The pro‐cell death Bcl‐2 family member, BNIP3, is localized to the nucleus of human glial cells: Implications for glioblastoma multiforme tumor cell survival under hypoxia

Abstract

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 wild‐type 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.