Hypersensitivity to ferroptosis in chromophobe RCC is mediated by a glutathione metabolic dependency and cystine import via solute carrier family 7 member 11.

Chromophobe (Ch) renal cell carcinoma (RCC) arises from the intercalated cell in the distal nephron. There are no proven treatments for metastatic ChRCC. A distinguishing characteristic of ChRCC is strikingly high levels of reduced (GSH) and oxidized (GSSG) glutathione. Here, we demonstrate that ChRCC-derived cells exhibit higher sensitivity to ferroptotic inducers compared with clear-cell RCC. ChRCC-derived cells are critically dependent on cystine via the cystine/glutamate antiporter xCT to maintain high levels of glutathione, making them sensitive to inhibitors of cystine uptake and cyst(e)inase. Gamma-glutamyl transferase 1 (GGT1), a key enzyme in glutathione homeostasis, is markedly suppressed in ChRCC relative to normal kidney. Importantly, GGT1 overexpression inhibits the proliferation of ChRCC cells in vitro and in vivo, suppresses cystine uptake, and decreases levels of GSH and GSSG. Collectively, these data identify ferroptosis as a metabolic vulnerability in ChRCC, providing a potential avenue for targeted therapy for these distinctive tumors.

Loss of FLCN inhibits canonical WNT signaling via TFE3.

Lower lobe predominant pulmonary cysts occur in up to 90% of patients with Birt-Hogg-Dubé (BHD) syndrome, but the key pathologic cell type and signaling events driving this distinct phenotype remain elusive. Through examination of the LungMAP database, we found that folliculin (FLCN) is highly expressed in neonatal lung mesenchymal cells. Using RNA-Seq, we found that inactivation of Flcn in mouse embryonic fibroblasts leads to changes in multiple Wnt ligands, including a 2.8-fold decrease in Wnt2. This was associated with decreased TCF/LEF activity, a readout of canonical WNT activity, after treatment with a GSK3-α/ß inhibitor. Similarly, FLCN deficiency in HEK293T cells decreased WNT pathway activity by 76% post-GSK3-α/ß inhibition. Inactivation of FLCN in human fetal lung fibroblasts (MRC-5) led to ~ 100-fold decrease in Wnt2 expression and a 33-fold decrease in Wnt7b expression-two ligands known to be necessary for lung development. Furthermore, canonical WNT activity was decreased by 60%. Classic WNT targets such as AXIN2 and BMP4, and WNT enhanceosome members including TCF4, LEF1 and BCL9 were also decreased after GSK3-α/ß inhibition. FLCN-deficient MRC-5 cells failed to upregulate LEF1 in response to GSK3-α/ß inhibition. Finally, we found that a constitutively active ß-catenin could only partially rescue the decreased WNT activity phenotype seen in FLCN-deficient cells, whereas silencing the transcription factor TFE3 completely reversed this phenotype. In summary, our data establish FLCN as a critical regulator of the WNT pathway via TFE3 and suggest that FLCN-dependent defects in WNT pathway developmental cues may contribute to lung cyst pathogenesis in BHD.

Mechanisms of acquired resistance to rapalogs in metastatic renal cell carcinoma.

The mechanistic target of rapamycin (mTOR) is an established therapeutic target in renal cell carcinoma (RCC). Mechanisms of secondary resistance to rapalog therapy in RCC have not been studied previously. We identified six patients with metastatic RCC who initially responded to mTOR inhibitor therapy and then progressed, and had pre-treatment and post-treatment tumor samples available for analysis. We performed deep whole exome sequencing on the paired tumor samples and a blood sample. Sequence data was analyzed using Mutect, CapSeg, Absolute, and Phylogic to identify mutations, copy number changes, and their changes over time. We also performed in vitro functional assays on PBRM1 in RCC cell lines. Five patients had clear cell and one had chromophobe RCC. 434 somatic mutations in 416 genes were identified in the 12 tumor samples. 201 (46%) of mutations were clonal in both samples while 129 (30%) were acquired in the post-treatment samples. Tumor heterogeneity or sampling issues are likely to account for some mutations that were acquired in the post-treatment samples. Three samples had mutations in TSC1; one in PTEN; and none in MTOR. PBRM1 was the only gene in which mutations were acquired in more than one post-treatment sample. We examined the effect of PBRM1 loss in multiple RCC cell lines, and could not identify any effect on rapalog sensitivity in in vitro culture assays. We conclude that mTOR pathway gene mutations did not contribute to rapalog resistance development in these six patients with advanced RCC. Furthermore, mechanisms of resistance to rapalogs in RCC remain unclear and our results suggest that PBRM1 loss may contribute to sensitivity through complex transcriptional effects.

