Mitochondrial Complex I Inhibitors Expose a Vulnerability for Selective Killing of Pten-Null Cells

Abstract

A hallmark of advanced prostate cancer (PC) is the concomitant loss of PTEN and p53 function. To selectively eliminate such cells, we screened cytotoxic compounds on Pten^-/-;Trp53^-/- fibroblasts and their Pten-WT reference. Highly selective killing of Pten-null cells can be achieved by deguelin, a natural insecticide. Deguelin eliminates Pten-deficient cells through inhibition of mitochondrial complex I (CI). Five hundred-fold higher drug doses are needed to obtain the same killing of Pten-WT cells, even though deguelin blocks their electron transport chain equally well. Selectivity arises because mitochondria of Pten-null cells consume ATP through complex V, instead of producing it. The resulting glucose dependency can be exploited to selectively kill Pten-null cells with clinically relevant CI inhibitors, especially if they are lipophilic. In vivo, deguelin suppressed disease in our genetically engineered mouse model for metastatic PC. Our data thus introduce a vulnerability for highly selective targeting of incurable PC with inhibitors of CI.

Keywords: ATP; ATP synthase; Pten; RapidCaP; complex I; deguelin; glucose; metabolism; mitochondria; prostate cancer.

Figure 1. Identification of Chemotherapeutics with Efficacy against Pten-Null Genotype

(A) Schematic of chemotherapeutic screen workflow. MEFs were harvested from mice, and cellular genomic DNA was recombined after infection with adenovirus encoding Cre recombinase. Cells were selected (using viral vector-encoded selection markers) to generate pure populations. Chemotherapeutic efficacy of 23 agents (each at three concentrations) was assessed, and effects on the two genotypes were measured via assessment of cell activity, viability, and number (tetrazolium dye-based assay developed by Biolog). (B) Viability measured over a 24 hr period of Pten-WT and Pten-null cells in the presence of four exemplary drugs. Graphs show data from two biological replicates. Error bars show SD. (C) Biolog system output for visualization of results from (B). (D) Top: overview of the 96-well results obtained at four concentrations per drug, with colored bars indicating location of no drug control (blue), rotenone(orange), and deguelin (red). Top right: magnification of highlighted wells. Bottom: post-screen IC₅₀ calculation identifies rotenone and deguelin as hits for specific toxicity in Pten-null MEFs. (E) Cell viability after deguelin treatment is measured in the Biolog system for each genotype and shows a strong effect in Pten-null but not Pten-WT cells. Graphs show data from two biological replicates. Error bars show SD

Figure 2. Deguelin Kills Pten-Null Cells by Targeting Mitochondrial Complex I

(A) Left: super-resolution imaging (OMX) of mitochondria stained with MitoTracker shows fragmentation induced by deguelin. Scale bars, 10 μm. Right: Quantification of mitochondrial lengths after treatment with deguelin. Note that both cell types respond to deguelin with fragmentation. Error bars show SEM (n = 3); one-way ANOVA with multiple comparisons test; *p < 0.0001. See also Figure S1C. (B) Left: relative OCR by complex I in intact Pten-null MEFs normalized to DMSO-treated control cells. Middle: relative complex I OCR in saponin-permeabilized Pten-null MEFs. Error bars are SD, n = 5 replicates per group; one-way ANOVA with Dunnett multiple comparisons; *p < 0.0001 compared with 0 μM deguelin. Right: relative mitochondrial complex II-driven oxygen consumption rate of saponin-permeabilized Pten-null MEFs in the presence of deguelin or the complex II inhibitor 3-nitropropionic acid (3-NPA) and normalized to DMSO-treated control cells. Error bars are ±SEM (n = 3 independent experiments); one-way ANOVA with Dunnett multiple comparisons; *p < 0.05 compared with 0 μM deguelin. (C) Relative mitochondrial oxygen consumption rate (OCR) of intact p53/pTen^−/− MEFs stably expressing either the empty vector (EV) or the yeast NDI1 gene was measured in the presence of 0.5 μM deguelin using the Seahorse Biosciences XF24 Extracellular Flux Analyzer and normalized to the corresponding untreated cells. Error bars are SEM (n = 3 independent experiments); one-way ANOVA with Bonferroni multiple comparisons; **p < 0.0001. (D) Pten-null cells stably expressing either the EV or the yeast NDI1 gene were cultured in media containing 25 mM galactose and treated with or without 0.5 μM deguelin for 24 hr. Error bars are SEM (n = 3 independent experiments); two-way ANOVA with Bonferroni multiple comparisons; **p < 0.0001.

Figure 3. Complex V Function Is Reversed in Pten-Null Cells

(A) Relative mitochondrial membrane potential of cells grown in 25 or 7 mM glucose and in the presence or absence of the complex V inhibitor oligomycin. Pten-WT cells show increased membrane potential upon oligomycin treatment at physiological glucose. In contrast, Pten-null cells show decreased membrane potential under these conditions, demonstrating reversed proton flow through complex V. Right: schematic of reversed complex V function and its inhibition by oligomycin, resulting in decreased membrane potential because complex V is exporting protons at the cost of ATP consumption. Error bars are SD (n = 8). See also Figures S2D and S2E. (B) MTT assay for viability upon deguelin treatment as a function of glucose concentration and Pten status. A red line on the x axis indicates physiological glucose range. Error bars are SD (n = 3). (C) Per cell glucose consumption in Pten-null or WT cells in the presence or absence of 0.5 μM deguelin. Pten-null cells treated with deguelin showed an increased dependence on glucose consumption compared with untreated. Error bars are SD (n = 3). (D) Pten-null and Pten-WT cells were grown in 7 mM glucose, and kill curves were determined for deguelin and rotenone at 24 and 48 hr post-treatment. Error bars are SD (n = 4).

Figure 4. Deguelin Can Be Used to Target Lethal Prostate Cancer

(A) LC-MS analysis of deguelin concentration in the prostate at up to 6 hr post-intra-peritoneal (i.p.) injection. (B) Staining of mitochondrial Tom20 in prostate tissue from deguelin-treated WT animals shows a loss of staining compared with untreated tissue (see also Figure S3C). E denotes location of the prostatic epithelium and L that of the prostatic lumen; see also Figure S3B. Scale bar, 10 μm. (C) Left: percentage change in bioluminescence luciferase imaging (BLI) of DMSO- or deguelin-treated mice at start and end of the 10 week trial. Middle: waterfall plot for overall disease burden as measured by relative change to starting BLI at 2 and 10 weeks, respectively. Line graph for overall disease burden, measured by relative percentage BLI signal change from start shows significant differences between control and treatment animals. The dosing increase from 0.4 to 4 mg/kg during the 10week period is indicated in gray; see also Figure S3D. Lines show moving averages (period = 3), and asterisks indicate p < 0.05 after linear regression analysis compared with DMSO. Linear regression analysis was performed on the full dataset, not the moving average. Right: deguelin was well tolerated, as judged by minor changes in relative body weight over the 10 week trial period. (D) Absence of abnormal proliferation, as judged by Ki-67 staining of prostate tissue from deguelin-treated mouse B confirms successful suppression of disease. Scale bars, 30 μm. Quantification shows percentage Ki-67 positive nuclei per gland. Error bars are SEM; unpaired t test (control, n = 8; treated, n = 6).