Magdalena Karbowniczek

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

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.

Constitutive mTOR activation in TSC mutants sensitizes cells to energy starvation and genomic damage via p53.

Miscoordination of growth and proliferation with the cellular stress response can lead to tumorigenesis. Mammalian target of rapamycin (mTOR), a central cell growth controller, is highly activated in some malignant neoplasms, and its clinical implications are under extensive investigation. We show that constitutive mTOR activity amplifies p53 activation, in vitro and in vivo, by stimulating p53 translation. Thus, loss of TSC1 or TSC2, the negative regulators of mTOR, results in dramatic accumulation of p53 and apoptosis in response to stress conditions. In other words, the inactivation of mTOR prevents cell death by nutrient stress and genomic damage via p53. Consistently, we also show that p53 is elevated in TSC tumors, which rarely become malignant. The coordinated relationship between mTOR and p53 during cellular stress provides a possible explanation for the benign nature of hamartoma syndromes, including TSC. Clinically, this also suggests that the efficacy of mTOR inhibitors in anti‐neoplastic therapy may also depend on p53 status, and mTOR inhibitors may antagonize the effects of genotoxic chemotherapeutics.

Estrogen promotes the survival and pulmonary metastasis of tuberin-null cells

Lymphangioleiomyomatosis (LAM) is an often fatal disease primarily affecting young women in which tuberin (TSC2)-null cells metastasize to the lungs. The mechanisms underlying the striking female predominance of LAM are unknown. We report here that 17-β-estradiol (E2) causes a 3- to 5-fold increase in pulmonary metastases in male and female mice, respectively, and a striking increase in circulating tumor cells in mice bearing tuberin-null xenograft tumors. E2-induced metastasis is associated with activation of p42/44 MAPK and is completely inhibited by treatment with the MEK1/2 inhibitor, CI-1040. In vitro, E2 inhibits anoikis of tuberin-null cells. Finally, using a bioluminescence approach, we found that E2 enhances the survival and lung colonization of intravenously injected tuberin-null cells by 3-fold, which is blocked by treatment with CI-1040. Taken together these results reveal a new model for LAM pathogenesis in which activation of MEK-dependent pathways by E2 leads to pulmonary metastasis via enhanced survival of detached tuberin-null cells.

Renal Angiomyolipomas from Patients with Sporadic Lymphangiomyomatosis Contain Both Neoplastic and Non-Neoplastic Vascular Structures

Renal angiomyolipomas are highly vascular tumors that occur sporadically, in women with pulmonary lymphangiomyomatosis (LAM), and in tuberous sclerosis complex (TSC). The goal of this study was to determine whether the distinctive vessels of angiomyolipomas are neoplastic or reactive. We studied angiomyolipomas with loss of heterozygosity (LOH) in the TSC2 region of chromosome 16p13 from patients with LAM. We found that angiomyolipomas contain five morphologically distinct vessel types: cellular, collagenous, hemangiopericytic, glomeruloid, and aneurysmatic. Using laser capture microdissection, we determined that four of the vessel types have TSC2 LOH and are therefore neoplastic. One vessel type, collagenous vessels, did not have LOH, and is presumably reactive. Recently, activation of S6 Kinase and its target S6 ribosomal protein (S6) was demonstrated in cells lacking TSC2 expression. We found that angiomyolipoma vessel types in which LOH were detected were immunoreactive with anti-phospho-S6 antibodies. Angiomyolipoma cells without LOH, including the endothelial component of the vessels, were not immunoreactive. To our knowledge, angiomyolipomas are the first benign vascular tumor in which the vascular cells, rather than the stromal cells, have been found to be neoplastic. Angiomyolipomas appear to reflect novel vascular mechanisms that may be the result of activation of cellular pathways involving S6 Kinase.

Rheb inhibits C-raf activity and B-raf/C-raf heterodimerization.

The Ras-Raf-MEK signaling cascade is critical for normal development and is activated in many forms of cancer. We have recently shown that B-Raf kinase interacts with and is inhibited by Rheb, the target of the GTPase-activating domain of the tuberous sclerosis complex 2 gene product tuberin. Here, we demonstrate for the first time that activation of Rheb is associated with decreased B-Raf and C-Raf phosphorylation at residues Ser-446 and Ser-338, respectively, concomitant with a decrease in the activities of both kinases and decreased heterodimerization of B-Raf and C-Raf. Importantly, the impact of Rheb on B-Raf/C-Raf heterodimerization and kinase activity are rapamycin-insensitive, indicating that they are independent of Rheb activation of the mammalian target of rapamycin-Raptor complex. In addition, we found that Rheb inhibits the association of B-Raf with H-Ras. Taken together, these results support a central role of Rheb in the regulation of the Ras/B-Raf/C-Raf/MEK signaling network.

Complement C5a receptor facilitates cancer metastasis by altering T cell responses in the metastatic niche

The impact of complement on cancer metastasis has not been well studied. In this report, we demonstrate in a preclinical mouse model of breast cancer that the complement anaphylatoxin C5a receptor (C5aR) facilitates metastasis by suppressing effector CD8(+) and CD4(+) T-cell responses in the lungs. Mechanisms of this suppression involve recruitment of immature myeloid cells to the lungs and regulation of TGFβ and IL10 production in these cells. TGFβ and IL10 favored generation of T regulatory cells (Treg) and Th2-oriented responses that rendered CD8(+) T cells dysfunctional. Importantly, pharmacologic blockade of C5aR or its genetic ablation in C5aR-deficient mice were sufficient to reduce lung metastases. Depletion of CD8(+) T cells abolished this beneficial effect, suggesting that CD8(+) T cells were responsible for the effects of C5aR inhibition. In contrast to previous findings, we observed that C5aR signaling promoted Treg generation and suppressed T-cell responses in organs where metastases arose. Overall, our findings indicated that the immunomodulatory functions of C5aR are highly context dependent. Furthermore, they offered proof-of-concept for complement-based immunotherapies to prevent or reduce cancer metastasis.