Impairment of gamma-glutamyl transferase 1 activity in the metabolic pathogenesis of chromophobe renal cell carcinoma.

Chromophobe renal cell carcinoma (ChRCC) accounts for 5% of all sporadic renal cancers and can also occur in genetic syndromes including Birt-Hogg-Dube (BHD) and tuberous sclerosis complex (TSC). ChRCC has a distinct accumulation of abnormal mitochondria, accompanied by characteristic chromosomal imbalances and relatively few "driver" mutations. Metabolomic profiling of ChRCC and oncocytomas (benign renal tumors that share pathological features with ChRCC) revealed both similarities and differences between these tumor types, with principal component analysis (PCA) showing a distinct separation. ChRCC have a striking decrease in intermediates of the glutathione salvage pathway (also known as the gamma-glutamyl cycle) compared with adjacent normal kidney, as well as significant changes in glycolytic and pentose phosphate pathway intermediates. We also found that gamma glutamyl transferase 1 (GGT1), the key enzyme of the gamma-glutamyl cycle, is expressed at ∼100-fold lower levels in ChRCC compared with normal kidney, while no change in GGT1 expression was found in clear cell RCC (ccRCC). Significant differences in specific metabolite abundance were found in ChRCC vs. ccRCC, including the oxidative stress marker ophthalmate. Down-regulation of GGT1 enhanced the sensitivity to oxidative stress and treatment with buthionine sulfoximine (BSO), which was associated with changes in glutathione-pathway metabolites. These data indicate that impairment of the glutathione salvage pathway, associated with enhanced oxidative stress, may have key therapeutic implications for this rare tumor type for which there are currently no specific targeted therapies.

TSC2 regulates microRNA biogenesis via mTORC1 and GSK3ß.

Tuberous sclerosis complex (TSC) is an autosomal dominant disease caused by germline inactivating mutations of TSC1 or TSC2. In TSC-associated tumors of the brain, heart, skin, kidney and lung, inactivation of both alleles of TSC1 or TSC2 leads to hyperactivation of the mTORC1 pathway. The TSC/mTORC1 pathway is a key regulator of cellular processes related to growth, proliferation and autophagy. We and others have previously found that mTORC1 regulates microRNA biogenesis, but the mechanisms are not fully understood. Microprocessor, a multi-protein complex including the nuclease Drosha, processes the primary miR transcript. Using a dual-luciferase reporter, we found that inhibition of mTORC1 or downregulation of Raptor decreased Microprocessor activity, while loss of TSC2 led to a striking increase (∼5-fold) in Microprocessor activity. To determine the global impact of TSC2 on microRNAs we quantitatively analyzed 752 microRNAs in Tsc2-expressing and Tsc2-deficient cells. Out of 259 microRNAs expressed in both cell lines, 137 were significantly upregulated and 24 were significantly downregulated in Tsc2-deficient cells, consistent with the increased Microprocessor activity. Microprocessor activity is known to be regulated in part by GSK3ß. We found that total GSK3ß levels were higher in Tsc2-deficient cells, and the increase in Microprocessor activity associated with Tsc2 loss was reversed by three different GSK3ß inhibitors. Furthermore, mTOR inhibition increased the levels of phospho-GSK3ß (S9), which negatively affects Microprocessor activity. Taken together these data reveal that TSC2 regulates microRNA biogenesis and Microprocessor activity via GSK3ß.

New Developments in the Pathogenesis of Pulmonary Cysts in Birt-Hogg-Dubé Syndrome.

