AR pathway inhibition has long been the treatment of choice for men with metastatic prostate cancer. While much attention has been devoted to mechanisms of acquired resistance, there has been little investigation of the considerable variability in primary response among patients. Here we show, by mRNA transcriptome analyses, that activation of the PI3K pathway is associated with repressed androgen signaling in mouse and human prostate cancers and that this may, in part, be responsible for the castrate resistant phenotype observed with these prostate tumors. Importantly, we demonstrate that this resistance is reversible because inhibition of the PI3K pathway restores AR signaling in PTEN deficient prostate cells. At least one mechanism appears to be through relief of negative feedback to HER kinases. Similarly, blockade of AR relieves feedback inhibition of AKT by the phosphatase PHLPP. This reciprocal feedback regulation of the PI3K and AR pathways provides a compelling explanation for the poor efficacy of single pathway therapy in PTEN null cancers and the substantially better effects of combined PI3K/AR pathway inhibition ().
Prior work has implicated PTEN
loss as a potential cause of castration resistance in mice and in humans (Gao et al., 2006
; Zhang et al., 2009
). Zhang and colleagues reported that Pten
prostate conditional null mice treated with surgical castration have a delay in tumor growth and minimal tumor regression (Zhang et al., 2009
). Although no human studies have formally addressed this question, there is evidence from presurgical treatment studies that tumors with PTEN loss are relatively refractory to bicalutamide (Ham et al., 2009
; Schulman et al., 2000
). Despite the evidence that PTEN loss can promote castration resistance, there is little insight into the mechanism. Some reports have suggested that PTEN loss activates AR, through PI3K-mediated stabilization of AR protein levels or AKT-mediated phosphorylation and transcriptional activation of AR. Conversely, other studies have demonstrated that PI3K activation promotes degradation of AR and inhibits AR transcriptional activity (Lin et al., 2004
; Lin et al., 2003
; Wang et al., 2008
). Our transcriptome studies make a strong case for the latter model. In addition, our finding that reduced expression of the AR target gene FKBP5 results in an increase in AKT activation (via PHLPP degradation) in PTEN null cancers further explains the survival advantage of these tumor cells in the setting of castration.
This work has immediate implications for the design of clinical trials evaluating PI3K pathway inhibitors in prostate cancer. Our preclinical data predict that single agent PI3K pathway inhibitor therapy will most likely result in disease stabilization rather that tumor regression, particularly in PTEN null tumors which represent ~40 percent of primary cancers and ~70 percent of metastases (Taylor et al., 2010
). Additionally, given that the primary serum marker used to monitor disease progression (prostate-specific antigen or PSA) is androgen regulated, patients treated with PI3K pathway inhibitors may experience a rise in PSA level if their tumors are PTEN deficient. Our data argue that combined therapy with an AR pathway inhibitor is required for maximal efficacy in PTEN null cancers. In patients with hormone-naïve disease this could be achieved using currently available antiandrogen therapy, but patients with castration resistant prostate cancer are likely to require next generation AR pathway inhibitors such as abiraterone or MDV3100.
Because BEZ235 inhibits both PI3K and mTORC1/2, our data do not delineate which target is most critical for the observed effects of combination therapy. Others reported beneficial effects of combined AR and mTORC1 (rapalog) inhibition in a similar Ptenlox/lox
model, but the magnitude of tumor response was less substantial since mice had significant amounts of residual tumor tissue at the time of sacrifice (Zhang et al., 2009
). In addition, these investigators monitored tumor volume by ultrasound, which makes it difficult to distinguish between shrinkage caused by true tumor regression versus a reduction in the cystic dilation that accompanies Pten−/−
prostate tumors. Kinkade et al also reported benefit from combining rapamycin with a MEK inhibitor in Nkx3.1−/−; Pten+/−
mice, but this experiment differs in that Pten+/−
mice have a less aggressive cancer phenotype than the Ptenlox/lox
model (Kinkade et al., 2008
). Side-by-side experiments using identical endpoints in the same model are required to properly compare these regimens. In the meantime, our in vitro
studies establish that dual PI3K/mTORC1/2 inhibition (BEZ235) is superior to mTORC1 inhibition (RAD001) when combined with AR blockade and that MEK inhibition (PD0325901) is relatively ineffective. Because BEZ235 inhibits mTORC1/2 more potently than PI3K, it is possible that the superiority of BEZ235 over RAD001 is solely through TORC1/2 blockade (Serra et al., 2008
). This question can be addressed using selective TORC1/2 inhibitors (Chresta et al., 2010
; Thoreen et al., 2009
Our finding that HER2/3 activation (and subsequent AR activation) is associated with PI3K pathway inhibition also has important clinical implications since a HER2 kinase inhibitor such as lapatinib could, in theory, replace the requirement for an antiandrogen in combination with a PI3K pathway inhibitor. Our studies with the preclinical HER2 inhibitor PKI-166 establish this principle in vitro
. Single agent trials with HER2 inhibitors (trastuzamab, pertuzumab, lapatinib) in men with castration resistant prostate cancer have been largely negative (Solit and Rosen, 2007
), but our data suggest that combination of these inhibitors with PI3K pathway inhibitors is required to elicit activity.
In summary, our results demonstrate that inhibition of the PI3K pathway in PTEN negative prostate cancer results in feedback signaling to the receptor tyrosine kinase HER2/HER3 leading to activation of AR. Conversely, blockade of AR results in activation of AKT through reduced levels of FKBP5 impairing the stability of PHLPP. This bidirectional crosstalk between two critical survival pathways in prostate cancer provides the molecular rationale for simultaneously targeting both pathways. The success of clinical trials evaluating PI3K pathway inhibitors in prostate cancer could be optimized by enrolling patients with documented activation of the PI3K pathway and treating in combination with appropriate AR pathway inhibition.