Adaptive prostate cancer cell metabolism following combined anti-androgen and biguanide treatment

Emerging treatments for advanced prostate cancer continue to target androgen signalling leading to prolonged androgen deprivation. This induces metabolic dysfunction and hyperinsulinaemia which is specifically associated with more rapid treatment failure. An increasing body of epidemiological and experimental evidence suggests drugs used to treat insulin resistance, such as the biguanide metformin, may reduce tumour growth and cell survival. We previously showed in androgen-deprived prostate cancer cells metformin modulates autophagy and increases apoptosis. This led us to examine more potent biguanides as adjuvant therapy in prostate cancer. Here we compare the transcriptional and metabolic response to combined AR targeting+biguanide therapy in androgen-deprived prostate cancer cells and in a xenograft mouse model of castrate-resistant disease. Androgen-dependent prostate cancer cells (LNCaP, DUCaP) were treated for 48hrs with androgen receptor antagonist, enzalutamide followed by biguanides metformin, buformin, phenformin and cycloguanil (half IC50). Despite having the lowest IC50 in androgen-deprived prostate cancer cells, phenformin was outperformed by metformin and buformin with respect to induction of oxidative stress, reduced mitochondrial function, inhibition of cell cycle and induction of autophagy. RNAseq analysis identified adaptive cellular metabolism in response to biguanides incorporating one-carbon metabolism, de novo serine synthesis and amino acid salvage pathways. In principle component analysis, phenformin-treated cells clustered with enzalutamide-only control cells but separated metformin, buformin and cycloguanil which co-clustered based on modulation of genes associated with amino acid and one-carbon metabolic pathways. LNCaP xenograft tumours were established in male NOD-SCID mice. Mice were castrated prior to commencing biguanide therapy to determine effects on tumour growth rate. Metformin increased survival time by 40% compared to vehicle control, but buformin and cyclguanil increased survival by 76% and 82% respectively with reduced tumour growth rate and no apparent toxicity. We present data to support the concept that potent biguanides may have therapeutic value by capitalising on increased mitochondrial stress induced by androgen targeting in prostate cancer cells. The data also supports the critical role of one-carbon metabolism in energetic triage of cancer cells.