# Differences in Starvation-Induced Autophagy Response and miRNA Expression Between Rat Mammary Epithelial and Cancer Cells: Uncovering the Role of miR-218-5p

## TL;DR

This study compares how healthy and cancerous rat mammary cells respond to starvation, focusing on the role of miR-218-5p in promoting cancer cell survival.

## Contribution

The study identifies miR-218-5p as a novel regulator of starvation-induced autophagy in breast cancer cells via its target SNX18.

## Key findings

- Starvation-induced autophagy increases cancer cell migration and invasion while reducing proliferation.
- miR-218-5p is upregulated in cancer cells under starvation and inhibits SNX18, promoting survival.
- A starvation-related miRNA signature was identified, with miR-218-5p showing BC-specific dysregulation.

## Abstract

Breast cancer is a complicated disease with varying molecular processes driving its malignant characteristics. Autophagy, the process by which cells break down old and damaged proteins and other substances, allows them to survive stressful conditions, such as nutrient deprivation. This process is regulated by various small, non-coding ribonucleotides, such as miRNAs. In this study, we demonstrate that rat mammary gland cancer cells exhibit distinct nutrient starvation responses compared to healthy mammary epithelial cells. Our results provide insights into the regulatory role of miRNAs in cancer cells’ resistance to starvation and their use of autophagy. We focus specifically on providing a mechanistic description of the role of miR-218-5p in the starvation response.

Background: Breast cancer (BC) is highly heterogeneous, with varying molecular characteristics, such as reliance on autophagy. Autophagy is a critical cellular degradation process that helps cells survive under stress, but its regulation can be influenced by altered microRNA (miRNA) expression. Studying miRNA changes during starvation-induced autophagy in both mammary epithelial cells and BC cells could reveal potential molecular therapy targets. Methods: Rat mammary gland healthy epithelial and cancer cells were subjected to starvation, and differences in proliferation, migration, invasion, autophagy, and expression of autophagy-associated miRNAs were determined. Afterward, we assessed the effects of miR-218-5p modulation on the aforementioned processes. Results: Starvation-induced autophagy reduced the proliferation of all cells and increased the invasive and migratory capacity of cancer cells (p ≤ 0.05). We identified a miRNA signature related to starvation, comprising twenty-seven miRNAs. One miRNA had a significantly elevated baseline expression, while another six, including miR-218-5p, had a significantly lower basal expression in cancer cells compared to healthy cells (p ≤ 0.05). However, starvation caused significant miRNA expression changes, with miR-218-5p being upregulated specifically in cancer cells (p = 0.20–0.01). Functional studies on the role of miR-218-5p show that its inhibition decreases migration and leads to autophagosome accumulation. The study of miR-218-5p molecular targets has shown that its inhibition of sorting nexin 18 (SNX18) may act as an important regulator of the starvation-induced response in cancer cells. Conclusions: The baseline expression of miRNA related to starvation and autophagy differs between rat mammary gland cancer and healthy cells. The response to starvation also varies between cancer cells and normal cells. Starvation induces BC-specific miRNA dysregulation, affecting particularly miR-218-5p, which acts via SNX18, promoting the cancer cells’ survival.