Mitophagy is an evolutionarily conserved process that selectively targets impaired mitochondria for degradation. Defects in mitophagy are often associated with diverse pathologies, including cancer. Because the main known regulators of mitophagy are frequently inactivated in cancer cells, the mechanisms that regulate mitophagy in cancer cells are not fully understood. Here, we identified an E3 ubiquitin ligase (ARIH1/HHARI) that triggers mitophagy in cancer cells in a PINK1-dependent manner. We found that ARIH1/HHARI polyubiquitinates damaged mitochondria, leading to their removal via autophagy. Importantly, ARIH1 is widely expressed in cancer cells, notably in breast and lung adenocarcinomas; ARIH1 expression protects against chemotherapy-induced death. These data challenge the view that the main regulators of mitophagy are tumor suppressors, arguing instead that ARIH1-mediated mitophagy promotes therapeutic resistance.

CSF-1 and IL-34 share the CSF-1 receptor and no differences have been reported in the signaling pathways triggered by both ligands in human monocytes. IL-34 promotes the differentiation and survival of monocytes, macrophages and osteoclasts, as CSF-1 does. However, IL-34 binds other receptors, suggesting that differences exist in the effect of both cytokines. In the present study, we compared the differentiation and polarization abilities of human primary monocytes in response to CSF-1 or IL-34. CSF-1R engagement by one or the other ligands leads to AKT and caspase activation and autophagy induction through expression and activation of AMPK and ULK1. As no differences were detected on monocyte differentiation, we investigated the effect of CSF-1 and IL-34 on macrophage polarization into the M1 or M2 phenotype. We highlighted a striking increase in IL-10 and CCL17 secretion in M1 and M2 macrophages derived from IL-34 stimulated monocytes, respectively, compared to CSF-1 stimulated monocytes. Variations in the secretome induced by CSF-1 or IL-34 may account for their different ability to polarize naïve T cells into Th1 cells. In conclusion, our findings indicate that CSF-1 and IL-34 exhibit the same ability to induce human monocyte differentiation but may have a different ability to polarize macrophages.

Autophagy is the process by which superfluous or damaged macromolecules or organelles are degraded by the lysosome. Pharmacologic and genetic evidence indicates that autophagy plays pleiotropic functions in cellular homeostasis, development, survival, and differentiation. The differentiation of human blood monocytes into macrophages is a caspase-dependent process when triggered ex vivo by colony stimulating factor-1. We show here, using pharmacologic inhibitors, siRNA approaches, and Atg7 ؊/؊ mice, that autophagy initiated by ULK1 is required for proper colony stimulating factor-1-driven differentiation of human and murine monocytes. We also unravel a role for autophagy in macrophage acquisition of phagocytic functions. Collectively, these findings highlight an unexpected and essential role of autophagy during monocyte differentiation and acquisition of macrophage functions. (Blood. 2012;119(19):4527-4531) IntroductionAutophagy (macroautophagy) is a general mechanism for degradation and recycling of macromolecules that is characterized by the formation of double-membrane vesicles called phagosomes that derive from phagophores. 1 Autophagy plays a crucial role in cellular homeostasis, development, differentiation, cell death and survival, and ageing. [2][3][4] Although autophagy functions mainly as a safeguard mechanism during nutrient starvation, excessive autophagy leads to cell death. 5 Monocytes have the unique property to migrate into tissues in response to inflammation where they are subjected to differentiation into morphologically and functionally heterogeneous cells, such as macrophages, myeloid dendritic cells, and osteoclasts, depending on the stimulus. 6 The differentiation of human peripheral blood monocytes into macrophages can be reproduced ex vivo by exposure to colony stimulating factor-1 (CSF-1; also known as M-CSF), a process that requires limited activation of caspase-8 and caspase-3. 7 Binding of CSF-1 to its receptor CSF-1R triggers successive waves of AKT activation, leading to the formation of a caspase-8 activating platform and differentiation of monocytes into macrophages. 7 In the present study, we investigated the implication of autophagy in monocyte differentiation and acquisition of macrophage functions. Methods Human monocyte culture and differentiationHuman peripheral blood monocytes were obtained from healthy donors with informed consent following the Declaration of Helsinki according to recommendations of an independent scientific review board. Experiments on mice were performed with the approval of the ethics committee of the University of Nice. Human and mouse monocytes were enriched with autoMACS Separator (Miltenyi Biotec). Macrophage differentiation was visualized using standard optics (Carl Zeiss), electronic microscopy, flow cytometry, and phagocytic assay. A detailed description is in supplemental Methods (available on the Blood Web site; see the Supplemental Materials link at the top of the online article). Immunoblot assaysWestern blot analysis has been described previo...