The BRAF Inhibitor Vemurafenib Activates Mitochondrial Metabolism and Inhibits Hyperpolarized Pyruvate-Lactate Exchange in BRAF-Mutant Human Melanoma Cells

Understanding the impact of BRAF signaling inhibition in human melanoma on key disease mechanisms is important for developing biomarkers of therapeutic response and combination strategies to improve long-term disease control. This work investigates the downstream metabolic consequences of BRAF inhibition with vemurafenib, the molecular and biochemical processes that underpin them, their significance for antineoplastic activity, and potential as noninvasive imaging response biomarkers. 1H NMR spectroscopy showed that vemurafenib decreases the glycolytic activity of BRAF-mutant (WM266.4 and SKMEL28) but not BRAFWT (CHL-1 and D04) human melanoma cells. In WM266.4 cells, this was associated with increased acetate, glycine, and myo-inositol levels and decreased fatty acyl signals, while the bioenergetic status was maintained. 13C NMR metabolic flux analysis of treated WM266.4 cells revealed inhibition of de novo lactate synthesis and glucose utilization, associated with increased oxidative and anaplerotic pyruvate carboxylase mitochondrial metabolism and decreased lipid synthesis. This metabolic shift was associated with depletion of hexokinase 2, acyl-CoA dehydrogenase 9, 3-phosphoglycerate dehydrogenase, and monocarboxylate transporters (MCT) 1 and 4 in BRAF-mutant but not BRAFWT cells and, interestingly, decreased BRAF-mutant cell dependency on glucose and glutamine for growth. Further, the reduction in MCT1 expression observed led to inhibition of hyperpolarized 13C-pyruvate-lactate exchange, a parameter that is translatable to in vivo imaging studies, in live WM266.4 cells. In conclusion, our data provide new insights into the molecular and metabolic consequences of BRAF inhibition in BRAF-driven human melanoma cells that may have potential for combinatorial therapeutic targeting as well as noninvasive imaging of response. Mol Cancer Ther; 15(12); 2987-99. ©2016 AACR.

Induction of morphological changes in death-induced cancer cells monitored by holographic microscopy

We are using the label-free technique of holographic microscopy to analyze cellular parameters including cell number, confluence, cellular volume and area directly in the cell culture environment. We show that death-induced cells can be distinguished from untreated counterparts by the use of holographic microscopy, and we demonstrate its capability for cell death assessment. Morphological analysis of two representative cell lines (L929 and DU145) was performed in the culture flasks without any prior cell detachment. The two cell lines were treated with the anti-tumour agent etoposide for 1-3days. Measurements by holographic microscopy showed significant differences in average cell number, confluence, volume and area when comparing etoposide-treated with untreated cells. The cell volume of the treated cell lines was initially increased at early time-points. By time, cells decreased in volume, especially when treated with high doses of etoposide. In conclusion, we have shown that holographic microscopy allows label-free and completely non-invasive morphological measurements of cell growth, viability and death. Future applications could include real-time monitoring of these holographic microscopy parameters in cells in response to clinically relevant compounds.

Model free approach to kinetic analysis of real-time hyperpolarized 13C magnetic resonance spectroscopy data

Real-time detection of the rates of metabolic flux, or exchange rates of endogenous enzymatic reactions, is now feasible in biological systems using Dynamic Nuclear Polarization Magnetic Resonance. Derivation of reaction rate kinetics from this technique typically requires multi-compartmental modeling of dynamic data, and results are therefore model-dependent and prone to misinterpretation. We present a model-free formulism based on the ratio of total areas under the curve (AUC) of the injected and product metabolite, for example pyruvate and lactate. A theoretical framework to support this novel analysis approach is described, and demonstrates that the AUC ratio is proportional to the forward rate constant k. We show that the model-free approach strongly correlates with k for whole cell in vitro experiments across a range of cancer cell lines, and detects response in cells treated with the pan-class I PI3K inhibitor GDC-0941 with comparable or greater sensitivity. The same result is seen in vivo with tumor xenograft-bearing mice, in control tumors and following drug treatment with dichloroacetate. An important finding is that the area under the curve is independent of both the input function and of any other metabolic pathways arising from the injected metabolite. This model-free approach provides a robust and clinically relevant alternative to kinetic model-based rate measurements in the clinical translation of hyperpolarized (13)C metabolic imaging in humans, where measurement of the input function can be problematic.

