Tumorigenesis in tuberous sclerosis complex is autophagy and p62/sequestosome 1 (SQSTM1)-dependent

Abstract

Tuberous sclerosis complex (TSC) is a tumor suppressor syndrome characterized by benign tumors in multiple organs, including the brain and kidney. TSC-associated tumors exhibit hyperactivation of mammalian target of rapamycin complex 1 (mTORC1), a direct inhibitor of autophagy. Autophagy can either promote or inhibit tumorigenesis, depending on the cellular context. The role of autophagy in the pathogenesis and treatment of the multisystem manifestations of TSC is unknown. We found that the combination of mTORC1 and autophagy inhibition was more effective than either treatment alone in inhibiting the survival of tuberin (TSC2)-null cells, growth of TSC2-null xenograft tumors, and development of spontaneous renal tumors in Tsc2 +/− mice. Down-regulation of Atg5 induced extensive central necrosis in TSC2-null xenograft tumors, and loss of one allele of Beclin1 almost completely blocked macroscopic renal tumor formation in Tsc2 +/− mice. Surprisingly, given the finding that lowering autophagy blocks TSC tumorigenesis, genetic down-regulation of p62/sequestosome 1 (SQSTM1), the autophagy substrate that accumulates in TSC tumors as a consequence of low autophagy levels, strongly inhibited the growth of TSC2-null xenograft tumors. These data demonstrate that autophagy is a critical component of TSC tumorigenesis, suggest that mTORC1 inhibitors may have autophagy-dependent prosurvival effects in TSC, and reveal two distinct therapeutic targets for TSC: autophagy and the autophagy target p62/SQSTM1.utophagy is increasingly recognized to play a critical role in tumor development and cancer therapy (1, 2). In autophagy, cells undergo membrane rearrangement to sequester a portion of cytoplasm, organelles, and intracellular proteins for delivery to a degradative lysosome for recycling. In situations of bioenergetic stress, autophagy promotes the survival of established tumors by supplying metabolic precursors; however, excessive autophagy has been associated with cell death (3, 4). Inhibition of autophagy using chloroquine (CQ), which blocks lysosomeautophagosome fusion and lysosomal protein degradation (5), suppresses the growth of Myc-induced lymphomas (6). In other situations, however, inhibition of autophagy promotes tumorigenesis; for example, haploinsufficinecy for the autophagy gene Beclin1 promotes tumorigenesis in mouse models (7,8), and allelic loss of Beclin1 is associated with human breast, ovarian, and prostate cancers (1).Tuberous sclerosis complex (TSC) is an autosomal dominant tumor suppressor gene syndrome caused by germline mutations in the TSC1 or TSC2 genes (9). Patients with TSC have multisystem manifestations, which can include neurologic disease (i.e., seizures, mental retardation, and autism), benign tumors in multiple organs, and pulmonary lymphangioleiomyomatosis (LAM).The TSC1-TSC2 protein complex acts as a cellular sensor, integrating signals from growth factors (10), hypoxia (11, 12), ATP availability (13), IκB kinase (IKK) (14), and the cell cycle (15) through direct phosphorylation by kinases ...

MTOR inhibition attenuates DNA damage and apoptosis through autophagy-mediated suppression of CREB1

“…Here, we present that the induced autophagy represents a protective role in WT MEFs as inhibition of autophagy enhanced etoposide-induced DNA damage and reduced the cell viability. Consistent with the observation from Henske's group, 61 inhibition of autophagy significantly suppressed the viability of rapamycin-pretreated MTOR-hyperactivated MEFs.…”

p62/SQSTM1 regulates cellular oxygen sensing by attenuating PHD3 activity through aggregate sequestration and enhanced degradation

“…An upregulation in p62 protein level is detected in e.g. breast cancer, glioblastoma cells, hepatocellular carcinomas and TSC-deficient renal cancers (Galavotti et al, 2012;Inami et al, 2011;Parkhitko et al, 2011;Takamura et al, 2011;Thompson et al, 2003). In line, p62 can support autophagy in tumors and enhance tumor survival.…”

Role of autophagy in the maintenance and function of cancer stem cells

“…-/-and Tsc2 +/-mice (Parkhitko et al, 2011). Apparently at odds with this, enhanced BECN1 inactivation augments tumor growth of established non-small-cell lung cancer with active mutation of epidermial growth factor (EGFR) (Wei et al, 2013b).…”