximal proteins, leading to cell death and the development of aging-associated proteopathies as well as metabolic or neurological disorders (Hegde and Zavodszky, 2019). Despite the pathophysiological importance of mislocalized secretory or membrane proteins in the cytosol, the molecular mechanisms underlying their cytotoxic effects remain unclear.

Proopiomelanocortin (POMC), a multifunctional prohormone, produces essential peptide hormones that influence various physiological processes, such as feeding behavior melanogenesis, steroidogenesis, and systemic energy homeostasis (Lindberg and Fricker, 2021, Lee et al., 2022, Sa et al., 2022). Disruption of POMC leads to metabolic imbalances and obesity (Lindberg and Fricker, 2021). Contrary to the prevailing notion that excess POMC production suppresses appetite and increases energy expenditure, its overexpression paradoxically increases obesity (Kim et al., 2018). This phenomenon is attributed to the production of improperly adjusted and misfolded POMC molecules within the ER, leading to the induction of ER stress (Kim et al., 2018). As a result, stress in the ER inhibits the secretion of end products, such as α-melanocyte-stimulating hormone because POMC overwhelms its ER’s quality control capacity (Kim et al., 2018). Consequently, stressed ERs most likely restrict their Sec61 translocons, slowing or halting the excessive import of POMCs into the ER lumen and allowing newly made POMCs to leak into the cytosol. Most POMC molecules in POMC neuronal cells are cotranslationally translocated into the ER lumen for integration into the secretory pathway, and misfolded POMCs are selectively retrotranslocated from the ER lumen to the cytosol for elimination by the ER-associated protein degradation pathway (Kim et al., 2018). However, the presence and functional implications of mislocalized, cytosol-accumulated POMCs remain unexplored.

Our recent study revealed intricate connections between misoriented cytosolic POMCs and the onset of proteotoxicity and ferroptosis, a regulated form of cell death involving excess iron-dependent and lipid peroxidation processes (Mun et al., 2023) (Fig. 1). In POMC neuronal cells that overexpress POMC, cytosol-exposed POMCs directly bind to Hspa5 (also known as Grp78 or Bip), an ER stress sensor within the Hsp70 chaperone family (Mun et al., 2023) (Fig. 1). Hspa5 inhibits ferroptosis by stabilizing a key ferroptosis suppressor, Gpx4 glutathione peroxidase, through Hspa5-Gpx4 complex formation in some cancer cells (Zhu et al., 2017). In contrast, in POMC neuronal cells, cytosol-exposed POMCs hijack Hspa5 from the Hspa5-Gpx4 complex with a stronger affinity for Hspa5 than for Gpx4. The resulting POMC-mediated hijacking of Hspa5 causes the degradation of Gpx4 by chaperone-mediated autophagy (CMA). Consequently, the CMA-mediated Gpx4 depletion results in elevated levels of lipid reactive oxygen species (ROS), specifically activating the PERK/eIF2α/ATF4/CHOP pathway among the unfolded protein response signaling pathways in a positive feedback loop (Mun et al., 2023) (Fig. 1). Concurrently, excess lipid ROS impairs the ER translocation of newly synthesized POMCs, exacerbating the harmful effects of cytosolic POMC accumulation. The effect is alleviated by a lipid ROS scavenger, which reduces cytosol-retained POMCs by facilitating their translocation into the ER lumen, with a special emphasis on the level of lipid ROS in this process (Mun et al., 2023) (Fig. 1).

Fig. 1.

In normal POMC neuronal cell trafficking, most POMCs are transported into the ER lumen for processing and secretion (left panel). However, an excess of POMC traffic can overwhelm the ER’s quality control capacity, leading to the mislocalization of some POMCs in the cytosol (right panel). Cytosolic POMC disrupts the Hspa5-Gpx4 complex, destabilizing Gpx4 through chaperone-mediated autophagy (CMA) and increasing lipid ROS levels. The elevated lipid ROS level, in turn, inactivates Marchf6 which targets cytosolic POMC for degradation. As a result, cytosolic POMC accumulates, further exacerbating lipid ROS production and triggering ferroptosis. As expected, POMC neuron-specific Marchf6-deficient mice developed an increased appetite, decreased energy expenditure, and subsequent weight gain.