Birt-Hogg-Dubé (BHD) syndrome is an autosomal dominant disorder caused by germline loss-of-function mutations in Folliculin gene (FLCN). BHD is characterized by lower lobe-predominant pulmonary cysts with risk of pneumothorax, benign skin tumors (fibrofolliculomas), and renal cell carcinoma, often of an unusual chromophobe/oncocytic hybrid histology. The FLCN protein functions in multiple signaling and metabolic pathways including positive regulation of mechanistic target of rapamycin complex 1 (mTORC1) activity via FLCN's GTPase (GAP) activity for Rag C, positive regulation of Wnt signaling (in mesenchymal cells), and negative regulation of TFE3 nuclear localization. Therefore, FLCN-deficient cells are predicted to have reduced mTORC1 and Wnt activity and enhanced TFE3 activity. Folliculin also has functions in autophagy, mitochondrial biogenesis, cell-cell adhesion, 5' AMP activated protein kinase activity, and other pathways. The specific contributions of these pathways to the lung manifestations of BHD are largely unknown. This review is focused on the pulmonary manifestations of BHD, highlighting selected recent advances in elucidating the cellular functions of FLCN and current hypotheses related to the pathogenesis of cystic lung disease in BHD, including the "stretch hypothesis." We also discuss important knowledge gaps in the field, including the genetic, cellular and physical mechanisms of cyst pathogenesis, and the timing of cyst initiation, which may occur during lung development.

Mechanisms of pulmonary cyst pathogenesis in Birt-Hogg-Dube syndrome: The stretch hypothesis.

Loss-of-function mutations in the folliculin gene (FLCN) on chromosome 17p cause Birt-Hogg-Dube syndrome (BHD), which is associated with cystic lung disease. The risk of lung collapse (pneumothorax) in BHD patients is 50-fold higher than in the general population. The cystic lung disease in BHD is distinctive because the cysts tend to be basilar, subpleural and lentiform, differentiating BHD from most other cystic lung diseases. Recently, major advances in elucidating the primary functions of the folliculin protein have been made, including roles in mTOR and AMPK signaling via the interaction of FLCN with FNIP1/2, and cell-cell adhesion via the physical interaction of FLCN with plakophilin 4 (PKP4), an armadillo-repeat containing protein that interacts with E-cadherin and is a component of the adherens junctions. In addition, in just the last three years, the pulmonary impact of FLCN deficiency has been examined for the first time. In mouse models, evidence has emerged that AMPK signaling and cell-cell adhesion are involved in alveolar enlargement. In addition, the pathologic features of human BHD cysts have been recently comprehensively characterized. The "stretch hypothesis" proposes that cysts in BHD arise because of fundamental defects in cell-cell adhesion, leading to repeated respiration-induced physical stretch-induced stress and, over time, expansion of alveolar spaces particularly in regions of the lung with larger changes in alveolar volume and at weaker "anchor points" to the pleura. This hypothesis ties together many of the new data from cellular and mouse models of BHD and from the human pathologic studies. Critical questions remain. These include whether the consequences of stretch-induced cyst formation arise through a destructive/inflammatory program or a proliferative program (or both), whether cyst initiation involves a "second hit" genetic event inactivating the remaining wild-type copy of FLCN (as is known to occur in BHD-associated renal cell carcinomas), and whether cyst initiation involves exclusively the epithelial compartment versus an interaction between the epithelium and mesenchyme. Ultimately, understanding the mechanisms of cystic lung disease in BHD may help to elucidate the pathogenesis of primary spontaneous pneumothorax, with more than 20,000 cases reported annually in the United States alone.

Tuberous sclerosis complex 2 loss increases lysophosphatidylcholine synthesis in lymphangioleiomyomatosis.

Lymphangioleiomyomatosis (LAM) is a destructive lung disease affecting women. LAM is caused by mutations in the tuberous sclerosis complex (TSC) genes. The TSC protein complex inhibits the mechanistic/mammalian target of rapamycin complex 1 (mTORC1), which is a master regulator of cellular metabolism. Using mass spectrometry-based lipid profiling, we analyzed plasma from patients with LAM and discovered elevated levels of four lysophosphatidylcholine (LPC) species (C16:0, C18:0, C18:1, and C20:4) compared with those in healthy control women. To investigate whether these lipids are generated in a TSC2-dependent manner, we profiled in vitro preclinical models of TSC/LAM and found significant LPC accumulation in TSC2-deficient cells relative to TSC2-expressing control cells. These lysoglycerophospholipid changes occurred alongside changes in other phospholipid and neutral lipid species. Treatment with rapamycin or torin1 or down-regulation of sterol regulatory element-binding protein (SREBP), a lipogenic transcription factor, did not suppress LPC in TSC2-deficient cells. Inhibition of distinct isoforms of phospholipase A2 decreased the proliferation of TSC2-deficient cells. Collectively, these results demonstrate that TSC2-deficient cells have enhanced choline phospholipid metabolism and reveal a novel function of the TSC proteins in choline lysoglycerophospholipid metabolism, with implications for disease pathogenesis and targeted therapeutic strategies.

