BETi enhance ATGL expression and its lipase activity to exert their antitumoral effects in triple-negative breast cancer (TNBC) cells
Authors: Rossi Teresa , Zamponi Raffaella , Chirico Mattea , Pisanu Maria Elena , Iorio Egidio , Torricelli Federica , Gugnoni Mila , Ciarrocchi Alessia , Pistoni Mariaelena 
Published: January 2023

Abstract
Background
Triple-Negative Breast Cancer (TNBC) is a subtype of breast cancer that differs from other types of breast cancers in the faster spread and worse outcome. TNBC presented limited treatment options. BET (Bromodomain and extra-terminal domain) proteins are epigenetic readers that control the expression of different oncogenic proteins, and their inhibition (BETi) is considered a promising anti-cancer strategy. Recent evidence demonstrated the involvement of BET proteins in regulation of metabolic processes.

Methods
MDA-MB231 cells treated with JQ1 followed by RNA-sequencing analysis showed altered expression of lipid metabolic genes; among these, we focused on ATGL, a lipase required for efficient mobilization of triglyceride. Different in vitro approaches were performed to validate the RNA-sequencing data (qRT-PCR, immunofluorescence and flow cytometry). NMR (Nuclear Magnetic Resonance) was used to analyze the lipid reprogramming upon treatment. ATGL expression was determined by immunoblot and qRT-PCR, and the impact of ATGL function or protein knockdown, alone and in combination with BETi, was assessed by analyzing cell proliferation, mitochondrial function, and metabolic activity in TNBC and non-TNBC cells culture models.

Results
TNBC cells treated with two BETi markedly increased ATGL expression and lipolytic function and decreased intracellular lipid content in a dose and time-dependent manner. The intracellular composition of fatty acids (FAs) after BETi treatment reflected a significant reduction in neutral lipids. The short-chain FA propionate entered directly into the mitochondria mimicking ATGL activity. ATGL KD (knockdown) modulated the levels of SOD1 and CPT1a decreasing ROS and helped to downregulate the expression of mitochondrial ß-oxidation genes in favor of the upregulation of glycolytic markers. The enhanced glycolysis is reflected by the increased of the mitochondrial activity (MTT assay). Finally, we found that after BETi treatment, the FoxO1 protein is upregulated and binds to the PNPLA2 promoter leading to the induction of ATGL. However, FoxO1 only partially prompted the induction of ATGL expression by BETi.

Conclusions
The anti-proliferative effect achieved by BETi is helped by ATGL mediating lipolysis. This study showed that BETi altered the mitochondrial dynamics taking advantage of ATGL function to induce cell cycle arrest and cell death.

Graphical Abstract

Schematic representation of BETi mechanism of action on ATGL in TNBC cells. BETi induce the
expression of FoxO1 and ATGL, lowering the expression of G0G2, leading to a switch in metabolic status. The
induced expression of ATGL leads to increased lipolysis and a decrease in lipid droplet content and bioavailability
of neutral lipid. At the same time, the mitochondria are enriched with fatty acids. This cellular status inhibits cell
proliferation and increases ROS production and mitochondrial stress. Interfering for ATGL expression, the
oxidative phenotypic status mildly reverted to a glycolytic status where neutral lipids are stored into lipid
droplets with a consequent reduction of oxidative stress in the mitochondrial.