Bcl-2 nineteen-kilodalton interacting protein (BNIP3) is a BH-3-only Bcl-2 family member whose expression levels increase during stress such as hypoxia through hypoxia-inducing factor-1-dependent or -independent mechanisms. When BNIP3 expression is induced, it localizes to the mitochondria and triggers a loss of membrane potential, and an increase in the reactive oxygen species production, which often leads to cell death. Cells under normal growth conditions suppress BNIP3 expression through transcriptional repression. There is considerable debate in the literature regarding what type of cell death is induced by BNIP3. It has been observed that BNIP3 could induce necrosis, autophagy and/or apoptosis. In contrast, other studies indicate that BNIP3 could promote cell survival. Besides its cell death regulation, BNIP3 plays a key role in the pathogenicity of many diseases. In cardiac infarction, loss of BNIP3 expression has been shown to reduce the number of damaged cardiomyocytes after ischemia and reperfusion. BNIP3 expression also plays an important role in the deregulation of cell death in many cancers. In this review, we will discuss the different and often contradictory mechanisms of BNIP3 regulation of cell death and the role that BNIP3 may play in diseases. Bcl-2 nineteen-kilodalton interacting protein (BNIP3) belongs to the Bcl-2 homology domain (BH3)-only Bcl-2 family members because it only contains a putative BH3 domain. 1 The other major domains found in BNIP3 are the PEST domain that targets BNIP3 for degradation, a conserved domain that is conserved from Caenorhabditis elegans to humans and a transmembrane (TM) domain, which targets BNIP3 to the mitochondria (Figure 1). 2 BH3-only containing proteins act as rheostats regulating apoptosis through their BH3 domain by binding to antiapoptotic Bcl-2 family members. However, BNIP3 differs from these members, as its BH3 domain fails to interact with antiapoptotic Bcl-2 family members. 3 Furthermore, deletions of the BH3 domain fail to affect the ability of BNIP3 to induce cell death. 4 Unlike other BH3-only family members, BNIP3 interacts with Bcl-2 and Bcl-X L through its TM domain and N terminus (amino acids 1-49). 3 Deletion of the TM domain blocks the ability of BNIP3 to induce cell death. 4 BNIP3 migrates at 30 and 60 kDa, indicating that BNIP3 is a protein that forms homodimers. This homodimerization is primarily through the TM domain of BNIP3. The unique structure of the TM domain suggests that BNIP3 dimers could act as proton channels in the outer mitochondrial membrane increasing ion conductance. 5 Serine 172 and histidine (His) 173 are residues that are present in the dimerization interface of BNIP3. These residues interact by hydrogen bonds and are essential for dimer formation. 6 Furthermore, mutation of the His 173 to alanine completely abrogated the ability of BNIP3 to induce cell death. 7 Bcl-2 and Bcl-X L can compete for binding to the TM domain, which blocks BNIP3 homodimerization and abrogates the ability of BNIP3 to induce cell death (Fi...

The Bcl-2 nineteen kilodalton interacting protein 3 (BNIP3) is a hypoxia-inducible proapoptotic member of the Bcl-2 family that induces cell death by associating with the mitochondria. Under normal conditions, BNIP3 is expressed in skeletal muscle and in the brain at low levels. In many human solid tumors, BNIP3 is upregulated in hypoxic regions but paradoxically, this BNIP3 expression fails to induce cell death. Herein, we have determined that BNIP3 is primarily localized to the nucleus of glial cells of the normal human brain, as well as in the malignant glioma cell line U251. Upon exposure of U251 cells to hypoxia, BNIP3 expression in the cytoplasm increases and localizes with the mitochondria, contributing to induction of cell death. In contrast, when BNIP3 is forcibly over expressed in the nucleus, it fails to induce cell death. Expression of N-terminal BNIP3 (lacking the transmembrane and conserved domains) in U251 cells blocks hypoxia-induced cell death acting as a dominant negative protein by binding to wildtype BNIP3 and blocking its association with the mitochondria. In glioblastoma multiforme (GBM) tumors, BNIP3 expression is increased in hypoxic regions of the tumor and is primarily localized to the nucleus in 80% of tumors. Hence, BNIP3 is sequestered in the nucleus within the brain but under hypoxic conditions, BNIP3 becomes primarily cytoplasmic, promoting cell death. In GBMs, BNIP3 expression is increased but it remains sequestered in the nucleus in hypoxic regions, thereby blocking BNIP3's ability to associate with the mitochondria, providing tumor cells with a possible survival advantage. ' 2005 Wiley-Liss, Inc.Key words: BNIP3; astrocytes; glial cells; glioblastoma multiforme; hypoxia; mitochondria; cell death; nuclear localization The Bcl-2 nineteen kilodalton interacting protein 3 (BNIP3) is a pro-cell death Bcl-2 family member that localizes to the mitochondria upon overexpression, causing opening of the permeability transition (PT) pore, loss of mitochondrial membrane potential (Dwm) and reactive oxygen species (ROS) production resulting in cell death. 1 BNIP3 also induces an autophagic response in cells contributing to cell death. 2,3 BNIP3-induced cell death is independent of mitochondrial cytochrome c release and caspase activation. 1,4 The BNIP3 protein has 4 major domains: a PEST domain that targets BNIP3 for degradation, a putative BH3 (Bcl-2 homology 3) domain that is homologous to other Bcl-2 family members, a CD (conserved) domain that is conserved from C. elegans to humans, and a TM (transmembrane) domain, which targets BNIP3 to the mitochondria and is essential for BNIP3-induced cell death. [5][6][7] Deletion of the CD or BH3 domains fails to affect BNIP3's ability to induce cell death, while deletion of the TM domain blocks BNIP3-induced cell death because of its inability to associate with the mitochondrial membrane. 8 BNIP3 is expressed in normal tissues such as skeletal muscle and brain and its expression is increased under hypoxic conditions correlating with hypoxia-induced...

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The Bcl-2 19 kDa interacting protein (BNIP3) is a pro-cell-death BH3-only member of the Bcl-2 family. We previously found that BNIP3 is localized to the nucleus in the majority of glioblastoma multiforme (GBM) tumors and fails to induce cell death. Herein, we have discovered that nuclear BNIP3 binds to the promoter of the apoptosis-inducing factor (AIF) gene and represses its expression. BNIP3 associates with PTB-associating splicing factor (PSF) and HDAC1 (histone deacetylase 1) contributing to transcriptional repression of the AIF gene. This BNIP3-mediated reduction in AIF expression leads to decreased temozolomide-induced apoptosis in glioma cells. Furthermore, nuclear BNIP3 expression in GBMs correlates with decreased AIF expression. Together, we have discovered a novel transcriptional repression function for BNIP3 causing reduced AIF expression and increased resistance to apoptosis. Thus, nuclear BNIP3 may confer a survival advantage to glioma cells and explain, in part, why BNIP3 is expressed at high levels in solid tumors, especially GBM.