Exploring mTOR-dependent regulation of mRNA translation in cancer

Abstract

Modulation of mRNA translation via the mammalian target of rapamycin (mTOR) pathway is primarily achieved by integrating internal or external signals onto the translation machinery, especially on the rate-limiting initiation step during mRNA translation. Subsets of transcripts are discriminated by structural and/or sequence features, and encode proteins involved in different biological functions. For example, a group of mRNAs with a stretch of uninterrupted 4-15 pyrimidines following a cytosine after the m7Gppp cap at their 5’ untranslated region (5’UTR) are termed as TOP mRNAs, which mainly encode ribosomal proteins and translation factors. By developing a luciferase reporter to quantify TOP mRNA translation, this thesis provides insight into mTOR-dependent or -independent modulators of TOP mRNA translation. These studies also suggest a vast kinase repertoire potentially modulating TOP-mRNA translation (Paper II).

Aberrations in mTOR pathway drive tumorigenesis and development. The first-generation allosteric mTOR inhibitors, rapamycin analogs (also called rapalogs), and the second generation ATP-competitive kinase mTOR inhibitors have been tested in a wide range of tumors as monotherapy or a component of combination therapy. However, either the poor potency of the first generation or the toxicity of the second generation makes the clinical benefit limited. Based on the prototype of one third-generation mTOR inhibitor, RapaLink-1, we produced a series of new mTORC1-selective bi-steric inhibitors through continuous and finely tuned pharmaceutical and chemical modifications, which selectively inhibit mTORC1 over mTORC2, but retain potent efficacy in suppressing tumor growth (Paper I). Findings within Paper I demonstrate that mTORC1 mediates modulation of mRNA translation.

Immune cells within the tumor microenvironment (TME) represent an indispensable factor during tumor cells’ escape from immune surveillance, which occurs via multiple mechanisms, including immune cell acquisition of pro-tumor phenotypes. Therefore, immunotherapy, which re-activates the immune system towards cancer cells, has emerged as an essential treatment option. Paper IV indicates that immune suppression mediated by tumor-associated macrophage (TAM) depends on the MNK2/eIF4E axis but not mTOR. The model suggests that MNK2 thereby controls translation of a subset of transcripts encoding proteins which in turn modulate the TAM phenotype. This finding extends our understanding of how mRNA translation contributes to immune cell phenotypes.

Expression of a functional Von Hippel-Lindau Tumor Suppressor (VHL) protein is commonly lost in renal cell carcinoma (RCC). In RCC, there is ample prognosis heterogeneity among patients treated with rapalogs, which inhibits mTORC1. Thus, it could be due to acquired resistance. Therefore, We examined whether alterations in gene expression in response to rapalogs associated with VHL status (Paper III). As expected, VHL re-expression not only caused wide-spread changes in mRNA levels but also alterations in mRNA translation. Moreover, translation of transcripts subsets was sensitive to rapamycin only under VHL proficiency or VHL deficiency. Further studies will aim to determine whether these differences affect downstream phenotypes.