LKB1 inactivation dictates therapeutic response of non-small cell lung cancer to the metabolism drug phenformin

Abstract

The LKB1 (also called STK11) tumor suppressor is mutationally inactivated in ∼20% of non-small cell lung cancers (NSCLC). LKB1 is the major upstream kinase activating the energy-sensing kinase AMPK, making LKB1-deficient cells unable to appropriately sense metabolic stress. We tested the therapeutic potential of metabolic drugs in NSCLC and identified phenformin, a mitochondrial inhibitor and analog of the diabetes therapeutic metformin, as selectively inducing apoptosis in LKB1-deficient NSCLC cells. Therapeutic trials in Kras-dependent mouse models of NSCLC revealed that tumors with Kras and Lkb1 mutations, but not those with Kras and p53 mutations, showed selective response to phenformin as a single agent, resulting in prolonged survival. This study suggests phenformin as a cancer metabolism-based therapeutic to selectively target LKB1-deficient tumors.

Figure 1. Phenformin selectively induces apoptosis in LKB1^−/− NSCLC tumor cells

(A) A549 human NSCLC cells expressing the pBabe vector (A549-pBabe), full length LKB1 (A549-LKB1^WT), or kinase dead LKB1 (A549-LKB1^KD) were treated with vehicle (DMEM), AICAR (2 mM), metformin (20 mM), phenformin (2 mM), 2DG (10 mM) or troglitazone (25 μM) for 48 hrs. Lysates were immunoblotted with the indicated antibodies. (B) A549 isogenic cell lines were treated for 24 hrs with vehicle (DMEM), 2 mM phenformin or 20 mM metformin. Lysates were immunoblotted with the indicated antibodies. (C) Fluorescence-activated cell sorting (FACS) on cells stained with AnnexinV-PE and 7AAD following 48 hr treatment with vehicle or 2 mM phenformin. (D) H460, H157 or A427 cell lines expressing the pBabe vector (pBabe), full length WT LKB1 (LKB1^WT), or kinase dead LKB1 (LKB1^KD) were treated for 24 hrs with vehicle (DMEM), 2 mM phenformin or 20 mM metformin. Lysates were immunoblotted with the indicated antibodies. Cancer gene driver mutations found in these cell lines listed under each.

Figure 2. Time course of tumor progression in K_luc, KL_luc and KP_luc lung tumor models

(A-C) Tumor progression in K_luc (A), KL_luc (B), and KP_luc (C) murine lung tumor measured by H&E staining or bioluminescence (bottom panel) at the indicated time points. Scale bars (black) = 300 μm. Scale bars (white) = 100 μm. Mice shown are representative of 10-12 mice per genotype. (D) Detailed analysis of H&E stained lung tumors from a KL_luc mouse (red circled mouse from bottom panel B) at 6 weeks post AdCre administration. Scale bars (red) = 1 mm, (black) = 500 μm, (white) = 100 μm. (E) Immunohistochemical analysis of representative lung tumors from K_luc, KL_luc and KP_luc mice 10 weeks post AdCre administration. Sections were stained with H&E or the indicated antibodies. Scale bars (black) = 500 μm. Scale bars (white) = 100 μm.

Figure 3. Phenformin treatment robustly activates AMPK pathway in K-Ras driven lung tumors in vivo

(A) Lung tissue lysates from wild type FVB mice following i.p. injection of saline (0.9%), metformin (300 mg/kg) or phenformin (150 mg/kg) and immunoblotted with the indicated antibodies. (B) Lysates of lung tumor nodules from K_luc, KL_luc and KP_luc mice treated with vehicle (water) or phenformin via 5 days ad lib feeding were immunoblotted with the indicated antibodies. (C) Representative images of hematoxylin and eosin (H&E), P-AMPK or P-4E-BP1 stained K_luc, KL_luc and KP_luc lung tumor sections. P-AMPK or P-4E-BP1 positively stained cells (in green) and nuclei (in blue). Scale bar = 100 μm. (D, E) Box plots representing the mean P-AMPK (D) or P-4E-BP1 (E) signal from K_luc, KL_luc and KP_luc lung tumors treated with vehicle or phenformin. n=6 mice analyzed per treatment group in each genotype (K_luc, KL_luc and KP_luc) treatment group. All data are represented as the mean ± SEM.

