Targeted inhibition of mitochondrial Hsp90 suppresses localised and metastatic prostate cancer growth in a genetic mouse model of disease

Abstract

Background: The molecular chaperone heat shock protein-90 (Hsp90) is a promising cancer drug target, but current Hsp90-based therapy has so far shown limited activity in the clinic.

Methods: We tested the efficacy of a novel mitochondrial-targeted, small-molecule Hsp90 inhibitor, Gamitrinib (GA mitochondrial matrix inhibitor), in the Transgenic Adenocarcinoma of the Mouse Prostate (TRAMP) model. The TRAMP mice receiving 3-week or 5-week systemic treatment with Gamitrinib were evaluated for localised or metastatic prostate cancer, prostatic intraepithelial neoplasia (PIN) or localised inflammation using magnetic resonance imaging, histology and immunohistochemistry. Treatment safety was assessed histologically in organs collected at the end of treatment. The effect of Gamitrinib on mitochondrial dysfunction was studied in RM1 cells isolated from TRAMP tumours.

Results: Systemic administration of Gamitrinib to TRAMP mice inhibited the formation of localised prostate tumours of neuroendocrine or adenocarcinoma origin, as well as metastatic prostate cancer to abdominal lymph nodes and liver. The Gamitrinib treatment had no effect on PIN or prostatic inflammation, and caused no significant animal weight loss or organ toxicity. Mechanistically, Gamitrinib triggered acute mitochondrial dysfunction in RM1 cells, with loss of organelle inner membrane potential and release of cytochrome-c in the cytosol.

Conclusions: The Gamitrinib has pre-clinical activity and favourable tolerability in a genetic model of localised and metastatic prostate cancer in immunocompetent mice. Selective targeting of mitochondrial Hsp90 could provide novel molecular therapy for patients with advanced prostate cancer.

Figure 1

Prostate histopathology of untreated TRAMP mice at 19.7 weeks of age (group 2). Prostatic samples were isolated from group 2 TRAMP mice, and analysed by H&E staining and light microscopy. Representative cases of prostatic neuroendocrine tumours (A, B; ventral prostate) or adenocarcinoma (C, D; dorsal prostate) in group 2 TRAMP mice associated with extensive PIN lesions and various degrees of inflammation are shown.

Figure 2

Quantification of prostatic lesions in untreated or Gamitrinib-treated TRAMP mice. Prostatic samples were harvested from untreated (control) or Gamitrinib (G-G4)-treated TRAMP mice, and analysed by H&E staining and light microscopy. The percentage of PIN lesions in representative matched samples of dorso-lateral prostate (DLP) from the various groups is shown. An inflammation or metastasis score was determined, and expressed as arbitrary units (U). Quantification of inflammation (NS), PIN (P=0.091) or tumour formation (P=0.0038) was carried out in TRAMP mice at 19.7 weeks of age (group 2). Quantification of metastasis to liver and loco-regional abdominal lymph nodes was determined in TRAMP mice at 24.9 weeks of age (group 1). Abbreviation: NS=not significant.

Figure 3

‘Mitochondriotoxic’ activity of Gamitrinib. (A) The TRAMP tumour-derived RM1 cells were labelled with the mitochondrial membrane potential-sensitive dye, JC1, incubated as indicated and analysed after 12 h by multiparametric flow cytometry. The percentage of cells in each quadrant is indicated. (B) RM1 cells were treated as indicated, and isolated cytosolic extracts were analysed by western blotting. COX-IV and β-actin were used as mitochondrial or cytosolic markers, respectively. Abbreviations: Cyto-c=cytochrome-c; PI=propidium iodide.