Metabolic regulation of FASN and the role of its product, palmitate, in breast cancer progression and metastasis

Fatty acid synthase (FASN) is the primary enzyme responsible for synthesizing palmitate and its expression can be regulated downstream of signaling cascades mediated through activated receptor tyrosine kinases. Insulin, which is often elevated during metabolic perturbations such as obesity, can affect the regulation of FASN through increasing its transcription. Breast cancer cells exposed to sera from breast cancer patients with obesity (body mass index > 30) revealed an increase in FASN mRNA and protein expression compared to sera from patients without obesity (body mass index < 20), which was dependent upon the insulin induced binding of the transcription factor, sterol response element binding protein-1 (SREBP-1), to its promoter region. Another circulating factor more bioavailable in states of obesity is insulin like growth factor-1 (IGF-1), which activates the insulin-like growth factor-1 receptor (IGF-1R). Exposure of IGF-1 to breast cancer cells, led to an increased FASN mRNA and protein expression that was blunted upon IGF-1R silencing and inhibition of its downstream mediator, mammalian target of rapamycin complex 1 (mTORC1), through rapamycin treatment. Further, inhibition of the mTORC1 substrate, serine arginine rich protein kinase-2 (SRPK2), through RNAi and small molecule inhibition caused a decrease in FASN mRNA stability, increased intron retention, and decreased de novo synthesized intracellular palmitate compared to control. To assess the consequences of palmitate production in breast cancer, the role of flotillin-1 palmitoylation in TNBC breast cancer progression was assessed due to its high expression in TNBC cell lines. TNBC cells expressing palmitoylation defective flotillin-1 constructs revealed flotillin-1 palmitoylation to be essential for its protein stability and metastatic capabilities in vivo. To assess the ability to target flotillin-1 palmitoylation therapeutically, we designed a competitive peptide, which demonstrated efficacy in blocking flotillin-1 palmitoylation in vitro without altering palmitoylation of other zDHHC5 substrates. Additionally, multiple TNBC tumor models displayed attenuated tumor growth and lung metastasis when expressing the same peptide construct modified to be expressed by doxycycline constructs. Collectively, these studies demonstrate the mechanistic role of extracellular factors in mediating metabolic regulation in breast cancer cells through FASN expression as well as phenotypic effects upon modulation of flotilln-1 protein palmitoylation in TNBC.