These data show that rapamycin induces only partial growth inhibition, whereas the dual inhibitors have a much greater suppression of proliferation. Open in a separate window Figure 2. Rapamycin, AZD8055, and Torin-1 inhibit proliferation of MTT cells in vitro. other catecholamine-dependent symptoms (using – and -adrenergic receptor antagonists), surgery is the main therapeutic option. For persisting disease, radiolabeled meta-iodobenzylguanidine therapy, peptide receptor radionucleotide therapy with radiolabeled somatostatin analogs (4), and certain types of chemotherapy may be helpful, but in advanced disease, in particular in patients transporting H100 succinate dehydrogenase subunit B (SDH-B) mutations, surgical resection is frequently ineffective and recurrence is usually frequent and eventually lethal (2). As a part of larger clinical trials for the evaluation of novel targeted therapies H100 in neuroendocrine tumors or as single case reports, a small number of patients with malignant pheochromocytomas and paragangliomas have shown at least temporary responses to the multiple tyrosine kinase inhibitor sunitinib (5). Other specific targeted therapies, including the tyrosine kinase inhibitor imatinib, were not found to be of significant benefit for these patients (6). Thus, there is an ongoing and urgent need for specific targeted therapies for such patients. The mammalian target of rapamycin (mTOR) is usually a serine/threonine protein kinase that is a grasp regulator of cell proliferation and survival (7), integrating complex upstream pathways and signals, including insulin, growth factors, and nutrient sensing, from the surrounding environment. The role of mTOR in malignancy is well established (8), and it represents a rational molecular target in oncology (9). Two major mTOR complexes (mTORCs) regulate its activity: mTORC1, which is usually allosterically inhibited by the macrolide antibiotic rapamycin (sirolimus) and contains the regulatory-associated protein raptor, and mTORC2 including the rapamycin-insensitive mTOR companion protein rictor (10). mTORC1 is mostly involved in growth factor-stimulated cellular proliferation and cellular homeostasis through phosphorylation of the ribosomal protein S6 kinase 1 (S6K1) and the eukaryotic translation initiation factor 4E-binding protein 1. It is allosterically inhibited by rapamycin, but the downstream substrate 4E-binding protein 1 is only partially dephosphorylated by rapamycin. This explains the H100 limited effect of rapalogs on protein synthesis. Rapamycin-resistant mTORC2 plays a prominent role in the regulation of the actin cytoskeleton and cellular motility. mTORC2 directly phosphorylates the serine/threonine protein kinase Akt/protein kinase B H100 at S473, linking this complex to the activation of the mTORC1 pathway. Activation of mTORC2 prospects to Akt phosphorylation and thus feeds forward in a positive fashion (11). Accumulating evidence has supported that this phosphoinositide 3-kinase (PI3K)/AKT/mTOR signaling pathway plays an important role in the pathogenesis of several neuroendocrine tumors, including pheochromocytoma (3, 12). For instance, S6K1, as a downstream target of the pathway, has been shown to be overexpressed in human pheochromocytoma, suggesting the potential use of inhibitors of this pathway in this disease (13). Recent reports also link the mTOR pathway to mutations in the gene, Rabbit Polyclonal to USP6NL which predisposes to the development of pheochromocytoma (14), emphasizing the importance of studying familial syndromes of pheochromocytoma to understand the pathogenic mechanisms involved in both sporadic and familial forms of the disease. Regrettably, studies using mTOR inhibitors in patients with pheochromocytoma have not clearly shown any therapeutic benefit. The mTOR inhibitor everolimus (RAD001; Novartis, Basel, Switzerland) failed to demonstrate a major clinical benefit in a small group of patients with pheochromocytoma (15). However, the inhibitors used in this study target only partially mTORC1, and in some solid tumors, treatment with these drugs has been associated with elevated Akt phosphorylation (16). These unpromising clinical studies were consistent with early experimental work showing that rapamycin inhibited proliferation of normal rat chromaffin cells stimulated by exogenous mitogens but was relatively ineffective against spontaneously proliferating PC12 rat pheochromocytoma cells (17). Recent data have recognized mTORC2 as the major kinase that phosphorylates Akt on Ser-473 (18, 19), and we have previously reported that levels of phospho-Akt are increased in pheochromocytoma compared with normal adrenal tissue (20). Moreover, there are several lines of evidence emphasizing a prominent role for mTORC2 in development of pheochromocytoma. For example, hypoxia-inducible factor H100 2, which is usually downstream of the mTORC2 pathway (21), is particularly overexpressed in some subtypes of pheochromocytoma (22C25). This suggests that drugs that would target both mTORC1 and mTORC2 might be of a greater benefit and demonstrate antitumor activity where brokers targeting only the mTORC1 have failed. Novel inhibitors targeting both mTORC1 and mTORC2 have been recently developed, including AZD8055 and Torin-1 (26C28). Compared with rapamycin and everolimus, they have high activity.
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