The same Bcl2 protein is produced by two human Bcl2 transcript isoforms that present different mechanisms of post-transcriptional regulation.

The Bcl-2 family of proteins has an essential role in apoptosis since it comprises both anti- and pro-survival factors. Bcl-2 prevents apoptosis in response to stress conditions such as DNA damage and endoplasmatic reticulum stress. Deregulated expression of Bcl-2 is observed in a variety of tumours such as leukemia, linfoma, neuroblastoma and non-small-cell lung carcinoma. Additionally, increased cell survival due to elevated levels of Bcl-2 confers resistance to anticancer therapies. With such an important role in regulating programmed cell death, expression of Bcl-2 is highly regulated at multiple levels, both transcriptionally and posttranscriptionally. There are three human Bcl2 transcript isoforms, two of which encoding the same protein but differing in the 5´ untranslated region (UTR) length. The upstream promoter originates a 5´UTR longer-transcript which harbors an upstream open reading frame and an internal ribosome entry site (IRES) that regulate Bcl-2 protein production. IRES is an RNA structure present mainly in the 5’UTRs, that assists protein synthesis initiation without the requirement of the cap structure and some canonical translation initiation factors. Since the main feature of the IRES-mediated translation initiation is its cap-independence, it bypasses this regulation step and allows the cell to continue to produce some proteins, although the overall reduction of protein synthesis. The shorter Bcl-2 transcript isoform, encoding the same protein, lacks a 425-bp 5´ sequence. This shorter Bcl-2 5´UTR has not been analyzed for IRES activity until now. To test the shorter 5’UTR for IRES activity, we have transfected HEK293 cells with renilla/firefly luciferase dicistronic reporter plasmids where the firefly cistron is linked to the Bcl-2 shorter 5’UTR. We found no IRES activity in the Bcl-2 shorter 5´UTR, which was reflected by no firefly luciferase protein activity in luminescent assays. Accordingly, we conclude that in the Bcl-2 shorter mRNA isoform, the 425-bp missing sequence is essential for IRES activity. Since IRES activity has been assumed to be a backup mechanism to bypass stressful conditions in which overall protein synthesis is impaired and as these two transcription variants encode the same protein, it remains to be elucidated the biological relevance of both Bcl-2 mRNA isoforms having such different mechanisms of function