Regulation of Metabolism by Mitochondrial MUL1 E3 Ubiquitin Ligase

Producing ATP through oxidative phosphorylation means mitochondria’s main job is supplying the body’s energy needs. They also play a crucial part in maintaining cellular homeostasis, controlling cell death, and regulating metabolism. In response to changes in the physiological environment or low oxygen levels, mitochondria can alter their bioenergetic and biosynthetic processes to satisfy the metabolic demands of the cell. In order to control metabolism, there must be constant and efficient communication between the mitochondria and the rest of the cell. A wide variety of proteins mediates this communication. MUL1 is such a protein involved in many cellular processes such as apoptosis, mitophagy, mitochondrial dynamics, and innate immunological response. MUL1’s diverse function suggests that it interacts with various distinct substrates and through its activity affects their function. With two transmembrane domains, a sizable intermembrane domain, and a RING finger domain facing the cytoplasm, MUL1 is anchored in the outer mitochondrial membrane, where it performs its ligase activity. It is capable of K48- or K63- ubiquitinating as well as SUMOylating a variety of particular substrates.

The importance of MUL1 in regulating critical cellular processes is mainly through post-translational modifications dependent on its E3 ligase activity. It is believed mitochondrial dysfunction which involves MUL1 may cause some pathologies including cancer and neurological diseases. Understanding the complete function of MUL1 will uncover new therapeutic targets for treating many diseases. In a new study published in Frontiers in Cell and Developmental Biology, University of Central Florida College of Medicine researchers Dr. Lucia Cilenti, Dr. Jacopo Gregorio, Dr. Camilla Ambivero and led by Professor Antonis Zervos in collaboration with Dr. Rohit Mahar and Associate Professor Matthew Merritt from the University of Florida reported a new role for MUL1 where MUL1 co-regulates both Akt2 and HIF-1α protein level to maintain a proper metabolic state through K48 polyubiquitination. Researchers discovered and defined a brand-new role for mitochondrial MUL1 E3 ubiquitin ligase in the control of metabolism and mitochondrial respiration. They also identified how the concurrent control of Akt2 kinase and the HIF-1α transcription factor mediates the role of MUL1 in metabolism.

The research team used cells where MUL1 is inactivated to discover new functions for MUL1. They looked at the MUL1 mediated K48 ubiquitination, which targets substrates for proteasomal destruction. ULK1, MFN2, HIF-1α, and Akt2 proteins were shown to accumulate in MUL1(-/-) cells. Moreover, since activation of Akt2 and HIF-1α is associated with metabolic characteristics that favour glycolysis, a characteristic of cancer cells known as the Warburg effect, the authors investigated Akt2 and HIF-1 coregulation by MUL1. They reported that inactivating MUL1 causes enhanced glycolysis and the inhibition of OXPHOS. Additionally, steady-state flux rates demonstrate higher YPC and PK flux activity in MUL1(-/-) cells. Furthermore, their findings suggest that the metabolic phenotype of MUL1(-/-) cells differs from the glycolytic state seen in cancer cells (the Warburg effect), in which elevated levels of pyruvate dehydrogenase and lactate dehydrogenase flip OXPHOS into glycolysis.

To examine in more detail the metabolic pathways,  the authors compared the metabolite and gene expression profiles of HEK293 WT (which expresses MUL1) and MUL1(-/-) cells. According to the authors these two cell lines’ metabolite and gene expression profiles differ significantly from one another. Lipidomic analysis revealed that MUL1(-/-) cells significantly accumulated neutral head groups comprising lipids like triglycerides and diacylglycerides. Their research demonstrates that MUL1 is important in the control of metabolism and that its downregulation can result in a unique metabolic state that hasn’t been previously identified. The new study also established that Akt2 and HIF-1α are the primary drivers of the MUL1(-/-) metabolic phenotype; any potential function MFN2 and/or ULK1 may play in this mechanism will be downstream of these two proteins pathways.

In conclusion, Professor Antonis Zervos and colleagues discovered an important new role for the mitochondrial MUL1 E3 ubiquitin ligase in the control of metabolism. The K48-polyubiquitinating activity of the ligase and the coregulation of the Akt2 and HIF-1 proteins are key components of the pathway. This novel metabolic phenotype is driven and maintained by the accumulation and joint activation of Akt2 and HIF-1α in MUL1(-/-) cells. It is characterized by activated pyruvate carboxylation and PK flow as well as enhanced aerobic glycolysis. These findings suggest that MUL1 ligase plays a critical role in the control of lipogenesis and mitochondrial metabolism. In a statement to Medicine Innovates, Professor Antonis Zervos, the lead author said “ This is an important discovery since it integrates mitochondrial dynamics and mitophagy with metabolic regulation in a way that has not been previously described. In addition, this novel function of MUL1 will help us understand the metabolic reprogramming that occurs in many human cancers”.