Prostate cancer is the second leading cause of death among men in the United States. Most prostate cancer deaths are due to advanced disease that is refractory to existing treatments. PTEN is a tumor suppressor gene that is often lost in advanced prostate cancer resulting in constitutive activation of the PI3K pathway. PI3K pathway subsequently activates multiple downstream targets including AKT1 and mTOR and promotes cell growth and inhibits apoptosis.
The importance of the PI3K-AKT1-mTOR pathway in tumor progression has been elegantly demonstrated in vivo
using transgenic mouse models. Loss of PTEN
or expression of activated AKT1 leads to malignant or pre-cancerous lesions in mouse prostate, respectively. Treating transgenic mice expressing myristoylated Akt1 under a probacin promoter with RAD001 (a Rapamycin analogue) for two weeks reverses the PIN phenotype, a precancerous lesion of the prostate (1
). This dramatic impact of mTOR inhibition reversing the neoplastic phenotype is also observed in transgenic mice with selective prostate knock out of Pten
). Thus, mTOR inhibition with rapamycin or rapamycin analogues is effective in PI3K-driven prostate cancer.
mTOR signaling is mediated primarily through two multi-protein complexes (TORC1 and TORC2) (reviewed in (3
)). TORC1 includes mTOR, raptor, and mLST8 (also known as GβL) (4
). When activated, TORC1 phosphorylates two primary down-stream effectors, eukaryotic initiation factor 4E (eIF-4E)-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase 1 (S6K1) (7
). Phosphorylation of 4E-BP1 results in release of eIF-4E and increased cap-dependent translation of a set of proteins involved in G1- to S-phase progression including cyclin D and MYC (10
). Phosphorylation of S6K1 results in the phosphorylation of the 40S ribosomal protein S6 and the enhancement of translation of mRNAs that possess a 5′-terminal oligopyrimidine tract (12
). The TORC2 complex is comprised of mTOR, rictor, and mLST8 and has been shown to phosphorylate AKT1 (14
) and Paxillin (15
) among others. While mTOR is activated in many human cancers, the relative biological importance of TORC1 and TORC2 activity in human cancers remains unknown.
The drug rapamycin binds to a cellular protein FKBP12, and this complex inhibits the TORC1 complex. Despite the dramatic effects observed after treatment of cells in tissue culture and transgenic animals with rapamycin and rapamycin-derivatives, limited clinical efficacy of single agent mTOR inhibition has been observed in advanced prostate cancer. Here, we used genetically engineered prostate epithelial cells and found that MYC expression profoundly reduces sensitivity to rapamycin. Importantly, MYC copy number gain is associated with increased 4EBP1 expression in human prostate tumors, MYC binds to regulatory regions of the 4EBP1 gene, and MYC increases 4EBP1 expression and decreases autophagy. Suppression of 4EBP1 re-sensitized cells to rapamycin and resulted in less autophagy. In fact, 4EBP1 expression limits autophagy induction by at least two inducers: rapamycin and tunicamycin. Thus, MYC over-expression is associated with increased 4EBP1 expression and decreased autophagy which may play a role in limiting the clinical effectiveness of single agent rapamycin or rapamycin analogues.