TFEB drives mTORC1 hyperactivation and kidney disease in Tuberous Sclerosis Complex.

Tuberous Sclerosis Complex (TSC) is caused by TSC1 or TSC2 mutations, leading to hyperactivation of mechanistic target of rapamycin complex 1 (mTORC1) and lesions  in multiple organs including lung (lymphangioleiomyomatosis) and kidney (angiomyolipoma and renal cell carcinoma). Previously, we found that TFEB is constitutively active in TSC. Here, we generated two mouse models of TSC in which kidney pathology is the primary phenotype. Knockout of TFEB rescues kidney pathology and overall survival, indicating that TFEB is the primary driver of renal disease in TSC. Importantly, increased mTORC1 activity in the TSC2 knockout kidneys is normalized by TFEB knockout. In TSC2-deficient cells, Rheb knockdown or Rapamycin treatment paradoxically increases TFEB phosphorylation at the mTORC1-sites and relocalizes TFEB from nucleus to cytoplasm. In mice, Rapamycin treatment normalizes lysosomal gene expression, similar to TFEB knockout, suggesting that Rapamycin's benefit in TSC is TFEB-dependent. These results change the view of the mechanisms of mTORC1 hyperactivation in TSC and may lead to therapeutic avenues.

mTORC1 upregulates B7-H3/CD276 to inhibit antitumor T cells and drive tumor immune evasion.

Identifying the mechanisms underlying the regulation of immune checkpoint molecules and the therapeutic impact of targeting them in cancer is critical. Here we show that high expression of the immune checkpoint B7-H3 (CD276) and high mTORC1 activity correlate with immunosuppressive phenotypes and worse clinical outcomes in 11,060 TCGA human tumors. We find that mTORC1 upregulates B7-H3 expression via direct phosphorylation of the transcription factor YY2 by p70 S6 kinase. Inhibition of B7-H3 suppresses mTORC1-hyperactive tumor growth via an immune-mediated mechanism involving increased T-cell activity and IFN-γ responses coupled with increased tumor cell expression of MHC-II. CITE-seq reveals strikingly increased cytotoxic CD38+CD39+CD4+ T cells in B7-H3-deficient tumors. In pan-human cancers, a high cytotoxic CD38+CD39+CD4+ T-cell gene signature correlates with better clinical prognosis. These results show that mTORC1-hyperactivity, present in many human tumors including tuberous sclerosis complex (TSC) and lymphangioleiomyomatosis (LAM), drives B7-H3 expression leading to suppression of cytotoxic CD4+ T cells.

TSC2 Interacts with HDLBP/Vigilin and Regulates Stress Granule Formation.

Tuberous sclerosis complex (TSC) is caused by mutations of either the TSC1 or TSC2 tumor suppressor gene. TSC causes tumors of the brain, heart, kidney, skin and lymphangioleiomyomatosis (LAM). Here we report that the TSC2 protein physically binds to high-density lipoprotein binding protein (HDLBP), also called vigilin, a core stress granule (SG) protein, and that TSC2 localizes to SGs. SGs contain mRNAs and translation initiation complexes, and regulate gene expression by sequestering specific transcripts, thereby serving a cytoprotective role. TSC2 has never before been shown to localize to SGs and knocking down vigilin impacts SG translocation of TSC2. TSC2-deficient cells showed a striking increase in the number of SGs after thermal shock and arsenite treatment relative to Tsc2-expressing cells. Our findings also show that murine kidney lysates from a model of TSC have increased levels of SG components including G3BP1 and Caprin1. G3BP1 and Caprin are elevated in renal angiomyolipomas (a renal tumor common in patients with TSC) compared with control normal kidney. G3BP1 is also elevated in TSC-associated subependymal giant cell astrocytomas. We found that genetic inhibition of G3BP1 inhibits the proliferation of TSC2-deficient cells in vitro. Finally, in a mouse model of TSC, genetic inhibition of SGs suppresses cell growth, suggesting that targeting SGs may have efficacy in the therapy of TSC. IMPLICATIONS: This study demonstrates that TSC2 physically interacts with HDLBP/vigilin, a component of SGs, that TSC2 localizes to SG and that TSC2-deficient cells have more SGs, suggesting that SGs represent a novel therapeutic target in TSC.

