Abstract
Preclinical models and clinical studies suggest that hypercholesterolemia promotes breast cancer progression 1,2. The expression of the low-density lipoprotein receptor (LDLR) has been positively associated with poorer recurrence-free survival in human breast cancer studies 3. Mechanistically, LDLR has been demonstrated to play a role in the increased tumor growth associated with hypercholesterolemia, as knock-down of LDLR led to decreased tumor growth in setting of elevated circulating LDL cholesterol. The aim of this study was to identify factors which up-regulate expression of LDLR in triple negative breast cancer (TNBC). In glioblastoma, hyper-activation of the epidermal growth factor receptor (EGFR) signaling pathway has been associated with greater LDLR expression and susceptibility to targeting of cholesterol metabolism4. As EGFR is frequently expressed in TNBC5, we examined if increased LDLR expression is associated with activation of the EGFR signaling pathway in TNBC. The expression of LDLR in the TNBC cell lines, MDA-MB-231 (231) and MDA-MB-468 (468) was examined pre- and post-EGF stimulation of the EGFR and in the presence of chemical inhibitors. Cells were grown in DMEM/10% FBS/1% Pen/strep (P/S), and experiments were performed under reduced serum conditions at 1.25%FBS/DMEM/1%P/S. In the absence of stimulation, LDLR protein expression was 3-fold higher in 231 vs 468 cell lines. This was despite mRNA expression being comparable at baseline, suggesting that the difference in protein expression was post-transcriptionally mediated. Treatment with 10 ng/mL EGF for 2 hours led to an increased activation of the EGFR, phosphorylation of Akt and extracellular signal regulated kinase (ERK) in both cell lines but induced an increase in LDLR protein and mRNA expression only in 468 cells. Treatment of 468 cells with EGF after exposure to actinomycin, a transcription inhibitor, revealed that EGF treatment resulted in reduced degradation of LDLR mRNA (p = 0.002) over 3 hours, suggesting that the EGF-induced increase in LDLR expression was by protection of LDLR mRNA from degradation. Chemical inhibition of the ERK pathway with 20 μM UO126 reduced both the EGF-induced increase in LDLR expression in 468 cells (p = 0.015) as well as the high baseline expression of LDLR by half in 231 cells (p = 0.001). Overall our results suggest that the EGFR/ERK signaling pathway regulates LDLR expression in TNBC, supporting the increased anabolic needs of this aggressive, swiftly expanding form of breast cancer.
References: 1Alikhani, N. et al., Oncogene32, 961-967 (2013), 2Pelton, K. et al., Am. J. Pathol.184, 2099-2110 (2014), 3Gallagher, E. J. et al., Oncogene36, 6462-6471 (2017), 4Guo, D. et al., Cancer Discov.1, 442-456 (2011), 5Reis-Filho, J. S. & Tutt, A. N. J. Histopathology52, 108-118 (2008).
Protein tyrosine kinase 2: a novel therapeutic target to overcome acquired EGFR-TKI resistance in non-small cell lung cancer