Pulmonary Alveolar Macrophages Contribute to the Premetastatic Niche by Suppressing Antitumor T Cell Responses in the Lungs

In contrast to tumor-associated macrophages, myeloid-derived suppressor cells, or inflammatory monocytes, functions of tissue resident macrophages, including alveolar macrophages (AM), in cancer were not well studied. Using a mouse model of breast cancer, we show that AM promote cancer metastasis to the lungs by suppressing antitumor T cells in this organ. AM accumulated in the premetastatic lungs through complement C5a receptor–mediated proliferation but not through recruitment from the circulation. AM preconditioned by breast tumors inhibited Th1 and favored generation of Th2 cells that had lower tumoricidal activity than Th1 cells. In addition, AM reduced the number and maturation of lung dendritic cells by regulating TGF-β in the lung environment. Depletion of AM reversed immunosuppression imposed by these cells and strengthened local Th1 responses, which significantly reduced lung metastatic burden. C5a receptor deficiency, which also lessens myeloid-derived suppressor cells in the premetastatic niche, synergized with the depletion of AM in preventing metastasis, leading to protection of mice from lung metastases. This study identifies AM as a new component of the premetastatic niche, which is harnessed by tumors to impose immunosuppression, and as a new target for cancer immunotherapies to eliminate or reduce metastasis. Because the lungs are the most common target for hematogenous metastasis, this research offers a plausible explanation for susceptibility of the lungs to cancer metastasis.

The evolutionarily conserved TSC/Rheb pathway activates Notch in tuberous sclerosis complex and Drosophila external sensory organ development

Mutations in either of the genes encoding the tuberous sclerosis complex (TSC), TSC1 and TSC2, result in a multisystem tumor disorder characterized by lesions with unusual lineage expression patterns. How these unusual cell-fate determination patterns are generated is unclear. We therefore investigated the role of the TSC in the Drosophila external sensory organ (ESO), a classic model of asymmetric cell division. In normal development, the sensory organ precursor cell divides asymmetrically through differential regulation of Notch signaling to produce a pIIa and a pIIb cell. We report here that inactivation of Tsc1 and overexpression of the Ras homolog Rheb each resulted in duplication of the bristle and socket cells, progeny of the pIIa cell, and loss of the neuronal cell, a product of pIIb cell division. Live imaging of ESO development revealed this cell-fate switch occurred at the pIIa-pIIb 2-cell stage. In human angiomyolipomas, benign renal neoplasms often found in tuberous sclerosis patients, we found evidence of Notch receptor cleavage and Notch target gene activation. Further, an angiomyolipoma-derived cell line carrying biallelic TSC2 mutations exhibited TSC2- and Rheb-dependent Notch activation. Finally, inhibition of Notch signaling using a gamma-secretase inhibitor suppressed proliferation of Tsc2-null rat cells in a xenograft model. Together, these data indicate that the TSC and Rheb regulate Notch-dependent cell-fate decision in Drosophila and Notch activity in mammalian cells and that Notch dysregulation may underlie some of the distinctive clinical and pathologic features of TSC.

Mutational Analysis of the von Hippel Lindau Gene in Clear Cell Renal Carcinomas from Tuberous Sclerosis Complex Patients

Tuberous sclerosis complex (TSC) is an autosomal-dominant disorder characterized by seizures, mental retardation, autism, and tumors of multiple organs. Renal disease in TSC includes angiomyolipomas, cysts, and renal cell carcinomas. It is known that somatic mutations in the von Hippel Lindau (VHL) tumor suppressor gene occur in most clear cell renal carcinomas. To determine whether TSC-associated clear cell carcinomas also contain VHL mutations, we analyzed six tumors for loss of heterozygosity in the VHL gene region of chromosome 3p and for mutations in the VHL gene. Four of the patients were women between the ages of 34 and 68 years, and two were males under the age of 21 years. The loss of heterozygosity analysis was performed using polymorphic microsatellite markers, and the mutational analysis was performed using direct sequencing. Chromosome 3p loss of heterozygosity was not detected, and no VHL mutations were identified. These findings suggest that mutations in the TSC1 and TSC2 genes lead to clear cell renal carcinogenesis via an alternate pathway not involving VHL mutations.

The Role of Tuberin in Cellular Differentiation: Are B-Raf and MAPK Involved?

Abstract: Tuberous sclerosis complex (TSC) is a tumor suppressor gene syndrome whose manifestations can include seizures, mental retardation, autism, and tumors in the brain, retina, kidney, heart, and skin. The products of the TSC1 and TSC2 genes, hamartin and tuberin, respectively, heterodimerize and inhibit the mammalian target of rapamycin (mTOR). This review focuses on the genetic and biochemical basis of the renal and pulmonary manifestations of TSC, angiomyolipomas, and lymphangiomyomatosis, respectively. Genetic analyses of sporadic angiomyolipomas revealed that all three components (smooth muscle, vessels, and fat) derive from a common progenitor cell, indicating the ability of cells lacking tuberin to differentiate into multiple lineages. Other genetic studies showed that the benign smooth muscle cells of pulmonary lymphangiomyomatosis have the ability to migrate to other organs. These findings suggest that tuberin and hamartin play a role in the regulation of cellular migration and differentiation. We have found that tuberin activates B‐Raf kinase and p42/44 MAPK and that cells lacking tuberin have low levels of B‐Raf activity. We hypothesize that aberrant B‐Raf activity in angiomyolipomas leads to abnormal cellular differentiation and migration.