Phosphorylation independent eIF4E translational reprogramming of selective mRNAs determines tamoxifen resistance in breast cancer

Abstract

Eukaryotic translation initiation factor 4E (eIF4E) selectively promotes translation of mRNAs with atypically long and structured 5′-UTRs and has been implicated in drug resistance. Through genome-wide transcriptome and translatome analysis we revealed eIF4E overexpression could promote cellular activities mediated by ERα and FOXM1 signalling pathways. Whilst eIF4E overexpression could enhance the translation of both ERα and FOXM1, it also led to enhanced transcription of FOXM1. Polysome fractionation experiments confirmed eIF4E could modulate the translation of ERα and FOXM1 mRNA. The enhancement of FOXM1 transcription was contingent upon the presence of ERα, and it was the high levels of FOXM1 that conferred Tamoxifen resistance. Furthermore, tamoxifen resistance was conferred by phosphorylation independent eIF4E overexpression. Immunohistochemistry on 134 estrogen receptor (ER + ) primary breast cancer samples confirmed that high eIF4E expression was significantly associated with increased ERα and FOXM1, and significantly associated with tamoxifen resistance. Our study uncovers a novel mechanism whereby phosphorylation independent eIF4E translational reprogramming in governing the protein synthesis of ERα and FOXM1 contributes to anti-estrogen insensitivity in ER + breast cancer. In eIF4E overexpressing breast cancer, the increased ERα protein expression in turn enhances FOXM1 transcription, which together with its increased translation regulated by eIF4E, contributes to tamoxifen resistance. Coupled with eIF4E translational regulation, our study highlights an important mechanism conferring tamoxifen resistance via both ERα dependent and independent pathways.

FASN inhibition as a potential treatment for endocrine-resistant breast cancer

“…This results in protein translation inhibition, which could explain the decrease in ERα protein levels observed upon FASN inhibition. This EnRS response to FASN inhibition was only observed in tamoxifen-resistant breast cancer; however, the speci c response could be due to the increased reliance upon translational machinery that has been recorded previously in acquired tamoxifen resistance [30,31]. Intriguingly, the increased expression of ROR2 recorded in the phospho-RTK assay could be a potential mediator of endoplasmic reticulum stress.…”

“…Pathways upregulated in tamoxifen resistance, such as mTOR, involve the phosphorylation of translational inhibitory complexes such as 4E binding proteins (4E-BP1) leading to an alleviation of eIF4E cap-binding protein. The mRNA translational initiation complex is composed of several eIF4F family members that include an RNA helicase (eIF4A), scaffolding protein (eIF4G), and the rate-limiting cap-binding protein (eIF4E) [30,31]. Moreover, many oncoproteins and growth factors such as cyclin D1 and vascular endothelial growth factor (VEGF) are encoded by mRNAs with longer than average 5' untranslated regions (UTRs) [30].…”

“…The mRNA translational initiation complex is composed of several eIF4F family members that include an RNA helicase (eIF4A), scaffolding protein (eIF4G), and the rate-limiting cap-binding protein (eIF4E) [30,31]. Moreover, many oncoproteins and growth factors such as cyclin D1 and vascular endothelial growth factor (VEGF) are encoded by mRNAs with longer than average 5' untranslated regions (UTRs) [30]. Translational complexes, such as eIF4E, have an enhanced a nity for longer UTRs.…”

“…Not only does p-eIF2α lead to a decrease in protein translation, but also allows the initiation complex to continue down the mRNA until it reaches the alternate reading frame [31]. The alternate reading frame of mRNA is the location of translation for EnRS response proteins, such as CHOP and ATF-4 [30,31]. Thus, the overexpression of ROR2 from treatment with TVB and tamoxifen could potentially be contributing to endoplasmic reticulum stress.…”

“…6a). In periods of EnRS, the protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) is activated resulting in serine phosphorylation of the eukaryotic initiation factor-2 complex in the alpha subunit (eIF2α) [30]. This phosphorylation results in a competitive inhibition of the guanine nucleotide exchange factor for its beta subunit (eIF2β), ultimately leading to an inhibition of protein translation [31].…”