Abstract PR009: mascRNA directs nutrient-insensitive activation of the LARS-mTOR pathway in breast cancer
Authors: Park Bohye , Stone Joshua K , Lim Steve , Harada Shuko , Ahn Erin 
Published: November 2024

Cellular growth and metabolism are tightly regulated by nutrient availability, yet cancer cells frequently lose these controls, enabling unchecked proliferation even in the absence of sufficient nutrients. While non-coding RNAs are known to regulate metabolic enzyme expression, their direct role in nutrient-sensing pathways in cancer remains unclear. MALAT1-associated small cytoplasmic RNA (mascRNA) is a 61-nucleotide small RNA with a tRNA-like structure, cleaved from the 3’ end of the long non-coding RNA MALAT1. mascRNA has been shown to promote hepatocellular carcinoma proliferation and enhance protein translation in HEK293 cells, though its underlying molecular mechanism is not fully understood. We assessed mascRNA levels in over 70 human breast cancer patient samples using BaseScope assays and found that mascRNA is detectable in the cytoplasm of tumor cells, but not in normal mammary tissues or stromal cells adjacent to tumors. However, the levels of mascRNA varied significantly among patient samples, irrespective of molecular subtypes or tumor stages, suggesting that mascRNA is associated with other aspects of tumor biology. Functional studies in breast cancer cell lines revealed that mascRNA overexpression promotes cell proliferation, invasion, and metabolic pathways linked to ATP generation. Specifically, mascRNA increases basal glycolysis and mitochondrial oxidative phosphorylation and significantly enhances mitochondrial spare capacity, particularly in triple-negative breast cancer (TNBC) cell lines. We also found that mascRNA overexpression activates both mTORC1 and AMPK, indicating a deviation from the normal negative feedback loop between these two pathways. To dissect the molecular mechanism, we performed mascRNA pull-down assays and mass-spectrometric analyses. We identified RNA splicing factors, tRNA-modifying enzymes, and several aminoacyl-tRNA synthetases as mascRNA-interacting proteins. Among these, leucyl-tRNA synthetase (LARS), a component of the multi-tRNA synthetase complex, is known for its non-canonical role in activating mTORC1 at the lysosomal membrane. We found that, in MDA-MB-231 TNBC cells, mascRNA overexpression led to increased LARS localization to the lysosomal membrane where it can activate mTORC1. Interestingly, while control MDA-MB-231 cells responded to glucose and leucine availability to regulate the lysosomal localization of LARS and mTOR, mascRNA- overexpressing cells constitutively maintained LARS and mTOR localization to the lysosome, as well as mTORC1 phosphorylation, regardless of nutrient status. In summary, our data demonstrate that mascRNA aberrantly activates the LARS-mTORC1 axis in breast cancer cells independent of nutrient levels, likely through direct interaction with LARS, promoting its localization to the lysosome. This suggests that mascRNA enables breast cancer cells, and possibly other cancer types, to sustain uncontrolled growth even in nutrient-deprived conditions. Elevated mascRNA levels may thus represent a highly efficient metabolic adaptation of cancer cells.

Citation Format: Bohye Park, Joshua K Stone, Steve Lim, Shuko Harada, Erin Ahn. mascRNA directs nutrient-insensitive activation of the LARS-mTOR pathway in breast cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: RNAs as Drivers, Targets, and Therapeutics in Cancer; 2024 Nov 14-17; Bellevue, Washington. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(11_Suppl):Abstract nr PR009.