TSC2 regulates lysosome biogenesis via a non-canonical RAGC and TFEB-dependent mechanism.

Tuberous Sclerosis Complex (TSC) is caused by TSC1 or TSC2 mutations, resulting in hyperactivation of the mechanistic target of rapamycin complex 1 (mTORC1). Transcription factor EB (TFEB), a master regulator of lysosome biogenesis, is negatively regulated by mTORC1 through a RAG GTPase-dependent phosphorylation. Here we show that lysosomal biogenesis is increased in TSC-associated renal tumors, pulmonary lymphangioleiomyomatosis, kidneys from Tsc2+/- mice, and TSC1/2-deficient cells via a TFEB-dependent mechanism. Interestingly, in TSC1/2-deficient cells, TFEB is hypo-phosphorylated at mTORC1-dependent sites, indicating that mTORC1 is unable to phosphorylate TFEB in the absence of the TSC1/2 complex. Importantly, overexpression of folliculin (FLCN), a GTPase activating protein for RAGC, increases TFEB phosphorylation at the mTORC1 sites in TSC2-deficient cells. Overexpression of constitutively active RAGC is sufficient to relocalize TFEB to the cytoplasm. These findings establish the TSC proteins as critical regulators of lysosomal biogenesis via TFEB and RAGC and identify TFEB as a driver of the proliferation of TSC2-deficient cells.

Lysosomal regulation of cholesterol homeostasis in tuberous sclerosis complex is mediated via NPC1 and LDL-R.

Tuberous sclerosis complex (TSC) is a multisystem disease associated with hyperactive mTORC1. The impact of TSC1/2 deficiency on lysosome-mediated processes is not fully understood. We report here that inhibition of lysosomal function using chloroquine (CQ) upregulates cholesterol homeostasis genes in TSC2-deficient cells. This TSC2-dependent transcriptional signature is associated with increased accumulation and intracellular levels of both total cholesterol and cholesterol esters. Unexpectedly, engaging this CQ-induced cholesterol uptake pathway together with inhibition of de novo cholesterol synthesis allows survival of TSC2-deficient, but not TSC2-expressing cells. The underlying mechanism of TSC2-deficient cell survival is dependent on exogenous cholesterol uptake via LDL-R, and endosomal trafficking mediated by Vps34. Simultaneous inhibition of lysosomal and endosomal trafficking inhibits uptake of esterified cholesterol and cell growth in TSC2-deficient, but not TSC2-expressing cells, highlighting the TSC-dependent lysosome-mediated regulation of cholesterol homeostasis and pointing toward the translational potential of these pathways for the therapy of TSC.

p62/SQSTM1 Cooperates with Hyperactive mTORC1 to Regulate Glutathione Production, Maintain Mitochondrial Integrity, and Promote Tumorigenesis.

p62/sequestosome-1 (SQSTM1) is a multifunctional adaptor protein and autophagic substrate that accumulates in cells with hyperactive mTORC1, such as kidney cells with mutations in the tumor suppressor genes tuberous sclerosis complex (TSC)1 or TSC2. Here we report that p62 is a critical mediator of TSC2-driven tumorigenesis, as Tsc2+/- and Tsc2f/f Ksp-CreERT2+ mice crossed to p62-/- mice were protected from renal tumor development. Metabolic profiling revealed that depletion of p62 in Tsc2-null cells decreased intracellular glutamine, glutamate, and glutathione (GSH). p62 positively regulated the glutamine transporter Slc1a5 and increased glutamine uptake in Tsc2-null cells. We also observed p62-dependent changes in Gcl, Gsr, Nqo1, and Srxn1, which were decreased by p62 attenuation and implicated in GSH production and utilization. p62 attenuation altered mitochondrial morphology, reduced mitochondrial membrane polarization and maximal respiration, and increased mitochondrial reactive oxygen species and mitophagy marker PINK1. These mitochondrial phenotypes were rescued by addition of exogenous GSH and overexpression of Sod2, which suppressed indices of mitochondrial damage and promoted growth of Tsc2-null cells. Finally, p62 depletion sensitized Tsc2-null cells to both oxidative stress and direct inhibition of GSH biosynthesis by buthionine sulfoximine. Our findings show how p62 helps maintain intracellular pools of GSH needed to limit mitochondrial dysfunction in tumor cells with elevated mTORC1, highlighting p62 and redox homeostasis as nodal vulnerabilities for therapeutic targeting in these tumors. Cancer Res; 77(12); 3255-67. ©2017 AACR.