MEK1/2 inhibition decreases lactate in BRAF-driven human cancer cells

The RAS/BRAF/MEK/ERK signaling pathway is a central driver in cancer with many BRAF and MEK inhibitors being evaluated in clinical trials. Identifying noninvasive biomarkers of early pharmacodynamic responses is important for development of these targeted drugs. As increased aerobic glycolysis is often observed in cancer, we hypothesized that MEK1/2 (MAP2K1/MAP2K2) inhibitors may reduce lactate levels as detected by magnetic resonance spectroscopy (MRS), as a metabolic biomarker for the pharmacodynamic response. MRS was used to monitor intracellular and extracellular levels of lactate in human cancer cells in vitro and in melanoma tumors ex vivo. In addition, we used (1)H MRS and a fluorescent glucose analog to evaluate the effect of MEK inhibition on glucose uptake. MEK1/2 signaling inhibition reduced extracellular lactate levels in BRAF-dependent cells but not BRAF-independent cells. The reduction in extracellular lactate in BRAF-driven melanoma cells was time-dependent and associated with reduced expression of hexokinase-II driven by c-Myc depletion. Taken together, these results reveal how MEK1/2 inhibition affects cancer cell metabolism in the context of BRAF oncogene addiction. Furthermore, they offer a preclinical proof-of-concept for the use of MRS to measure lactate as a noninvasive metabolic biomarker for pharmacodynamic response to MEK1/2 inhibition in BRAF-driven cancers.

Abstract 5083: Inhibition of MEK1/2 signaling in human BRAFV600E melanoma cells reduces glucose uptake and lactate dehydrogenase activity resulting in a time-dependent decrease in lactate production

Background: Oncogenic transformation is associated with changes in cell metabolism. The MEK1/2 activator BRAF is mutated in ∼70% of malignant melanoma. This provides a significant focus for advancing mechanism-based therapy and several inhibitors of the pathway are now undergoing clinical development. Our aim was to investigate whether MEK1/2 signaling inhibition in BRAFV600E human melanoma cells results in magnetic resonance spectroscopy (MRS) detectable changes in glycolysis that could serve as non-invasive biomarkers of target suppression. For this we assessed lactate production, glucose uptake and lactate dehydrogenase (LDH) activity in response to treatment.

Methods: WM266.4 melanoma cells (BRAFV600E) were treated with 1 µM of the MEK1/2 inhibitor CI-1040 and lactate production was monitored using 1H MRS at 30 min and 2, 6, 16, 24, 48 h on a 11.7 T Bruker Avance spectrometer. LDH activity was measured using a dynamic nuclear polarization assay by suspending 107 control or treated cells (24h) in a neutralized solution of hyperpolarized 1-13C pyruvic acid and unpolarized lactate followed by serial 13C MRS acquisitions. Glucose uptake at 24 h was evaluated by adding 10 µM of 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG) to control and treated cells for the last 2 h of treatment. Median fluorescence intensities of 2-NBDG uptake were obtained using a BD FACSAriaTM Flow Cytometer. Statistical analysis was performed using Student's t-test with p ≤ 0.05 considered to be significant.

Results: 1H MRS analysis indicated that lactate levels were unchanged at 30 min to 6 h but decreased significantly at 16 h (79±3%), 24 h (76±4%) and 48 h (80±6%) as compared to controls (n=3, p ≤ 0.006). LDH activity in control and treated cells was 0.83±0.26 and 0.48±0.08 nmol/s/106 cells, respectively (n=5, p=0.02) i.e. reduced by ∼40% following MEK1/2 inhibition. Similarly, CI-1040 treatment led to a reduction in 2-NBDG uptake to 88±2% relative to the control (n=3, p=0.003).

Conclusions: Our findings demonstrate that MEK1/2 signaling inhibition leads to a time-dependent decrease in lactate production through modulation of both LDH activity and glucose uptake. These results show lactate as a potential non-invasive biomarker of response to MEK1/2 targeted therapeutics in human BRAFV600E melanoma cells. Further studies are required to establish the molecular processes linking MEK1/2 inhibition to decreased lactate production.

Funding: Marie Curie Action: Early Stage Training (grant MEST-CT-2005-020718) and CRUK (grants C309/A8274, C1060/A5117 and C1060/A6916). We also acknowledge the support received from the CRUK and EPSRC Cancer Imaging Centre in association with the MRC and Department of Health (England) grant C1060/A10334 and NHS funding to the NIHR Biomedical Research Centre.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5083.

Effect of polyamine deficiency on proteins involved in Okazaki fragment maturation

Polyamine depletion causes S phase prolongation, and earlier studies indicate that the elongation step of DNA replication is affected. This led us to investigate the effects of polyamine depletion on enzymes crucial for Okazaki fragment maturation in the two breast cancer cell lines MCF-7 and L56Br-C1. In MCF-7 cells, treatment with N(1),N(11)-diethylnorspermine (DENSPM) causes S phase prolongation. In L56Br-C1 cells the prolongation is followed by massive apoptosis. In the present study we show that L56Br-C1 cells have substantially lower basal expressions of two Okazaki fragment maturation key proteins, DNA ligase I and FEN1, than MCF-7 cells. Thus, these two proteins might be promising markers for prediction of polyamine depletion sensitivity, something that can be useful for cancer treatment with polyamine analogues. DENSPM treatment affects the cellular distribution of FEN1 in L56Br-C1 cells, but not in MCF-7 cells, implying that FEN1 is affected by or involved in DENSPM-induced apoptosis.