Abstract
The anti-apoptotic BAG-1 protein isoforms are known to be overexpressed in colorectal tumors and are considered to be potential therapeutic targets. The isoforms are derived from alternative translation initiations occuring at four in-frame start codons of a single mRNA transcript. Its 5′UTR also contains an internal ribosome entry site (IRES) regulating the cap-independent translation of the transcript. An RNA G-quadruplex (rG4) is located at the 5′end of the BAG-1 5′UTR, upstream of the known cis-regulatory elements. Herein, we observed that the expression of BAG-1 isoforms is post-transcriptionally regulated in colorectal cancer cells and tumors, and that stabilisation of the rG4 by small molecules ligands reduces the expression of endogenous BAG-1 isoforms. We demonstrated a critical role for the rG4 in the control of both cap-dependent and independent translation of the BAG-1 mRNA in colorectal cancer cells. Additionally, we found an upstream ORF that also represses BAG-1 mRNA translation. The structural probing of the complete 5′UTR showed that the rG4 acts as a steric block which controls the initiation of translation at each start codon of the transcript and also maintains the global 5′UTR secondary structure required for IRES-dependent translation.
Highlights
The BAG-1 protein (Bcl2-associated athanogene 1) was initially identified as an interactor of the anti-apoptotic protein BCL-2 [1], and is known to be an inhibitor of the intrinsic apoptotic pathway [2,3]
The RNA G4 (rG4) located in the 5 -untranslated region (5 UTR) of the BAG-1 mRNA was identified during an analysis of the rG4s associated with the colorectal cancer (CRC) pathway that could affect mRNA translation [30].The initial step was to confirm the post-transcriptional regulation of the BAG-1 mRNA in human CRC cells
We demonstrated that the expression of BAG-1 protein isoforms is controlled at a posttranscriptional level in colorectal cells and in tumors
Summary
The BAG-1 protein (Bcl2-associated athanogene 1) was initially identified as an interactor of the anti-apoptotic protein BCL-2 [1], and is known to be an inhibitor of the intrinsic apoptotic pathway [2,3]. Overall, depending on the interactions with the various partners, BAG-1 integrates signals from multiple pathways modulating survival, and gene transcription, cell proliferation, and growth [12]. All BAG-1 protein isoforms are translated from the same mRNA transcript via two mechanisms. The first mechanism is called ‘leaky scanning’ and it consists of differential rates of translation initiation at either one of the four inframe start codons present in the 501 nucleotides (nts) long 5 -untranslated region (5 UTR) of the mRNA [13,14], depending on the strength of the initiation context surrounding the start codon.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
