One aspect of this project relates to determining the role of the ubiquitin system in mitochondrial membrane dynamics and how this in turn relates to cellular energetics in cancer and to apoptosis. This project was begun in yeast and is now being expanded to mammalian systems. We have determined that in yeast that the E3 SCF-Mdm30 plays a critical role in targeting the mitofusin, Fzo1, for degradation and that this degradation is integral to the process of mitochondrial outer membrane fusion. In mammals we have established a pathway leading from cell stress to activation of JNK, which leads to phosphorylation of mammalian mitofusin 2 (Mfn2). This in turn leads to the recruitment of the large HECT domain ubiquitin ligase Huwe1, Mfn2 is then ubiquitinated and degraded by the proteasome leading to mitochondrial fragmentation and cell death. Thus we have established a new pathway from cell stress to degradation of a specific mitochondrial protein to apoptosis. Ongoing work on Mfn2 relates to understanding the role of other ubiquitin ligases in regulating Mfn2 degradation. Other newly initiated studies in mitochondria are focused on the degradation of other mitochondrial membrane proteins by the ubiquitin-proteasome system. A major aspect of this project relates to degradation from the endoplasmic reticulum. Together with collaborators we are studying E2s and E3s critical to this process. gp78/RNF45, also known as the tumor autocrine motility factor receptor (AMFR), was discovered by our laboratory to be an ubiquitin ligase resident to the endoplasmic reticulum. We have determined that gp78 plays essential roles in the degradation of multiple substrates functioning together with an E2 known as MmUBC7 (Ube2G2). We have now determined that multiple domains within gp78 function together to mediate its ubiquitin ligase activity. These include its RING finger, an ubiquitin-binding Cue domain and a novel region that specifically recruits Ube2g2 independent of the RING finger, referred to as the G2BR. Moreover, we now know that expressing the G2BR in isolation can block ERAD and induce ER stress. We are exploring the potential for such expression or other means of blocking the interaction between the gp78 and Ube2g2 as a means to target cells that are predisposed to ER stress, such as multiple myeloma cells, to undergo apoptosis. gp78 levels are correlated with the metastatic potential of tumors including melanomas and lung cancers. In vivo studies in mice using knockdowns of gp78 and re-expression in cells, in xenograft models, have now determined that gp78 does in fact play an important role in the metastatic potential of multiple different sarcomas and that this potential for metastasis requires intact ubiquitin ligase function of this protein. We have also determined that gp78 targets the metastasis suppressor, KAI1 (CD82) for degradation in sarcomas. This provides at least a partial explanation for our findings. We have generated mouse models to study gp78, studies are underway to determine the role of this ubiquitin ligase in breast cancer including in the metastasis of spontaneously arising tumors. gp78 has been reported in the literature as being responsible for the targeting of the rate-limiting enzyme in cholesterol synthesis, HMG-CoA reductase, for degradation. This was an important finding in considering approaches to regulation of sterols. However, we have found, through a thorough analysis of multiple different cell lines, that this observation is an artifact. On the other hand, gp78 does target Insig-1, a major negative regulator of sterol biosynthesis, for degradation. This now makes gp78 an attractive target, and an alternative to statins, for treatment of hypercholesterolemia. gp78 represents a potentially important target in both cancer and metastasis. Importantly, gp78 has also been reported to target the major liver cytochrome oxidase, CYP34A for degradation. Studies in mouse models are ongoing to evaluate the in vivo significance of this finding.
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