EGFR-dependent and independent activation of Akt/mTOR cascade in bone and soft tissue tumors

Abstract

To gain the insight into the involvement of signaling mediated by the mammalian target of rapamycin (mTOR) in the phenotype and biological profiles of tumors and tumor-like lesions of the bone and soft tissue, we analyzed the expression and phosphorylation (activation) of mTOR and its correlation with the status of upstream and downstream modulator proteins Akt, p70S6-kinase (S6K), and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), which we refer to collectively as mTOR cassette proteins. Immunohistochemical analysis of 140 cases showed activation of Akt in 55% (61% in malignant and 27% in benign), and mTOR expression in 61% (66% in malignant and 39% in benign). The preponderance of mTOR activation was found in tumors of peripheral nerve sheath (malignant peripheral nerve sheath tumor and schwannoma), skeletal muscle origin (rhabdomyosarcoma), and in those exhibiting epithelial nature (chordoma and synovial sarcoma). Together with the result of immunoblotting analysis, it was shown that many of those particular tumors with mTOR activation exhibited activation of Akt, S6K, and 4E-BP1, suggesting the constitutive activation of the Akt/mTOR pathway. In addition, although activation of the Akt/mTOR pathway was largely independent of activation of epidermal growth factor receptor (EGFR), mutation of EGFR was frequently accompanied by constitutive activation of Akt–mTOR–S6K/4E-BP1. By clinicopathological analysis, activation of Akt correlates with statistically higher probability of metastasis. We conclude that mTOR-mediated signaling proteins function not only in the proliferation of the tumor cells, but also in the differentiation and/or maintenance of morphological phenotypes in tumors of rhabdomyoblastic and nerve sheath cell origin. Furthermore, mTOR signaling may also modulate morphogenesis of tumors exhibiting epithelial nature. Additionally, activated Akt may have a function in metastasis. Overall, these results suggest that inhibitors of mTOR cassette may be useful as novel components of combined chemotherapy for a defined subset of bone and soft tissue sarcomas.