NF‐κB Promotes Alternative Splicing of Bnip3 During the Cellular Adaptation to Hypoxia

Bnip3 is a hypoxia‐regulated gene that has been reported to promote either cell death or cell survival depending on the cellular context. The transcription factor NF‐κB has been previously shown to repress Bnip3 expression to promote cell survival. However, more recent data suggests Bnip3 is subjected to alternative splicing to produce pro‐death and pro‐survival isoforms, suggesting a molecular mechanism for regulating the balance between death and survival in hypoxic cells. Mitochondrial matrix calcium accumulation has been implicated in regulated necrotic cell death, while nuclear calcium signaling has been implicated in transcriptional activation, cellular adaptation, and hypertrophy. We hypothesize that alternative Bnip3 splice variants prevent cell death through calcium‐mediated signaling between the mitochondria and nucleus. Treatment of the human cell line (HCT‐116) with the pro‐survival prostaglandin receptor agonist, misoprostol, resulted in NF‐κB nuclear accumulation concurrent with reduced expression of full length Bnip3 (Bnip3FL) and increased expression of a Bnip3 splice variant lacking the third exon (Bnip3ΔExon3). To dissect these pathways, we overexpressing Bnip3FL and/or Bnip3ΔExon3, and measured the effects on cell viability, mitochondrial function, calcium signaling, and intracellular localization of select pro‐survival transcriptional regulators. Bnip3FL caused an 8‐fold increase in cell death, an effect rescued by expression of Bnip3ΔExon3. Using genetically‐encoded calcium biosensors targeted to either the endoplasmic reticulum (ER), mitochondria, or to the nucleus, we demonstrate that Bnip3ΔExon3 differentially regulates intracellular calcium signaling. Both Bnip3FL and Bnip3ΔExon3 induced calcium release from the ER, which was inhibited by pharmacological blockade of the inositol triphosphate (IP‐3) calcium channel. However, Bnip3FL preferentially induced mitochondrial calcium uptake, which was inhibited by Bnip3ΔExon3. Importantly, Bnip3ΔExon3 preferentially directed calcium to accumulate in the nucleus. Our data also demonstrates that Bnip3ΔExon3 preserved mitochondrial function by preventing Bnip3FL‐induced mitochondrial depolarization and by promoting closure of the permeability transition pore. Finally, we investigated whether Bnip3ΔExon3 could alter down‐stream calcium‐activated transcriptional regulators. Using fluorescent‐tagged proteins we observed nuclear accumulation of the transcription factor NFATc3 (nuclear factor of activated T‐cells) and nuclear export of the histone deacetlyase 5 (HDAC5), when co‐expressed with Bnip3ΔExon3. These findings indicate that Bnip3ΔExon3 may prevent cell death by promoting calcium‐dependent signaling pathways that converge on the nucleus, and provide a novel mechanism that may serve to protect cells from hypoxic injury.