Hegde, V. | Search bioRxiv

bioRxiv2026

Eukaryotic Initiation Factor 5B (eIF5B)-Driven Translational Control Impacts Oral Squamous Cell Carcinoma Pathophysiology

The non-canonical translation of specific mRNAs has been implicated in oncogenesis and cancer progression. We previously identified eukaryotic Initiation Factor 5B (eIF5B) as a key factor in Internal Ribosome Entry Site (IRES)-mediated translation of a subset of mRNAs encoding anti-apoptotic proteins. Here, we demonstrate that EIF5B is predominantly expressed in cancer cells compared to other cell types in the Oral Squamous Cell Carcinoma (OSCC) microenvironment. Higher EIF5B mRNA and protein expression are associated with poor patient outcomes. We show that eIF5B depletion in OSCC cells blunted pro-growth, pro-inflammatory, and pro-angiogenic signaling pathways and significantly increased TNF-related apoptosis-inducing ligand (TRAIL)-induced cell death. This is achieved through decreased translation of mRNAs encoding critical factors associated with OSCC pathophysiology. Importantly, the level of interaction of eIF5B with tRNAiMet was significantly higher in OSCC cells compared to non-cancerous fibroblasts. This suggests that OSCC cells (but not non-cancerous fibroblasts) rely heavily on eIF5B for translation initiation. In an in vivo flank xenograft model using nude mice, eIF5B knockdown in UMSCC-29 cells led to a significant reduction in tumor volume compared to control tumors. Also, the immunohistochemical analysis of the xenografted tumor sections demonstrated decreased staining intensity of critical factors associated with OSCC pathophysiology in eIF5B-depleted tumors relative to controls. Collectively, our data demonstrate that OSCC cells are uniquely dependent on eIF5B-tRNA interactions to sustain translation of pro-survival mRNAs. Targeting eIF5B disrupts these oncogenic programs, sensitizing OSCC cells to apoptosis and suppressing pro-angiogenic and pro-growth signaling.

cancer biology↗

bioRxiv2024

Eukaryotic initiation factor 3F (eIF3F) regulates the IRES-mediated translation of Bcl-xL via its interaction with programmed cell death 4 (PDCD4) protein

Programmed cell death 4 (PDCD4) protein is a well-characterized tumor suppressor protein. PDCD4 inhibits mRNA translation by inhibiting the activity of an RNA helicase, eukaryotic initiation factor 4A (eIF4A). We have previously reported that PDCD4 interacts with the internal ribosome entry site (IRES) element that is found within the 5 untranslated region (UTR) of mRNA encoding B-cell lymphoma extra-large (Bcl-xL) protein. PDCD4s interaction with the Bcl-xL IRES element inhibits the IRES-mediated translation initiation on Bcl-xL mRNA. However, S6 kinase (S6K)-mediated phosphorylation of PDCD4 activates its degradation by proteasomal degradation pathway and derepress IRES-mediated translation initiation of Bcl-xL mRNA. Interestingly, eIF3F (one of the 13 subunits of eIF3) was reported to recruit S6K to phosphorylate eIF3. Therefore, we were intrigued by the possibility of co-regulation of PDCD4 and eIF3F by S6K and the regulation of IRES-mediated translation initiation by PDCD4-eIF3F. To this end, using co-immunoprecipitation (co-IP), we demonstrated that PDCD4 interacts with several subunits of eIF3. Reciprocal co-IP, endogenous IP, and in vitro pull-down assays demonstrated that eIF3F directly interacts with PDCD4 in an RNA-independent manner. In order to functionally characterize the PDCD4-eIF3F complex, we depleted PDCD4 from the glioblastoma (GBM) cells, which resulted in decreased levels of eIF3F. Also, depletion of eIF3F from GBM cells reduced the levels of PDCD4 protein. However, this was not observed in non-cancer cells. Overexpression of PDCD4 resulted in enhanced levels of eIF3F, and vice versa. We further confirmed that the interaction of eIF3F and PDCD4 proteins prevents each others proteasomal degradation. By performing RNA-IP, we showed that PDCD4 and eIF3F interact with Bcl-xL RNA independently. Moreover, our IRES-bi-cistronic reporter assay and polysome profiling experiments demonstrated that eIF3F regulates IRES-mediated translation of Bcl-xL mRNA, likely via its interaction with PDCD4. SignificanceThis study uncovers the fundamental mechanism of the internal ribosome entry site (IRES)- mediated translation regulation of B-cell lymphoma extra-large (Bcl-xL) mRNA by programmed cell death 4 (PDCD4) protein, and the eukaryotic initiation factor 3F (eIF3F). Our results show that PDCD4 and eIF3F interact with each other directly and they also interact with Bcl-xL mRNA independently. We show that PDCD4 works via eIF3F to regulate Bcl-xL levels. We also show that the PDCD4-eIF3F-dependent mechanism of Bcl-xL mRNA translation is implicated in glioblastoma (GBM) cells, including the primary brain tumor stem cells (BTSCs), and would likely affect the GBM pathophysiology.

molecular biology↗