Rab2 Revealed: Deciphering the Golgi’s Role in Cancer through the Lens of Noncanonical Autophagy

The Golgi complex, a central component of the cell’s secretory pathway, is involved in various cellular functions like protein modifications, lipid sorting, and signaling. The study highlights the role of the small GTPase Rab2, predominantly located in the cis/medial Golgi and the ER-Golgi intermediate compartment (ERGIC). Rab2 is critical for maintaining the Golgi’s structural integrity, and its overexpression is linked to changes in Golgi morphology, commonly observed in several human cancers.

In a new study published in Experimental Cell Research Journal by Associate Professors Ellen Tisdale and Cristina Artalejo from the Department of Pharmacology at Wayne State University School of Medicine, the authors investigated how Rab2 “gain of function” affects the structure and activity of membrane compartments in the early secretory pathway, potentially contributing to oncogenesis. They used Normal Rat Kidney (NRK) cells as their model system. These cells were transfected with Rab2B cDNA to overexpress Rab2B, a variant of Rab2. After transfection, they observed the morphology of pre- and early Golgi compartments using immunofluorescence and microscopy. They assessed the impact of Rab2B overexpression on the transport rate of Vesicular Stomatitis Virus Glycoprotein (VSV-G) in the secretory pathway. The researchers monitored the cells for the presence of the autophagic marker protein LC3, especially focusing on LC3-lipidation on Rab2-containing membranes. They conducted both morphological and biochemical studies to confirm LC3-lipidation.

The authors further investigated the role of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in the process of LC3-lipidation. They explored the dependency of LC3 recruitment on GAPDH, which used a non-canonical LC3-conjugation mechanism. Moreover, the researchers examined the association between Golgi structural alterations and changes in Golgi-associated signaling pathways. A particular focus was on assessing elevated Src activity in cells overexpressing Rab2. Furthermore, the team investigated the specifics of the noncanonical autophagy mechanism utilized in LC3-lipidation on Rab2-containing membranes. They explored the independence of this process from upstream regulators of autophagy and nutrient status.

The researchers observed that overexpression of Rab2B in NRK cells led to significant alterations in the morphology of pre- and early Golgi compartments. This overexpression reduced the transport rate of the reporter molecular VSV-G in the secretory pathway, indicating Rab2’s pivotal role in cellular homeostasis and membrane trafficking. Furthermore, these morphological changes in Golgi structure are associated with altered signaling pathways, notably an increase in Src kinase activity, which is a potential contributor to oncogenic processes.

When the authors investigated the relationship between Rab2 overexpression and autophagy, particularly focusing on the autophagic marker protein LC3, the researchers found that Rab2 ectopic expression stimulates LC3-lipidation on Rab2-containing membranes, employing a non-canonical LC3-conjugation mechanism that is non-degradative. This process is essential for the cell to maintain altered Golgi structures, implicating a protective role in membrane homeostasis. A novel finding of the study is the involvement of GAPDH in the LC3-lipidation process.  GAPDH, traditionally viewed as a cytosolic enzyme, interacts with Rab2 to mediate the recruitment of LC3 and vacuolar- ATPase assembly to membrane compartments, playing a vital role in noncanonical autophagy. This finding is significant as GAPDH is overexpressed in various cancer types, suggesting a possible link to cancer pathogenesis through its involvement in noncanonical autophagy.  Dr. Artalejo, a trained neuroscientist notes that these studies are equally applicable to neurodegenerative diseases where GAPDH plays a role and Golgi fragmentation occurs.

In conclusion, the study by Tisdale and Artalejo expand our understanding of the role of Rab2 in noncanonical autophagy and its implications for oncogenesis. The discovery of GAPDH’s involvement in this process adds an additional layer of complexity to our understanding of cellular mechanisms in cancer. The role of Rab2 in Golgi morphology alterations and its interaction with LC3 and GAPDH opens new avenues for cancer research. It suggests potential targets for therapeutic interventions, especially in cancers where Rab2 overexpression is evident.