Targeting autophagy overcomes Enzalutamide resistance in castration-resistant prostate cancer cells and improves therapeutic response in a xenograft model

Abstract

Macro-autophagy is associated with drug resistance in various cancers and can function as an adaptive response to maintain cell survival under metabolic stresses, including androgen deprivation. Androgen deprivation or treatment with androgen receptor (AR) signaling inhibitor (ARSI), Enzalutamide (MDV-3100, ENZA) or bicalutamide induced autophagy in androgen-dependent and in castration-resistant CaP (castration-resistant prostate cancer (CRPC)) cell lines. The autophagic cascade triggered by AR blockage, correlated with the increased light chain 3-II/I ratio and ATG-5 expression. Autophagy was observed in a subpopulation of C4-2B cells that developed insensitivity to ENZA after sustained exposure in culture. Using flow cytometry and clonogenic assays, we showed that inhibiting autophagy with clomipramine (CMI), chloroquine or metformin increased apoptosis and significantly impaired cell viability. This autophagic process was mediated by AMP-dependent protein kinase (AMPK) activation and the suppression of mammalian target of rapamycin (mTOR) through Raptor phosphorylation (Serine 792). Furthermore, small interfering RNA targeting AMPK significantly inhibited autophagy and promoted cell death in CaP cells acutely or chronically exposed to ENZA or androgen deprivation, suggesting that autophagy is an important survival mechanism in CRPC. Lastly, in vivo studies with mice orthotopically implanted with ENZA-resistant cells demonstrated that the combination of ENZA and autophagy modulators, CMI or metformin significantly reduced tumor growth when compared with control groups (P<0.005). In conclusion, autophagy is as an important mechanism of resistance to ARSI in CRPC. Antiandrogen-induced autophagy is mediated through the activation of AMPK pathway and the suppression of mTOR pathway. Blocking autophagy pharmacologically or genetically significantly impairs prostate cancer cell survival in vitro and in vivo, implying the therapeutics potential of autophagy inhibitors in the antiandrogen-resistance setting.

Atg7 cooperates with Pten loss to drive prostate cancer tumor growth

“…Given that prior studies in a xenograft model suggest that autophagy is also a mechanism of resistance to AR signaling inhibitors, future preclinical studies of autophagy modulation using our genetic model to further test the effect of Atg7 deficiency on castration or androgen axis targeting sensitivity and resistance with and without additional agents such as chemotherapy would be important. (Farrow et al 2014;Nguyen et al 2014).…”

Overcoming Autophagy to Induce Apoptosis in Castration-Resistant Prostate Cancer

“…After the extensive in vitro studies of the effect of saracatinib and enzalutamide on CRPC cell lines, we have concluded that autophagy was the usual pathway for cancer cell survival. This treatment-mediated autophagy is through activation of AMPK and repression of mTOR [1][2][3][4][5]. Knocking down AMPK with siRNA reversed the survival mechanism and led cells to undergo apoptosis.…”

Androgen Deprivation Therapy (ADT), Metabolic Syndrome and Metastatic Prostate Cancer: In Vivo and In Vitro Assessments of Effects of Metformin

“…Enzalutamide is a next generation androgen targeting agent that is used for CRPC and confers overall survival advantage. It has been reported that enzalutamide inhibits AR signalling, which induces autophagy and through this mechanism it causes treatment resistance and subsequent recurrence [345]. Overall, the findings suggested that metformin may reduce expression of autophagy genes.…”

“…It has previously been described that enzalutamide induces a catabolic process that allows temporal cellular regression, senescence and ultimately the promotion of treatment resistance and propagation [345]. Enzalutamide treatment allowed a trend for increased BCLN1 expression in LNCaP and MSK3 cells, which were then reversed by metformin.…”

“…It has previously been described that enzalutamide induces the catabolic process that allows temporal cellular regression, senescence and ultimately the promotion of treatment resistance and propagation [345].…”