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In the previous issue of Breast Cancer Research, Jin and colleagues [1] demonstrated that the lipogenic enzyme fatty acid synthase (FASN) is phosphorylated when it is in complex with the tyrosine kinase receptor HER2. HER2-dependent activating phosphorylation of FASN was related to tumor progression and was sensitive to the dual HER1/HER2 tyrosine kinase inhibitor lapatinib [1]. Adding to knowledge of the therapeutic value of the phosphorylation status of FASN in HER2-driven breast cancer (BC), we now provide experimental evidence that the expression status of FASN in the extracellular milieu (that is, extracellular FASN) may function as a novel biomarker that distinctively predicts molecular functioning of lapatinib.  AMP-activated protein kinase (AMPK)-activating drugs, by mimicking an elevated AMP/ATP ratio in BC cells, drastically augment the release of extracellular FASN in HER2-positive BC cells [2]. Lapatinib-induced deprivation of tumor cell energy activates AMPK to trigger an entire cascade of metabolic events, including suppression of FASN expression and activity [1,3]. We hypothesized that a differential ability to initiate AMPK-sensed metabolic stress responses may provide information about the efficacy of HER-targeting drugs via changes in the extracellular FASN status. Enzyme-linked immunosorbent assay (ELISA)-based quantitative analyses revealed that lapatinib treatment drastically enhanced extracellular FASN concentration (by at least 8.0-fold) (Figure ​(Figure1a).1a). Immunoblotting assessment of AMPK phosphorylation at Thr172 confirmed that lapatinib treatment induced a strong activation of AMPK (Figure ​(Figure1b).1b). A weak but detectable upregulation of PP-AMPKThr172 was observed upon treatment with gefitinib. Trastuzumab, cetuximab, and erlotinib - all of which are unable to promote FASN release - failed to activate AMPK. AMPK knockdown using short interfering RNA (siRNA) transfection [2] fully prevented lapatinib-induced FASN release (Figure S1). Immunoblotting and cell imaging analyses confirmed that FASN was depleted from the cytosol of lapatinib-treated HER2 overexpressors and accumulated in their extracellular milieu (Figure S1).    Figure 1  Lapatinib's molecular functioning involves AMPK-dependent release of extracellular fatty acid synthase (FASN). (a) HER1 or HER2 inhibitors (or both) differentially regulate extracellular expression of FASN. FASN concentration in cell culture supernatants ...    Treatment with lapatinib dramatically increased FASN release (by approximately 17 times) in lapatinib-responsive SKBR3 TzbR cells (Figure ​(Figure1c),1c), which were selected for long-term outgrowth in trastuzumab-containing culture medium. Extracellular FASN remained unaltered in response to trastuzumab or lapatinib in JIMT-1 BC cells, which exhibit de novo cross-refractoriness to multiple HER1/2-targeted therapies (Figure ​(Figure1c).1c). Lapatinib treatment significantly activated AMPK and promoted an enormous accumulation of extracellular FASN in lapatinib-hypersensitive MCF-7/HER2 cells (Figure ​(Figure1d).1d). Equimolar concentrations of lapatinib failed to activate AMPK or to alter FASN release into the extracellular milieu of MCF-7/HER2 LapR cells, which were obtained by continuously subculturing parental MCF-7/HER2 cells with high-dose lapatinib.  High levels of circulating FASN can be found in peripheral blood of HER2-overexpressing BC patients [4]. The exclusive ability of lapatinib to actively promote the extracellular release of FASN via AMPK-sensed energy depletion in metabolically demanding HER2-driven BC cell growth might provide a molecular rationale to evaluate the predictive value of circulating FASN in HER2-positive BC patients receiving lapatinib.

Circulating fatty acid synthase: an exploratory biomarker to predict efficacy of the dual HER1/HER2 tyrosine kinase inhibitor lapatinib