We explored whether the anti-
prostate cancer (PC) activity of the
androgen receptor-axis-targeted agents (ARATs)
abiraterone and
enzalutamide is enhanced by
metformin. Using complementary biological and molecular approaches, we determined the associated underlying mechanisms in pre-clinical
androgen-sensitive PC models. ARATs increased androgren receptors (ARs) in LNCaP and AR/ARv7 (AR variant) in VCaP cells, inhibited cell proliferation in both, and induced
poly(ADP-ribose) polymerase-1 (PARP-1) cleavage and death in VCaP but not LNCaP cells.
Metformin decreased AR and ARv7 expression and induced cleaved PARP-1-associated death in both cell lines.
Metformin with
abiraterone or
enzalutamide decreased AR and ARv7 expression showed greater inhibition of cell proliferation and greater induction of cell death than single agent treatments. Combination treatments led to increased cleaved PARP-1 and enhanced PARP-1 activity manifested by increases in
poly(ADP-ribose) (PAR) and nuclear accumulation of
apoptosis inducing factor (AIF). Enhanced
annexin V staining occurred in LNCaP cells only with
metformin/ARAT combinations, but no
caspase 3 recruitment occurred in either cell line. Finally,
metformin and
metformin/ARAT combinations increased lysosomal permeability resulting in
cathepsin G-mediated PARP-1 cleavage and cell death. In conclusion,
metformin enhances the efficacy of
abiraterone and
enzalutamide via two PARP-1-dependent,
caspase 3-independent pathways, providing a rationale to evaluate these combinations in
castration-sensitive PC.