BNIP3-dependent mitophagy promotes cytosolic localization of LC3B and metabolic homeostasis in the liver.

Mitophagy formed the basis of the original description of autophagy by Christian de Duve when he demonstrated that GCG (glucagon) induced macroautophagic/autophagic turnover of mitochondria in the liver. However, the molecular basis of liver-specific activation of mitophagy by GCG, or its significance for metabolic stress responses in the liver is not understood. Here we show that BNIP3 is required for GCG-induced mitophagy in the liver through interaction with processed LC3B; an interaction that is also necessary to localize LC3B out of the nucleus to cytosolic mitophagosomes in response to nutrient deprivation. Loss of BNIP3-dependent mitophagy caused excess mitochondria to accumulate in the liver, disrupting metabolic zonation within the liver parenchyma, with expansion of zone 1 metabolism at the expense of zone 3 metabolism. These results identify BNIP3 as a regulator of metabolic homeostasis in the liver through its effect on mitophagy and mitochondrial mass distribution.Abbreviations: ASS1, arginosuccinate synthetase; BNIP3, BCL2/adenovirus E1B interacting protein 3; CV, central vein; GCG - glucagon; GLUL, glutamate- ammonia ligase (glutamine synthetase); HCQ, hydroxychloroquine; LIR, LC3-interacting region; MAP1LC3B/LC3B, microtubule-associated protein 1 light chain 3 beta; mtDNA:nucDNA, ratio of mitochondrial DNA to nuclear DNA; PV, periportal vein; TOMM20, translocase of outer mitochondrial membrane protein 20.

Active and dynamic mitochondrial S-depalmitoylation revealed by targeted fluorescent probes.

The reversible modification of cysteine residues by thioester formation with palmitate (S-palmitoylation) is an abundant lipid post-translational modification (PTM) in mammalian systems. S-palmitoylation has been observed on mitochondrial proteins, providing an intriguing potential connection between metabolic lipids and mitochondrial regulation. However, it is unknown whether and/or how mitochondrial S-palmitoylation is regulated. Here we report the development of mitoDPPs, targeted fluorescent probes that measure the activity levels of "erasers" of S-palmitoylation, acyl-protein thioesterases (APTs), within mitochondria of live cells. Using mitoDPPs, we discover active S-depalmitoylation in mitochondria, in part mediated by APT1, an S-depalmitoylase previously thought to reside in the cytosol and on the Golgi apparatus. We also find that perturbation of long-chain acyl-CoA cytoplasm and mitochondrial regulatory proteins, respectively, results in selective responses from cytosolic and mitochondrial S-depalmitoylases. Altogether, this work reveals that mitochondrial S-palmitoylation is actively regulated by "eraser" enzymes that respond to alterations in mitochondrial lipid homeostasis.

Small molecules inhibit STAT3 activation, autophagy, and cancer cell anchorage-independent growth.

Triple-negative breast cancers (TNBCs) lack the signature targets of other breast tumors, such as HER2, estrogen receptor, and progesterone receptor. These aggressive basal-like tumors are driven by a complex array of signaling pathways that are activated by multiple driver mutations. Here we report the discovery of 6 (KIN-281), a small molecule that inhibits multiple kinases including maternal leucine zipper kinase (MELK) and the non-receptor tyrosine kinase bone marrow X-linked (BMX) with single-digit micromolar IC50s. Several derivatives of 6 were synthesized to gain insight into the binding mode of the compound to the ATP binding pocket. Compound 6 was tested for its effect on anchorage-dependent and independent growth of MDA-MB-231 and MDA-MB-468 breast cancer cells. The effect of 6 on BMX prompted us to evaluate its effect on STAT3 phosphorylation and DNA binding. The compound's inhibition of cell growth led to measurements of survivin, Bcl-XL, p21WAF1/CIP1, and cyclin A2 levels. Finally, LC3B-II levels were quantified following treatment of cells with 6 to determine whether the compound affected autophagy, a process that is known to be activated by STAT3. Compound 6 provides a starting point for the development of small molecules with polypharmacology that can suppress TNBC growth and metastasis.

Expanding perspectives on the significance of mitophagy in cancer.

Mitophagy is a selective mode of autophagy in which mitochondria are specifically targeted for degradation at the autophagolysosome. Mitophagy is activated by stresses such as hypoxia, nutrient deprivation, DNA damage, inflammation and mitochondrial membrane depolarization and plays a role in maintaining mitochondrial integrity and function. Defects in mitophagy lead to mitochondrial dysfunction that can affect metabolic reprogramming in response to stress, alter cell fate determination and differentiation, which in turn affects disease incidence and etiology, including cancer. Here, we discuss how different mitophagy adaptors and modulators, including Parkin, BNIP3, BNIP3L, p62/SQSTM1 and OPTN, are regulated in response to physiological stresses and deregulated in cancers. Additionally, we explore how these different mitophagy control pathways coordinate with each other. Finally, we review new developments in understanding how mitophagy affects stemness, cell fate determination, inflammation and DNA damage responses that are relevant to understanding the role of mitophagy in cancer.

In Brief: Mitophagy: mechanisms and role in human disease.

Mitophagy is a selective form of macro-autophagy in which mitochondria are specifically targeted for autophagic degradation. Mitophagy plays an important role in cellular homeostasis by eliminating dysfunctional mitochondria and reducing mitochondrial mass as an adaptive response to stress. Cells execute mitophagy through several non-redundant mechanisms, including the PINK1/Parkin partnership, which modulates turnover of depolarized mitochondria, and stress-induced BNIP3, NIX, and FUNDC1 molecular adaptors, which interact directly with LC3 to promote mitophagy. These pathways are deregulated in human diseases, including cancer, neurodegeneration, metabolic disorders, muscle atrophy, ageing, and inflammation, reflecting the importance of mitophagy as a cellular housekeeping function. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.