Pharmacological blockade of Fatty Acid Synthase (FASN) reverses acquired autoresistance to trastuzumab (Herceptin™) by transcriptionally inhibiting ‘HER2 super-expression’ occurring in high-dose trastuzumab-conditioned SKBR3/Tzb100 breast cancer cells

Abstract

Abstract. Elucidating the mechanisms underlying resistance to the human epidermal growth factor receptor 2 (HER2)-targeted antibody trastuzumab (Tzb; Herceptin™) is a major challenge that is beginning to be addressed. This dilemma is becoming increasingly important as recent studies strongly support a role for Tzb in the adjuvant setting for HER2-overexpressing early-stage breast cancers. We previously reported that pharmacological and RNA interference-induced inhibition of tumor-associated fatty acid synthase (FASN; Oncogenic antigen-519), a key metabolic enzyme catalyzing the synthesis of long-chain saturated fatty acids, drastically down-regulates HER2 expression in human breast cancer cells bearing HER2 gene amplification. Given that FASN blockade was found to suppress HER2 overexpression by attenuating the promoter activity of the HER2 gene, we here envisioned that this mechanism of action may represent a valuable strategy in breast cancers that have progressed while under Tzb. We created a preclinical model of Tzb resistance by continuously growing HER2-overexpressing SKBR3 breast cancer cells in the presence of clinically relevant concentrations of Tzb (20-185 μg/ml Tzb). This pool of Tzb-conditioned SKBR3 cells, which optimally grows now in the presence of 100 μg/ml trastuzumab (SKBR3/Tzb100 cells), exhibited HER2 levels notably higher (~2-fold) than those found in SKBR3 parental cells. Real-time polymerase chain reaction studies showed that up-regulation of HER2 mRNA levels closely correlated with HER2 protein up-regulation in SKBR3/ Tzb100 cells, thus suggesting that 'HER2 super-expression' upon acquisition of autoresistance to Tzb resulted, at least in part, from up-regulatory effects in the transcriptional rate of the HER2 gene. SKBR3/Tzb100 cells did not exhibit crossresistance to C75, a small-compound specifically inhibiting FASN activity. On the contrary, SKBR3/Tzb100 cells showed a remarkably increased sensitivity (~3-fold) to the cytotoxic effects occurring upon C75-induced inhibition of FASN enzymatic activity. Both HER2 mRNA and HER2 protein 'super-expression', which have not been reported in earlier Tzb-resistant breast cancer models, were entirely suppressed following pharmacological blockade of FASN activity. Moreover, while Tzb was still able to reduce HER2 protein expression by ~20% in SKBR/Tzb100 cells, C75 and Tzb coexposure synergistically down-regulated HER2 protein levels by >85%. The nature of the interaction between Tzb and C75 in Tzb-resistant SKBR3/Tzb100 cells was also found to be strongly synergistic when analyzing the extent of apoptotic cell death using ELISA-based detection of histone-associated DNA fragments. In summary, a) the molecular mechanism(s) contributing to Tzb resistance in our SKBR3/Tzb100 model appear to be clearly different to those previously reported as we found important transcriptional up-regulatory transcriptional changes in HER2 gene expression levels relative to parental cells; b) since FASN inhibition acts on HER2 gene expression via reduction of its tr...

Elovl6 is a poor prognostic predictor in breast cancer

“…Under hypoxic conditions, FAS is upregulated following activation of Akt and SREBP-1 (19). FAS expression in breast cancer confers poor prognosis and is associated with HER2 expression in aggressive breast cancer (20,21), whilst FAS inhibition reverses the resistance of trastuzumab in HER2(+) breast cancer cells (22). As a long fatty acid elongase, Elovl6 contributes to de novo synthesis of fatty acids, and is understood to modulate insulin resistance (23).…”

Fatty acid synthase activity regulates HER2 extracellular domain shedding into the circulation of HER2-positive metastatic breast cancer patients

“…Indeed, serum HER2 ECD can be found significantly elevated in patients with tumor exhibiting activated HER2, a predictor of the efficacy of trastuzumabbased therapy in patients with MBC (32). We recently provided experimental evidence that FASN blockade efficiently reverses acquired autoresistance to trastuzumab in high-dose trastuzumab-conditioned SKBR3 cells (48). Subsequent studies should explore how circulating levels of sFASN relate to clinical prognostic variables and/or clinical responses to trastuzumab-based therapies.…”

The Electronics of HER2/neu Positive Breast Cancer Cells

“…This represents a new physiological alteration that is transcriptionally independent from the NR1D1 and PBP axis since knockdown of PBP does not change mRNA levels of MDH1 or ME1. The importance of fatty acid synthesis in this type of cancer is underscored by the finding that FASN inhibition reverses Herceptin resistance in SKBR3 cells (Vazquez-Martin et al 2007). To date, there is not much known about the contribution of coenzyme A (CoA) metabolism to this lipogenic phenotype.…”