PI3K/mTOR signaling promotes tumor progression by activating cell proliferation, growth and survival [4
] and has been implicated in tumor metastasis. Here we provide evidence that a novel catalytic PI3K/mTOR inhibitor may exert anti-tumor effects by opposing metastasis. Inhibition of effector kinases by the PI3K/mTOR inhibitor, PF1502, decreased C-terminal phosphorylation of p27, reduced its cytoplasmic localization, and attenuated tumor cell motility, invasion and metastasis. That these effects occur at a drug dose that failed to inhibit cell proliferation suggests that processes governing tumor metastasis downstream of PI3K/mTOR may be independent of those driving proliferation. Thus, PI3K/mTOR inhibition, in lines that appear resistant to the compound’s antiproliferative action and indeed at a dose that fails to prevent primary tumor growth, may have potential to prevent or attenuate the establishment of micrometastatic foci that initiate systemic tumor spread.
While there is evidence that both PI3K and mTOR pathways may contribute to metastasis, the mechanisms mediating this are not well understood. In human breast cancers, pAKT and p4EBP1 were greater in matched distant metastases compared to primary tumors [26
]. mTORC2-dependent AKT activation increased ovarian cancer motility in vitro and metastasis in vivo [8
], and PI3K inhibition prevented metastasis in a murine thyroid cancer model [27
]. mTORC1 increased migration in gastric cancer cells [28
], mTORC2 drove migration and invasion in a glioma model [29
] and both Rictor and Raptor expression are required for metastasis of a colorectal cancer xenograft [30
]. While none of these reports distinguished between effects on tumor growth versus direct effects on metastasis, they raise the possibility that PI3K/mTOR inhibition may not only impair biosynthetic processes driving tumor growth, but also modulate the metastatic process.
The CDK inhibitor p27 plays dual roles to regulate both cell proliferation and motility. While p27 restrains normal cell proliferation through cyclin-Cdk2 inhibition [9
], it can also modulate cell motility through mechanisms involving its C-terminal domain [31
]. p27 can bind RhoA to inactivate RhoA/ROCK and drive cell motility [16
]. Motility effects of p27 may exist in normal cells, since p27-dependent migration is essential for normal cortical neuron development in vivo in murine embryos [33
]. In cancers, oncogenic PI3K/mTOR activation increases p27 phosphorylation at T157 and/or T198 and promotes cytoplasmic p27 mislocalization [10
]. Indeed, PI3K-mediated p27 phosphorylation at T198 enhances its binding to and inhibition of RhoA [14
The balance between growth inhibitory and acquired pro-motility/metastatic functions may determine effects of p27 in different cancers. For example, p27 knockdown increased proliferation and enhanced primary tumor formation in a Ras-driven mouse tumor model, but impaired tumor invasion [35
]. In many cancers, p27 protein levels are reduced due to miRNA-mediated loss of synthesis or accelerated proteolysis [9
]. In contrast, tumors with activated AKT exhibit abundant cytoplasmic p27 [9
]. Cytoplasmic p27 was implicated in local invasion in an AKT-driven human glioma xenograft [36
] and overexpression of cytoplasmic p27CK−
enhanced murine melanoma metastasis [37
Here we investigated further the link between PI3K/mTOR activation, p27 and cancer metastasis. The well-characterized highly bone-metastatic 1833 model showed activation of PI3K and mTOR effector kinases and a p27-dependent increase in cell motility and invasion in vitro. The dual PI3K/mTOR inhibitor drug, PF1502, inhibited PI3K as evidenced by decreased pPDK1, and effectively impaired activation of both TORC1 and TORC2 substrates. It also reduced C-terminal p27 phosphorylation, cytoplasmic p27 accumulation, and phenocopied effects of p27 knockdown to impair cell adhesion, motility, and invasion in vitro. The p27CK-DD mutant, a T157/T198 phosphomimetic, increased motility and invasion of parental 231 cells. It also promoted resistance to in vitro effects of PI3K/mTOR inhibition on invasion and motility in both 231 and 1833 cells. That the phosphomimetic p27 did not fully reverse PF1502 effects on motility/invasion may reflect that aspartate at T157/T198 fails to fully mimic phosphorylation, or that p27-independent mediators also contribute to PI3K/mTOR driven tumor metastasis.
The present in vivo experimental approach was not intended to mimic patient therapy, but rather to assay the requirement for PI3K/mTOR pathway activity at the time of injection for subsequent primary and metastatic tumor outgrowth. Notably, prior PI3K/mTOR inhibition by PF1502 that failed to restrain cell proliferation during or after one week in culture in vitro, and did not reduce primary orthotopic tumor growth, significantly impaired the subsequent outgrowth of bone metastatic tumors after intracardiac injection in vivo. In addition, nodal metastasis from orthotopic primary tumors also showed a trend toward reduction with drug pre-treatment prior to injection. Thus, drug effects to impair metastasis in this model appear to occur despite the lack of an antiproliferative effect, and may modulate tumor cell extravasation and establishment of metastatic foci, as suggested by early IVIS data in the first few days post-injection. Present data extend prior work implicating PI3K and mTOR pathways in tumor growth and metastasis [30
], and suggest that drug effects on cell proliferation or growth may be separable from those driving metastasis. Indeed, malignant progression of cells resistant to antiproliferative effects of a novel PI3K/mTOR kinase inhibitor may still be interdicted at the level of metastasis—an effect that might be missed by traditional phase 1 clinical trials focused on tumor size reduction.
A relationship between cytoplasmic p27 and metastasis is supported by the novel observation that there is a positive correlation between the % of primary breast cancer cells showing cytoplasmic p27 staining with the number of nodes affected at diagnosis. While loss of nuclear p27 is strongly associated with poor patient outcome [9
], few studies have addressed the prognostic import of cytoplasmic p27, which is observed in up to 60 % of human cancers. Cytoplasmic p27 is correlated with poor outcome in colon and prostate cancers and lymphoma [9
]. p27 localization to cytoplasm in primary breast cancers was associated with AKT activation[10
]. The present study supports our earlier report associating cytoplasmic p27 in primary breast cancer with a reduced metastasis free interval [10
], and provides the first association with reduced patient survival.
PI3K and/or mTOR inhibitors have shown significant therapeutic promise in a several cancers [3
]. Dual PI3K/mTOR catalytic-site inhibitors have begun to enter clinical trials [3
] and have shown improved anti-tumor effects over allosteric mTORC1 inhibitors in pre-clinical models of breast [19
], pancreatic [22
], and renal cell cancers [39
], melanoma [40
], glioma [21
], multiple myeloma [42
], and acute myeloid leukemia [43
]. The present work suggests the antiproliferative effects of PI3K/mTOR inhibitor drugs may be distinct from their anti-metastatic action. The latter may result in part from their effects to abrogate cytoplasmic p27 accumulation. This work has implications for the clinical application of this new class of dual catalytic-site PI3K/mTOR inhibitors: in addition to their effects to check primary tumor growth, PI3K/mTOR inhibitors may also help prevent establishment of metastasis by clinically occult tumor cells, proving a rationale for trials investigating their potential to prevent systemic metastases early in the disease course. Furthermore, the presence of cytoplasmic p27 may identify tumors with PI3K/mTOR activation and provide a potential biomarker of PI3K/mTOR inhibitor therapeutic efficacy.