Chapter 2 Profiling mTOR Pathway in Neuroendocrine Tumors

The serine/threonine kinase mammalian target of rapamycin (mTOR) plays a central role in regulating critical cellular processes such as growth, proliferation, and protein synthesis. The study of cancer predisposing syndromes within which neuroendocrine tumors (NETs) may arise has furnished clues on the involvement of mTOR pathway in sporadic diseases so far. Recent comprehensive analyses have definitely shown activation of mTOR pathway in both experimental and human sporadic NETs. Upstream regulators of mTOR (PTEN and TSC2) have been found mutated in sporadic PNETs. Activation of mTOR pathways in NETs is already demonstrated by expression profiles analysis that revealed downregulation of TSC2 gene and alterations of TSC2 and PTEN protein expression in the vast majority of tumors well-differentiated tumors. Moreover, a global microRNA expression analysis revealed the overexpression, in highly aggressive tumors, of a microRNA (miR-21) that targets PTEN reducing its expression and therefore leading to mTOR activation as well. Overall, these clues have furnished the rationale for the use of mTOR inhibitors the treatment for PNETs. With the recent approval of everolimus (mTOR-targeted drug) for the treatment of advanced PNETs, this paradigm has been effectively translated into the clinical setting. In this review, we discuss mTOR pathway involvement in NETs, the clinical evidence supporting the use of mTOR inhibitors in cancer treatment, and the current clinical issues that remain to be elucidated to improve patients’ management. The pathway of the mammalian target of rapamycin (mTOR) plays a central role both in cell proliferation and in the survival rate. Physiologically, it finely tunes anabolic and catabolic processes according to the available energy sources to S. Cingarlini M. Bonomi C. Trentin G. Tortora (&) Medical Oncology, Department of Medicine, University of Verona and Azienda Ospedaliera Universitaria Integrata, P.le Scuro 10, 37134 Verona, Italy e-mail: giampaolo.tortora@univr.it V. Corbo A. Scarpa ARC-NET Research Centre and Department of Pathology and Diagnostics, University of Verona and Azienda Ospedaliera Universitaria Integrata, Verona, Italy E. Raymond et al. (eds.), Management of Neuroendocrine Tumors of the Pancreas and Digestive Tract, DOI: 10.1007/978-2-8178-0430-9_2, Springer-Verlag France 2014 9 warrant cell proliferation and homeostasis [1]. mTOR is also involved in many pathological conditions other than cancer such as diabetes, neurodegeneration, and obesity. Aberrant signaling caused by molecular alterations within the cascade may contribute to cancer development and progression [2–4]. The great amount of extracellular and intracellular inputs converging on it (or on its singular components) makes mTOR a crucial crossroad whose outputs influence essential cellular functions (such as protein/lipid synthesis, autophagy, or cytoskeletal organization). Growth factors stimuli (acting on mTORC1 and triggering the downstream anabolic signaling), energy depletion and low oxygen levels (activating mainly AMPK and thereby inhibiting mTOR complex either directly or through TSC2), DNA damage (which leads to a PTENand TSC2mediated inactivation of mTOR), and amino acids levels (whose presence is essential for mTOR signaling but whose exact mechanism of action is still unraveled) are some of the most significant examples of the plethora of inputs and outputs coming to and from mTOR [1]. In neuroendocrine tumors (NETs), nearly all the members of PI3K/Akt/mTOR pathway, from the upstream RTK inducers to its final effectors, can be molecularly altered and one or more than one of the above-mentioned alterations can be detected in the same cancer cell. The involvement of mTOR pathway in neuroendocrine tumorigenesis is suggested by a series of evidences: • Familial syndromes such as multiple endocrine neoplasia type 1 (MEN1), von Hippel–Lindau (VHL) syndrome, type 1 neurofibromatosis (NF1), and tuberous sclerosis complex (TSC). Single pathogenic molecular alterations may trigger the development of NETs with a higher incidence if compared to the generic population. Inactivation of VHL is associated with an increased steady-state level of HIF-1, whose expression is dependent on mTOR-mediated translational regulation [5, 6]. Loss of NF1 is associated with constitutive mTOR activation (depending upon Ras and PI3K) [7]. Loss of function mutations of either TSC1 or TSC2, whose encoded proteins form the TSC complex, can negatively regulate mTOR. • Sporadic disease: The majority of primary pancreatic neuroendocrine tumors (PNETs) show reduced protein levels of either one or both of the two main inhibitors of the mTOR pathway, TSC2 and PTEN [2]. Allelic loss of PTEN at the level of the chromosome arm 10q is frequent, and somatic inactivating mutations affecting PTEN and TSC2 genes have been reported in nearly 10 % of PNETS [8–10]. Reduced PTEN expression may also be ascribed to the miR-21 overexpression, a noncoding microRNA regulating protein expression on a posttranscriptional level [11]. Oncogene mutations affecting mTOR pathway are rarely, if ever, observed [12, 13]. • A phase III clinical trial showing that the mTOR inhibitor everolimus gave a clinically meaningful benefit in treated patients. 10 S. Cingarlini et al.