Autophagy | SLC16A3/MCT4-Dependent Secretory Autophagy Drives Lactate Secretion and Metastasis in Triple-Negative Breast Cancer

This study reveals a novel mechanism by which EGF signaling regulates lactate efflux through secretory autophagy-mediated trafficking of SLC16A3, providing innovative experimental design ideas for research on metabolic reprogramming in TNBC, and suggesting that targeting autophagic vesicle transport may be superior to direct inhibition of the transporter.

 

Literature Overview

This article, 'Secretory autophagy mediates SLC16A3/MCT4-dependent lactate secretion to drive metastatic progression in triple-negative breast cancer,' published in the journal Autophagy, systematically investigates how EGF signaling in triple-negative breast cancer (TNBC) promotes lactate efflux and drives metastasis via a non-canonical autophagy pathway—secretory autophagy. By integrating cell models, animal experiments, and clinical samples, the study uncovers a previously unknown signaling axis. This work not only expands our understanding of autophagy functions but also identifies new therapeutic targets for metabolic intervention in TNBC.

Background Knowledge

Triple-negative breast cancer (TNBC) has a poor clinical prognosis and is prone to recurrence and metastasis due to the lack of ER, PR, and HER2 expression, making it ineligible for endocrine or targeted therapies. Acidification of the tumor microenvironment (TME) is a hallmark feature, primarily caused by lactate accumulation from glycolysis. Lactate is transported extracellularly via SLC16A3 (also known as MCT4), maintaining intracellular pH homeostasis and promoting immune evasion and invasion. However, the regulatory mechanisms controlling SLC16A3 membrane localization remain unclear, especially under EGF signaling activation. Current targeting strategies mostly focus on inhibiting SLC16A3 activity, but with limited efficacy, suggesting that upstream regulatory pathways may be more critical. This study investigates whether autophagy is involved in SLC16A3 membrane trafficking, particularly in TNBC with high EGFR expression, to determine if a non-degradative secretory autophagy pathway coordinates metabolism and metastasis.

 

 

Research Methods and Experiments

The authors used multiple TNBC cell lines (e.g., MDA-MB-231, 4T1) and non-TNBC controls (MCF-7), combined with EGF stimulation and autophagy modulation (3-MA, CQ, ATG5/ULK1 knockout), to systematically analyze the relationship between autophagy and lactate secretion. Using an RFP-GFP-LC3 dual-fluorescence reporter system, they observed that EGF induces autophagosome accumulation but inhibits autophagosome–lysosome fusion, indicating activation of non-canonical secretory autophagy. TIRF microscopy revealed LC3 puncta moving toward the plasma membrane in a SNARE protein SEC22B-dependent manner. Membrane protein biotinylation coupled with mass spectrometry identified SLC16A3 and its chaperone BSG enriched on autophagosomal membranes, with protease protection assays confirming their localization on the outer membrane. Co-immunoprecipitation and proximity ligation assays (PLA) further validated direct interaction between LC3 and SLC16A3, dependent on an LIR motif (F340). In vivo, an orthotopic breast cancer model was established using 4T1 cells, with knockdown of Atg7 or pharmacological interventions (3-MA, spautin-1) to assess lung metastasis. Clinically, multiplex immunofluorescence analysis of 166 patient samples validated the correlation between co-expression of EGFR, LC3, and SLC16A3 and patient prognosis.

Key Conclusions and Perspectives

• EGF activates EGFR signaling via the ERK pathway to induce autophagy, but suppresses its degradative function, shifting toward a secretory autophagy mode—this finding highlights the importance of non-degradative autophagy functions in TNBC models.
• Secretory autophagy directs autophagosomes to the plasma membrane via a SEC22B-dependent pathway, serving as a vehicle to transport SLC16A3 and BSG to the cell surface—this mechanism provides a new intervention node for targeting SLC16A3 membrane localization.
• SLC16A3 directly binds LC3 through its LIR motif, dependent on ATG5- and ULK1-mediated autophagosome formation—this interaction is critical for its membrane localization and lactate efflux, suggesting that LIR-mimetic peptides could be developed as potential inhibitors.
• In the 4T1 orthotopic model, inhibiting autophagy significantly reduced lactate accumulation and lung metastasis, outperforming direct SLC16A3 inhibition—indicating autophagy is a more upstream and effective therapeutic target.
• Clinical samples show that co-expression of EGFR, LC3, and SLC16A3 is significantly associated with poor prognosis, supporting the pathological relevance of this axis in TNBC patients and suggesting its potential as a combined biomarker for clinical monitoring.

Research Significance and Prospects

This study extends autophagy beyond its traditional degradative role into secretory regulation, offering a new dimension for understanding metabolic reprogramming in TNBC. Targeting the secretory autophagy–SLC16A3 axis may overcome the limitations of current lactate pathway inhibitors, particularly in EGFR-high populations.

In drug development, designing small molecules or peptides that specifically disrupt the LC3–SLC16A3 interaction, or inhibitors targeting SEC22B-mediated vesicle trafficking, could represent novel anti-metastatic strategies. Additionally, this axis could serve as a biomarker panel for clinical monitoring to identify high-risk TNBC patients.

For disease modeling, generating conditional knockout mice for Atg5 or Slc16a3, combined with TNBC transplantation models, will help further dissect the role of this pathway in tumor initiation, immune microenvironment remodeling, and therapy resistance, advancing the development of precision intervention strategies.

 

 

Conclusion

This study establishes secretory autophagy as a critical bridge linking EGFR signaling to SLC16A3-dependent lactate secretion, playing a central role in TNBC metastasis. By revealing the autophagosome as a novel secretory vehicle, it deepens our understanding of tumor metabolic regulation and provides a new therapeutic paradigm shifting from 'inhibiting transporters' to 'blocking vesicle trafficking.' In the laboratory, this mechanism provides a clear molecular pathway for studying TME acidification in TNBC, suggesting researchers should monitor autophagy status and subcellular localization of SLC16A3 when building models. Clinically, the EGFR–autophagy–SLC16A3 axis holds promise as a prognostic biomarker panel for high-risk TNBC patients and may guide the development of novel combination therapies. In the future, therapeutic strategies targeting this pathway may improve survival in TNBC patients, filling a critical gap in current treatment options and becoming a cornerstone in reshaping TNBC care.

 

Shan-Ying Wu, Hung-Ju Lin, Kai-Ying Lan, Yuan-Chieh Yeh, and Sheng-Hui Lan. Secretory autophagy mediates SLC16A3/MCT4-dependent lactate secretion to drive metastatic progression in triple-negative breast cancer. Autophagy.