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.

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.

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-X_L, p21^WAF1/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.

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.

Abstract 228: Tumor suppressor functions of BNIP3 and mitophagy in pancreatic ductal adenocarcinoma

The role of mitochondrial dysfunction in cancer is an emerging research field fuelled by the growing realization that mitochondrial dynamics are subject to oncogenic control, with mitochondrial biogenesis and mitophagy in particular modulated directly or indirectly by key oncogenes (c-Myc, B-Raf, K-ras) and tumor suppressors (p53, pRB). Work from our lab has focused on understanding how defects in mitophagy influence tumorigenesis with a particular focus on how BNIP3 loss during tumor progression influences disease outcome. BNIP3 functions as a dimer at the outer mitochondrial membrane to target mitochondria for degradation at the autophagolysosome through its interaction with processed LC3 at nascent phagophores. BNIP3 is a transcriptional target of HIF-1, E2F and FoxO3A transcription factors and is induced by activated Ras and suppressed by the RB and p53 tumor suppressors. Various studies have shown BNIP3 expression to be induced at early stages of human cancers and then lost as cancers progress to malignancy. In particular, epigenetic silencing of BNIP3 in human pancreas cancer has been correlated with reduced median survival. However, the mechanisms by which loss of BNIP3 may be contributing to cancer progression are not well delineated and a better understanding of how BNIP3-dependent mitophagy modulates pancreatic cancer in particular could provide new insight to the development of this deadly disease. We set out to examine how loss of BNIP3 affects the progression of pancreatic ductal adenocarcinoma using human cell lines and genetically engineered mouse models. We show that loss of BNip3 markedly accelerates tumor initiation and progression to invasive PDAC in the Pdx1-Cre;LSL-KRasG12D mouse model with metastases to the liver and lungs evident in BNip3 null mice but not in age-matched wild-type mice. Supporting a role for BNIP3 in suppressing pancreatic tumorigenesis, we also showed that re-expression of exogenous BNIP3 in Miapaca-2 cells (in which endogenous BNIP3 is silenced) resulted in reduced growth while knockdown of BNIP3 in CFPAC1 cells (that express BNIP3) promotes cell growth. Work described here examines the underlying mechanisms explaining these tumor suppressor functions of BNIP3 in pancreatic ductal adenocarcinoma.

Citation Format: Aparajita H. Chourasia, Maya Z. Springer, Kay F. Macleod. Tumor suppressor functions of BNIP3 and mitophagy in pancreatic ductal adenocarcinoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 228.