In a study published in the journal Nature Communications, scientists have identified a lipid-degrading enzyme, fatty acid amide hydrolase, as a prognostic biomarker in patients with luminal breast cancer. In both in vitro and in vivo experimental setups, this enzyme has been found to prevent cancer progression and lung metastasis.
Study: Identification of fatty acid amide hydrolase as a metastasis suppressor in breast cancer. Image Credit: Guschenkova/Shutterstock.com
Background
Breast cancer is a highly heterogeneous disease, with many subtypes showing distinct pathological features and clinical outcomes. Among different subtypes, luminal breast cancer accounts for more than 70% of all breast cancer cases.
Although typically associated with favorable clinical outcomes, luminal breast cancer exhibits a high risk of long-term metastatic relapse even after 15 years of treatment completion.
Impaired functioning of the endocannabinoid system has been observed in many cancers. This cell communication system includes G-protein coupled cannabinoid receptors and their endogenous ligands anandamide and 2-arachidonoylglycerol.
In mouse models of cancers, pharmacological activation of cannabinoid receptors has been found to exert anticancer response.
Fatty acid amide hydrolase is an enzyme responsible for anandamide degradation, thus determining the bioavailability and functions of this endogenous ligand of cannabinoid receptors. In the current study, scientists have explored the clinical significance of fatty acid amide hydrolase (FAAH) with luminal breast cancer.
Important observations
The scientists analyzed transcriptomic data from more than 15,000 breast cancer patients to determine the clinical status of the endocannabinoid system in different breast cancer subtypes.
The findings revealed that FAAH is the only gene showing differential expression patterns between different breast cancer subtypes. A strong association was observed between high FAAH expression and luminal breast cancer subtype at both mRNA and protein levels.
In human breast tumor samples, high FAAH expression showed a significant association with estrogen receptor-positive (ER+) tumors and highly differentiated tumors (low histological grade), further highlighting the involvement of FAAH in luminal breast cancer.
FAAH expression and cancer prognosis
The scientists explored whether the level of FAAH expression can predict clinical outcomes in breast cancer patients.
The findings revealed that lower FAAH expression is associated with a higher risk of distant metastasis and lower overall survival. In particular, low FAAH expression in breast tumors increased the risk of lung metastasis but not bone or brain metastasis.
Further analysis revealed that FAAH is downregulated during the metastatic progression of luminal breast cancer and that higher FAAH expressions in both primary and metastatic tumors are associated with higher survival rates.
The scientists conducted in vivo experiments using a mouse model of metastatic breast cancer to explore further the association between FAAH expression and breast cancer aggressiveness. The scientists genetically inactivated FAAH expression in experimental mice and compared them with FAAH-expressing control mice.
The histological analysis of early detected tumors revealed that control mice developed low-grade adenocarcinomas and experimental mice developed solid high-grade carcinomas.
At the molecular level, control mice-derived tumors showed luminal breast cancer features, whereas experimental mice-derived tumors mostly showed basal subtype features.
Further characterization of tumors revealed that tumors derived from FAAH-deficient experimental mice have significantly increased metastatic rate and upregulated expression of markers associated with poor prognosis.
FAAH expression and pro-oncogenic and pro-metastatic features of human breast cancer cells
The scientists downregulated FAAH expression in a human luminal breast cancer cell line. They upregulated FAAH expression in a human basal breast cancer cell line to understand the significance of FAAH in tumor progression.
The findings revealed that silencing of FAAH in luminal cancer cells leads to reduced expression of epithelial markers and increased expression of mesenchymal markers.
This indicates the initiation of epithelial-to-mesenchymal transition (EMT), a hallmark process associated with cancer cell migration and invasion. The overexpression of FAAH in basal cancer cells led to the opposite phenotype.
The scientists analyzed the growth of xenografts derived from FAAH-deficient luminal cells and FAAH-overexpressing basal cells in immunocompromised mice. The findings revealed that FAAH-deficient tumors have significantly higher growth rates than FAAH-overexpressing tumors.
Moreover, tumors generated from FAAH-overexpressing basal cancer cells showed a significantly reduced ability to induce lung metastasis compared to tumors generated from parental cells with low FAAH expression.
The identification and functional characterization of differentially expressed genes in FAAH-knockout luminal cancer cells further confirmed that FAAH reduces breast cancer cell aggressiveness by inhibiting the expression of migration- and invasion-related genes.
Further analysis of differentially expressed genes revealed that FAAH inhibits breast cancer cell metastasis by deregulating chemokine signaling through the CXCR4-CXCL12 axis.
Mechanistically, metanandamide, a non-hydrolyzable structural analog of anandamide, increased CXCR4 expression and EMT phenotypes in FAAH-expressing luminal cells by activating cannabinoid receptors. These features were similar to the effects of FAAH genetic silencing.
These observations collectively indicate that an endogenous anandamide tone is ultimately controlled by FAAH activity, which modulates breast cancer cells' tumorigenic and metastatic potential.
Study significance
The study identifies a lipid-degrading enzyme FAAH as a prognostic marker in luminal breast cancer. Given its significant involvement in controlling tumor progression, scientists believe that FAAH can be a potential therapeutic target in metastatic breast cancer.
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