Marchf6 (also known as March6, Teb4, or Rnf176) is the largest ER transmembrane E3 ubiquitin ligase. It exhibits upregulation in POMC neuronal cells, playing a critical role in mitigating ER stress and averting ferroptotic toxicity induced by POMCs (Mun et al., 2023) (Fig. 1). Notably, Marchf6 operates as a NADPH (nicotinamide adenine dinucleotide phosphate) sensor, preventing ferroptosis (Nguyen et al., 2022b, Yang et al., 2023) and as a pivotal component of both the Ac/N-degron pathway (targeting the N-terminal acetyl group of substrates) and the ER-Associated protein Degradation-Cytosol (ERAD-C) pathway (targeting misfolded substrates) (Nguyen et al., 2022a, Yang et al., 2023). Our recent findings also reveal that Marchf6 serves as an E3 ligase in the ER stress-induced pre-emptive quality control system for the degradation of POMCs (Mun et al., 2023) (Fig. 1). Importantly, the absence of Marchf6 results in elevated levels of lipid ROS and ER stress. This action impedes efficient POMC translocation into the ER lumen, thereby exacerbating cellular toxicity due to the accumulation of cytosol-misoriented POMCs. Consequently, POMC neuron-specific Marchf6-deficient mice exhibit increased food intake, decreased energy expenditure, and weight gain (Mun et al., 2023) (Fig. 1). Consistently, Marchf6 perturbations have been implicated in various diseases, including neurodegeneration, cancers, and metabolic syndromes (Nguyen et al., 2022b, Scott et al., 2021, Yang et al., 2023). Heterozygous mutations in the POMC signal sequence or its neighboring N-POMC (N-terminal POMC peptide) region are also associated with early-onset obesity (Gutierrez Guarnizo et al., 2023, Mencarelli et al., 2012). Thus, the proposed molecular mechanism elucidating Marchf6-mediated degradation of POMC provides new insights into previously unexplored facets of overloaded secretory proteins, Marchf6 anomalies, ER stress, and ferroptosis.

Professional secretory cells in various tissues, such as the pituitary gland, pancreas, liver, thyroid, and bone marrow, rely on a sophisticated ER for secretory protein synthesis and processing (Szenci et al., 2023). Notably, mutations in the proinsulin signal peptide, observed in late-onset diabetes, lead to exposure of the prohormone to the cytosol. Cytosol-retainable proinsulin induces ER stress, β-cell dysfunction, and ultimately cell death, culminating in the development of diabetes (Guo et al., 2014). However, more detailed investigations are imperative to determine whether Marchf6 functions as a broad guardian against various metabolic diseases, including diabetes. In order to address the harmful detrimental effects of cytosolic proinsulin retention, it is imperative to investigate the potential of Marchf6 in alleviating ER stress and safeguarding against ferroptosis.

Due to the lack of direct medications that improve POMC expression, alternative approaches, such as metreleptin (a leptin hormone analog), have been explored to indirectly reduce body weight in cases of leptin-deficient obesity (Angelidi et al., 2022). However, hyperactivation of POMC neurons induced by the leptin analog may lead to unforeseen neurological complications. Our study suggests that incorporating adjuncts to prevent lipid peroxidation (Mun et al., 2023) could mitigate the adverse effects of antiobesity drugs that might induce POMC overload. For example, combining lipophilic antioxidants such as vitamin E with antiobesity drugs has shown promising results (Wong et al., 2017). This comprehensive strategy can alleviate complications, emphasizing the importance of further exploration of cytosol-localized secretory preproteins and their interaction with Marchf6, ER stress, ferroptosis, and potential therapeutic targets for associated pathophysiological consequences, such as diabetes, neurodegeneration, immunological disorders, and aging.

Author Contributions

S.-H.M. and C.-S.H. designed and wrote the manuscript, and C.-S.H. secured funding.