Akt-Dependent Proapoptotic Effects of Dietary Restriction on Late-Stage Management of a Phosphatase and Tensin Homologue/Tuberous Sclerosis Complex 2–Deficient Mouse Astrocytoma

Abstract

Purpose: Malignant astrocytomas exhibit constitutive Akt phosphorylation due to reduced phosphatase and tensin homologue (PTEN) tumor suppressor expression or to increased growth factor receptor tyrosine kinase activation. Many astrocytomas are also tuberous sclerosis complex 2 (TSC2) protein deficient and exhibit constitutive mammalian target of rapamycin (mTOR) activity. Astrocytomas harboring PTEN/Akt/TSC2 pathway mutations are dependent on glycolysis to satisfy their bioenergetic requirements. Therapies that disrupt energy homeostasis can potentially manage astrocytoma growth and progression. Although dietary restriction (DR) reduces glycolysis and manages early-stage astrocytoma growth, no prior studies have identified the mechanisms involved or determined if DR can also manage late-stage tumor growth. Experimental Design: The effects of a late-onset intermittent DR feeding paradigm were examined in adult C57BL/6J mice bearing the syngeneic CT-2A malignant astrocytoma grown orthotopically or subcutaneously. Results: In contrast to contralateral normal brain, CT-2A was PTEN/TSC2 protein deficient; exhibited constitutive Akt, mTOR, and BAD phosphorylation; and overexpressed insulin-like growth factor-I (IGF-I), IGF-I receptor, hypoxia-inducible transcription factor-1a (HIF-1a), type 1 glucose transporter protein (GLUT1), and pyruvate kinase. DR initiated 10 to 14 days after tumor implantation (late onset) reduced CT-2A growth, delayed malignant progression, and significantly extended survival. DR suppressed phosphorylation of Akt and BAD while reducing expression of IGF-I, HIF-1a, and GLUT1. DR also enhanced procaspase-9/procaspase-3 cleavage but had no effect mTOR phosphorylation. Conclusions: Our findings indicate that IGF-I/Akt signaling is associated with the antiapoptotic and glycolytic phenotype of the CT-2A astrocytoma and that DR targets this pathway. Moreover, PTEN/TSC2 deficiency may impair adaptation to the DR-induced disruption of energy homeostasis, thus enhancing apoptosis. Our findings highlight the efficacy of late-onset DR in managing astrocytoma growth and suggest that DR may be an effective broad-spectrum inhibitor of Akt signaling in PTEN/TSC2^deficient astrocytomas.Malignant astrocytomas are the most common primary brain tumors and represent a leading cause of cancer-related deaths in children and the elderly (1 -4). The inability to effectively manage astrocytomas has been due, in part, to the unique anatomic and metabolic environment of the brain that prevents the complete resection of tumor tissue and impedes the delivery of therapeutic agents. Although glucose is the preferred metabolic fuel for healthy neurons and glia, these cells can transition to noncarbohydrate fuels (ketone bodies) for energy under low glucose conditions (5, 6). In contrast, astrocytomas lack metabolic flexibility and are more susceptible than normal brain tissues to cell death mediated by oxidative stress or ATP depletion in response to reduced glucose availability (7 -10). Hence, therapies that can ex...

Cancer as a Metabolic Disease

“…The RTG response will activate MYC, Ras, HIF-1α, Akt, and mTor etc, which are required to facilitate and to sustain up-regulation of substrate level phosphorylation [61,110,113,167,168]. In addition to facilitating the uptake and metabolism of alternative energy substrates through substrate level phosphorylation, MYC and Ras further stimulate cell proliferation [136,169,170].…”

“…For example, sustained glycolysis requires participation of mTOR, MYC, Ras, HIF-1α, and the IGF-1/PI3K/Akt signaling pathways [28,110,112,113,128,168]. The up-regulation of these genes and pathways together with inactivation of tumor suppressor genes like p53, which is required to initiate apoptosis, will disengage the apoptotic-signaling cascade thus preventing programmed cell death [142].…”

“…Vascularity is necessary in order to provide the tumor with essential energy nutrients to include glucose and glutamine, and to remove toxic tumor waste products such as lactic acid and ammonia [49]. In addition to its role in up-regulating glycolysis in response to hypoxia, HIF-1α is also the main transcription factor for vascular endothelial growth factor (VEGF), which stimulates angiogenesis [168,[180][181][182]. HIF-1α is part of the IGF-1/ PI3K/Akt signaling pathway that also indirectly influences expression of β FGF, another key angiogenesis growth factor [168,183].…”

“…In addition to its role in up-regulating glycolysis in response to hypoxia, HIF-1α is also the main transcription factor for vascular endothelial growth factor (VEGF), which stimulates angiogenesis [168,[180][181][182]. HIF-1α is part of the IGF-1/ PI3K/Akt signaling pathway that also indirectly influences expression of β FGF, another key angiogenesis growth factor [168,183]. Hence the sustained vascularity of tumors can be linked mechanistically to the metabolic requirements of substrate level phosphorylation necessary for tumor cell survival.…”

“…Dietary energy restriction specifically targets the IGF-1/PI3K/Akt/HIF-1α signaling pathway, which underlies several cancer hallmarks to include cell proliferation, evasion of apoptosis, and angiogenesis [168,175,176,250,251,254,[258][259][260][261][262][263][264][265]. Calorie restriction also causes a simultaneous down-regulation of multiple Figure 2 Linking the hallmarks of cancer to impaired energy metabolism.…”