Background: Mutations and alternative splicing often silence tumor suppressor gene expression, promoting tumor development. Results: A novel Bax isoform (Bax⌬2) generated from the combination of a microsatellite deletion and unexpected splicing promotes unconventional cell death. Conclusion: Bax⌬2 is an MSI tumor-specific pro-death Bax isoform. Significance: Alternative splicing of mutated gene can restore tumor suppressor function and can be detrimental to the tumors.

Mutation or aberrant splicing can interrupt gene expression. Tumor suppressor Bax is one of the susceptible genes prone to microsatellite frameshifting mutations in coding regions. As a result, tumors exhibiting microsatellite instability (MSI) often present a "Bax-negative" phenotype. We previously reported that some Bax-negative cells in fact contain a functional Bax isoform (BaxΔ2), generated when unique alternative splicing "salvages" the shifted reading frame introduced by a microsatellite mutation. Here we compared Bax alternative splicing profiles in a range of cell lines and primary tumors with and without Bax microsatellite mutations. We found that MSI tumors exhibit a high Bax alternative splicing frequency, especially in exon 2, and produce a family of alternatively spliced isoforms that retain many important Bax functional domains. Surprisingly, these BaxΔ2 family isoforms can rescue Bax from all common microsatellite frameshift mutations. Production of BaxΔ2 requires specific cis mutations, while trans components are not cell-type specific. Furthermore, all BaxΔ2 family isoforms are more potent cell death inducers than the parental Bax without directly targeting mitochondria. These results indicate that the BaxΔ2 family can potentially salvage Bax tumor suppressor expression otherwise lost to mutation.

Androgen receptor (AR) is able to promote stress-induced cell death independently of its transcription activity in androgen-independent prostate cancer cells. Yet, the underlying mechanism is incompletely understood. Here, we report that stress-induced proteasomal degradation of AR contributes to its pro-death activity. Upon exposure to ultraviolet light and staurosporine, AR underwent proteasomal degradation. Blockade of AR degradation significantly suppressed stress-induced apoptosis in androgen-independent prostate cancer cells. Ectopic expression of the AR N-terminal (AR-N) domain, which lacks DNA- and ligand-binding abilities, led to cell death without any additional death stimuli. Truncation analysis revealed that AR-N domain contains several sub-domains that regulate the pro-death activity of AR, specifically the first 105 amino acids, which function as a minimal pro-death domain acting upstream of caspases. The pro-apoptotic activity of AR N-terminal fragments was suppressed by ectopic expression of Bcl-2 or selected caspase inhibitors. Thus, our results reveal a novel mechanism by which AR promotes stress-induced cell death in androgen-independent prostate cancer cells.

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