Figure 4. Phenformin preferentially inhibits LKB1-deficient lung tumors in vivo

(A) Representative ventral view images of bioluminescence from K_luc, KL_luc and KP_luc mice. Bioluminescence imaging (BLI) performed on K_luc (veh n=11, phen n=12 mice), KL_luc (veh n=10, phen n=10 mice) and KP_luc (veh n=7, phen n=7 mice) following three weeks of treatment with vehicle (water) or phenformin via ad lib feeding. (B) Average BLI for each treatment group of the K_luc, KL_luc and KP_luc mice at three weeks treatment. (C) Representative images of tumor burden in K_luc, KL_luc and KP_luc mice as shown by H&E stained lung sections (top panel) or the same images tissue segmented and pseudo-colored with Inform software (bottom panel). Scale bar = 1 mm. (D) Box plots representing the mean tumor burden following 3 weeks of treatment. Tumor burden was calculated by averaging the red pseudocolored tumor area from H&E stained whole lung sections shown in (C) bottom panel. (E) Representative images of H&E, Ki67 or TUNEL stained K_luc, KL_luc and KP_luc lung tumor sections. Ki67 or TUNEL positively stained cells (in green) and nuclei (in blue). Scale bar = 100 μm. (F, G) Box plots representing the mean Ki67 (F) and TUNEL scores (G) from K_luc, KL_luc and KP_luc mice treated with vehicle or phenformin. (*) Statistical significance (p-value < 0.05) calculated using a non-parametric one-way ANOVA (Tukey test). All data are represented as the mean ± SEM.

Figure 5. Phenformin significantly increased overall survival and slowed tumor progression in KL_luc mice shown by ¹⁸FDG-PET and μCT imaging

(A, B) Kaplan-Meier survival curves for KL_luc mice (A) or KP_luc mice (B) treated with vehicle (n=5) or phenformin (n=5) for each tumor genotype respectively with a **p-value = 0.008 for KL_luc mice determined by Wilcoxon log rank test. (C) Representative images of micro FDG-PET and CT scans of lungs from KL_luc and KP_luc mice at baseline (pre-treatment) or following 4 and 6 weeks treatment with vehicle or phenformin. Heart is labeled (H) and tumors are labeled with arrows (white or blue). (D, E) Mean tumor volume measured by micro CT at 4 and 6 weeks treatment for KL_luc (D) and KP_luc (E) mice respectively. *** p-value = 0.0009 for KL_luc mice as determined by Wilcoxon rank sum test. (F, G) Mean FDG-PET SUVmax measured at 4 and 6 weeks treatment for KL_luc (F) and KP_luc mice (F) respectively. **p-value = 0.005 for KL_luc mice as determined by Wilcoxon rank sum test. n=5 mice per treatment group of each genotype. All data are represented as the mean ± SEM.

Figure 6. Phenformin induces a significant therapeutic response in KL_luc lung tumors

(A) Hematoxylin and eosin (H&E) stained whole lung sections of KL_luc mice treated with vehicle (saline) or phenformin, scale bar = 4 mm. (B-E) Representative images of lung tumors from KL_luc mice treated with vehicle (saline) or phenformin. (B) Tumor necrosis, scale bar = 50 μm (C) Activated caspase 3, scale bar= 50 μm. Inset represents magnification of caspase 3 stained cells, scale bar = 25 μm. (D) Ki67 stain, scale bar = 50 μm. Inset represents magnification of Ki67 stained cells, scale bar = 25 μm. (E) Mixture of bronchiolar-epithelial adenocarcinoma (BEAC) and mucinous bronchiolar-epithelial adenocarcinoma (mBEAC) shown with arrow, scale bar = 200 μm. Inset represents magnification of mBEAC cells, scale bar = 50 μm. (F) Mean percentage of necrotic tumor area in vehicle and phenformin treated KL_luc mice. **p-value = 0.002. (G) Mean percentage of tumor cell apoptosis from vehicle and phenformin treated KL_luc mice. ***p-value < 0.0001. (H) Ki67 index for vehicle and phenformin treated KL_luc mice. (I) Total BEAC and mBEAC tumor area (μm²) in vehicle and phenformin treated KL_luc mice **p-value < 0.006. n=5 KL_luc mice per treatment group (vehicle or phenformin). Statistical analysis performed using an unpaired Student’s t test. All data are represented as the mean ± SEM.

Figure 7. Mitochondrial defects in LKB1^−/− NSCLC confer sensitivity to phenformin

(A) Lysates from A549-pBabe, A549-LKB1^WT, and A549-LKB1^KD cells were treated with vehicle (DMEM), phenformin (2 mM) or metformin (20 mM) for 4 hrs and immunoblotted with the indicated antibodies. (B) FACS analysis of cells from (A) stained with Mitotracker Red. (C) Cells from (A) stained with JC-1 dye following 4 hrs of treatment with vehicle (DMEM), phenformin (2 mM), metformin (20 mM) or CCCP (100 μM). Mitochondrial membrane potential ( Ψ, red/green ratios) were measured by emission of red and green fluorescence using FACS. (D) Oxygen consumption rate (OCR) of cells from (A). (E) Cells from (A) were stained with MitoSOX Red following 16 hrs of treatment with vehicle (DMEM) or phenformin (2 mM) and analyzed by FACS. (F) Lung tumor lysates from K_luc, KL_luc, and KP_luc mice treated 5 days with vehicle or phenformin ad lib feeding were immunoblotted with the indicated antibodies. (G) Model illustrating mitochondrial defects and phenformin induced cell death in LKB1^−/− deficient tumor cells. All data are represented as the mean ± SEM.