I. Research Background
Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer associated with high metastatic potential and mortality. Owing to the limited targeted therapies available for TNBC patients, there is an urgent demand to identify promising therapeutic targets and treatment regimens.
Metabolic reprogramming in primary tumors and metastatic lesions has attracted growing attention due to differences in energy demand and nutritional microenvironment. Invasive tumors exhibit plasticity in carbon source utilization to meet bioenergetic requirements. Although cancer cells display elevated glycolysis, they also utilize mitochondrial respiration to generate substantial ATP.
Previous studies have demonstrated that reprogramming toward mitochondrial fatty acid β-oxidation (FAO) serves as a dominant metabolic pathway in TNBC. Recent research further reveals a hybrid metabolic state in cancer cells, in which cells engage both glycolysis and oxidative phosphorylation (OXPHOS). This hybrid metabolic phenotype is critical for tumor progression and adaptation to distant metastatic niches. Therefore, targeting mitochondrial metabolism represents an attractive therapeutic strategy for TNBC.
As first-line agents for type 2 diabetes mellitus (T2DM), biguanides including metformin have recently been investigated for anticancer applications. Epidemiological observations of reduced cancer incidence among diabetic patients receiving metformin have motivated numerous clinical trials evaluating metformin in multiple malignancies including TNBC.
Nevertheless, metformin fails to reach therapeutically effective concentrations within tumor tissues. At clinically relevant doses, it neither suppresses tumor proliferation nor provides survival benefits. Accordingly, identifying appropriate combinatorial regimens to overcome the limited efficacy caused by poor bioavailability of biguanides is an urgent priority.
The anticancer activity of biguanides is mainly attributed to the inhibition of OXPHOS via suppressing mitochondrial electron transport chain (ETC) Complex I. Several recent studies have explored metabolic modulation induced by biguanides in cancer treatment. Importantly, one clinical study identified two metabolically distinct tumor subgroups in breast cancer patients treated with metformin, presenting drastically different clinical outcomes: the oxidative phosphorylation transcriptional response (OTR) group with upregulated transcription of OXPHOS-related genes, and the fluorodeoxy-D-glucose (FDG) response group with increased ¹⁸F-FDG uptake. After metformin treatment, tumors in the OTR subgroup exhibited enhanced cell proliferation and drug resistance. Further analyses confirmed that therapeutic-dose metformin administration in primary breast cancer elevates FAO in OTR tumors, and FAO activation correlates with increased proliferation.
On this basis, the research team proposed that identifying accompanying targets of metabolic reprogramming during metformin therapy for TNBC is essential for developing effective combination therapies.
II. Scientific Hypothesis and Research Rationale
Existing literature indicates that FAO can activate Src kinase in invasive TNBC. Accordingly, the research team hypothesized that low-dose biguanide-driven AMPK-ACC-FAO signaling may trigger the Src pathway in TNBC.
A comprehensive analysis using multiple models was performed to uncover the crosstalk between mitochondria and oncogenic pathways induced by metformin in TNBC. The bioavailability of metformin in vivo recapitulates the effects of low-dose treatment observed in vitro in TNBC xenografts.
III. Key Experimental Findings
Dose-dependent bidirectional effects on tumor phenotypes Low-dose metformin activates Src signaling, whereas high-dose metformin exerts inhibitory effects on the Src pathway. Low-dose metformin remodels metabolism in TNBC to activate the FAO-Src kinase signaling axis.
FAO acts as a pivotal intermediate mediator Pharmacological or genetic inhibition of FAO significantly potentiates the antitumor properties of biguanides, verifying the indispensable role of FAO in mediating the pro-tumor effects triggered by low-dose metformin.
Synergistic antitumor activity of metformin combined with Src inhibitor Based on the above findings, the team proposed a novel therapeutic strategy: clinically dosed metformin combined with dasatinib (a Src inhibitor). The combination therapy synergistically suppresses tumor growth in patient-derived xenograft (PDX) models of TNBC and inhibits TNBC metastasis. Notably, this synergistic effect is abrogated in mice fed a high-fat diet.
IV. Research Conclusions
Biguanides exert opposing effects on TNBC progression in a dose-dependent manner: low doses promote tumor progression, while high doses suppress tumor growth.
Low-dose metformin activates Src kinase in TNBC via the AMPK-FAO signaling pathway, which explains the limited efficacy of single-agent metformin at clinical doses.
The combination of clinically dosed metformin and the Src inhibitor dasatinib restrains TNBC progression and metastasis, offering a potential novel therapeutic option for metastatic TNBC with scarce treatment alternatives.
V. Research Significance and Clinical Implications
This study uncovers the dual-sided effect of metformin in cancer treatment, breaking the unilateral view focused merely on its anticancer activity and elucidating the mechanism underlying unsatisfactory outcomes of single-agent metformin in clinical trials.
From a translational perspective, this work proposes a combination strategy targeting bypass pathways of mitochondrial metabolism to block the pro-tumor FAO-Src cascade activated by low-dose metformin. In addition, the study reveals that systemic lipid metabolism status (e.g., high-fat diet) modulates the efficacy of this combination regimen, providing important references for subsequent clinical trial design and screening of potential beneficiaries.
