Pro-apoptotic Bim Induction in Response to Nerve Growth Factor Deprivation Requires Simultaneous Activation of Three Different Death Signaling Pathways

Abstract

Bim is a pro-apoptotic member of the Bcl-2 family that is induced and contributes to neuron death in response to nerve growth factor (NGF) deprivation. Past work has revealed that Bim is downstream of multiple independent transcriptional pathways in neurons, including those culminating in activation of the c-Jun, FoxO, and Myb transcription factors. This study addresses the issue of whether the three signaling pathways are redundant with respect to Bim induction or whether they act cooperatively. Examination of the proximal Bim promoter reveals binding sites for FoxO, Mybs, and, as shown here, c-Jun. We find that mutation of any one of these types of sites abolishes induction of a Bim promoter-driven reporter in response to NGF deprivation. Moreover, down-regulation of either c-Jun, FoxOs, or Mybs by short hairpin RNAs blocks induction of Bim promoter-reporter activity triggered by withdrawal of NGF. This was the case for reporters driven by either the proximal promoter or a promoter that also includes additional regulatory elements in the first intron of the Bim gene. Such short hairpin RNAs also suppressed the induction of endogenous Bim protein. These findings thus indicate that the Bim promoter acts as a coincidence detector that optimally responds to the simultaneous activation of three different pro-apoptotic transcriptional pathways. Such a mechanism provides a "fail-safe" that prevents neurons from dying by accidental activation of any single pathway. It also permits neurons to utilize individual pathways such as JNK signaling for other purposes without risk of demise.

Statins induce apoptosis in ovarian cancer cells through activation of JNK and enhancement of Bim expression

“…Further investigation revealed that the expres sion of Bim, a proapoptotic protein, is signiWcantly induced in the cells by statins. These Wndings are consistent with the observation that Bim induction in neurons deprived of NGF is regulated by c-Jun [42]. SP600125 is a competitive inhibitor of ATP binding to JNK and some other kinases [43].…”

AMP kinase–mediated activation of the BH3-only protein Bim couples energy depletion to stress-induced apoptosis

“…Similarly, treatment with the pharmacological AMPK inhibitor compound C afforded protection against excitotoxic toxic to nonneuronal cells (Meisse et al, 2002;Kefas et al, 2003). Therefore, AMPK can be added to the growing list of kinases and transcription factors that have dual functions in the regulation of cell survival or cell death depending on the type of stress and type of cell, as reported for JNK, Foxo3a, and NF-B (nuclear factor B; O'Neill and Kaltschmidt, 1997;Biswas et al, 2007;Mojsilovic-Petrovic et al, 2009). A key task for future studies will be to elucidate the mechanisms of cell fate switches by these signaling molecules.…”

“…Several different factors have been implicated in the regulation of the bim gene in neurons, including JNK/c-Jun (Harris and Johnson, 2001;Whitfield et al, 2001), Foxo3a (Sunters et al, 2003;Biswas et al, 2007), and C-myb (Biswas et al, 2007). Previous studies have demonstrated the activation of JNK during excitotoxic injury and the neuroprotection achieved by JNK inhibition in these scenarios (Borsello et al, 2003;Chen et al, 2003;Centeno et al, 2007).…”

“…Indeed, AMPK activation has also been linked to subsequent downstream JNK activity during apoptosis (Meisse et al, 2002;Kefas et al, 2003;Lee et al, 2008). However, it should be noted that the regulation of bim expression is highly complex (Biswas et al, 2007), and other pathways independent of JNK signaling may well be required for Bim-dependent excitotoxic apoptosis. A recent study has shown that AMPK can phosphorylate and enhance the transcriptional activity of Foxo3a (Greer et al, 2007).…”

A feed-forward loop involving Trib3, Akt and FoxO mediates death of NGF-deprived neurons

“…4,5 Active Akt phosphorylates FoxOs, causing their cytoplasmic retention while loss of Akt activity results in dephosphorylation of FoxO proteins and their consequent nuclear translocation where they activate pro-apoptotic genes, such as Bim. 6,7 In addition to FoxOs, several other transcription factors and their associated pathways contribute to neuron death caused by NGF deprivation. These include the Jun N-terminal kinase (JNK)/c-Jun pathway, the apoptotic cell-cycle pathway, NF-Y and p53/p63/p73.…”