Induced mitochondrial deficit by NDUFS3 transient silencing reduces RAB7 expression and causes lysosomal dysfunction...

RAB7 is a small GTPase with multiple cellular roles, regulating late endocytic trafficking and lysosomal biogenesis, influencing mitochondria-lysosome crosstalk, and contributing to many mitochondrial processes. Mitochondrial dysfunctions are widely reported in cancer and the development of cancer therapeutic strategies targeting mitochondria gained momentum in recent years. Mitochondrial impairment can cause alterations of mitochondria-lysosome crosstalk and can influence lysosomal function. Here, we used cell models of pancreatic cancer, one of the deadliest cancers worldwide, to cause a transient mild mitochondrial deficit lowering NDUFS3 protein levels in order to investigate the consequences on RAB7 and on the late endocytic pathway and, thus, the contribution of the mitochondria-lysosomes communication alterations to cancer progression.

NDUFS3 and RAB7 downregulation was obtained by RNA interference (RNAi). Seahorse assays, Western blot analysis, mitochondrial staining, and Transmission Electron Microscopy (TEM) were used to assess silencing effects on mitochondrial structure and functioning. Western blotting was used to investigate expression of late endocytic pathway proteins and of the invasion marker vimentin. Confocal microscopy was used to analyze the mitochondrial network and lysosomal assessment. Zymography was performed to evaluate the ability to digest the extracellular matrix linked to cancer migration. SRB and colony assays were performed to assess cancer viability and proliferation. Wound healing assay and FluoroBlok membranes were used to determine migration and invasiveness.

In pancreatic cancer cells, transient silencing of the NDUFS3 protein caused mitochondrial deficit, slower oxidative metabolism, and mitochondrial morphology alterations. In this context, we observed RAB7 downregulation and impairment of the late endocytic pathway. In addition, NDUFS3-silenced RAB7-downregulated cells showed less invasive tumorigenic potential revealed by reduced levels of vimentin and other Epithelial-to-Mesenchymal Transition proteins, decreased viability, migration and invasiveness. Moreover, we found that modulation of RAB7 expression may regulate vimentin levels and influence mitochondrial morphology and levels of mitochondrial proteins.

Overall, our data show that mitochondrial deficit determines alterations of the crosstalk with lysosomes, leading to dysfunctions, and that this process is regulated by RAB7 acting as an oncogene. This highlights the synergic role of RAB7 and mitochondrial dysfunction, focusing on a cellular mechanism that may boost the effect of mitochondrial dysfunction in the cells, leading to the reduction of the tumorigenic potential.