High-throughput drug screen identifies chelerythrine as a selective inducer of death in a TSC2-null setting.

UNLABELLED: Tuberous sclerosis complex (TSC) is an autosomal dominant syndrome associated with tumors of the brain, heart, kidney, and lung. The TSC protein complex inhibits the mammalian or mechanistic target of rapamycin complex 1 (mTORC1). Inhibitors of mTORC1, including rapamycin, induce a cytostatic response in TSC tumors, resulting in temporary disease stabilization and prompt regrowth when treatment is stopped. The lack of TSC-specific cytotoxic therapies represents an important unmet clinical need. Using a high-throughput chemical screen in TSC2-deficient, patient-derived cells, we identified a series of molecules antagonized by rapamycin and therefore selective for cells with mTORC1 hyperactivity. In particular, the cell-permeable alkaloid chelerythrine induced reactive oxygen species (ROS) and depleted glutathione (GSH) selectively in TSC2-null cells based on metabolic profiling. N-acetylcysteine or GSH cotreatment protected TSC2-null cells from chelerythrine's effects, indicating that chelerythrine-induced cell death is ROS dependent. Induction of heme-oxygenase-1 (HMOX1/HO-1) with hemin also blocked chelerythrine-induced cell death. In vivo, chelerythrine inhibited the growth of TSC2-null xenograft tumors with no evidence of systemic toxicity with daily treatment over an extended period of time. This study reports the results of a bioactive compound screen and the identification of a potential lead candidate that acts via a novel oxidative stress-dependent mechanism to selectively induce necroptosis in TSC2-deficient tumors. IMPLICATIONS: This study demonstrates that TSC2-deficient tumor cells are hypersensitive to oxidative stress-dependent cell death, and provide critical proof of concept that TSC2-deficient cells can be therapeutically targeted without the use of a rapalog to induce a cell death response.

Folliculin regulates cell-cell adhesion, AMPK, and mTORC1 in a cell-type-specific manner in lung-derived cells.

Germline loss-of-function BHD mutations cause cystic lung disease and hereditary pneumothorax, yet little is known about the impact of BHD mutations in the lung. Folliculin (FLCN), the product of the Birt-Hogg-Dube (BHD) gene, has been linked to altered cell-cell adhesion and to the AMPK and mTORC1 signaling pathways. We found that downregulation of FLCN in human bronchial epithelial (HBE) cells decreased the phosphorylation of ACC, a marker of AMPK activation, while downregulation of FLCN in small airway epithelial (SAEC) cells increased the activity of phospho-S6, a marker of mTORC1 activation, highlighting the cell type-dependent functions of FLCN. Cell-cell adhesion forces were significantly increased in FLCN-deficient HBE cells, consistent with prior findings in FLCN-deficient human kidney-derived cells. To determine how these altered cell-cell adhesion forces impact the lung, we exposed mice with heterozygous inactivation of Bhd (similarly to humans with germline inactivation of one BHD allele) to mechanical ventilation at high tidal volumes. Bhd(+/-) mice exhibited a trend (P = 0.08) toward increased elastance after 6 h of ventilation at 24 cc/kg. Our results indicate that FLCN regulates the AMPK and mTORC1 pathways and cell-cell adhesion in a cell type-dependent manner. FLCN deficiency may impact the physiologic response to inflation-induced mechanical stress, but further investigation is required. We hypothesize that FLCN-dependent effects on signaling and cellular adhesion contribute to the pathogenesis of cystic lung disease in BHD patients.

Folliculin, the product of the Birt-Hogg-Dube tumor suppressor gene, interacts with the adherens junction protein p0071 to regulate cell-cell adhesion.

Birt-Hogg-Dube (BHD) is a tumor suppressor gene syndrome associated with fibrofolliculomas, cystic lung disease, and chromophobe renal cell carcinoma. In seeking to elucidate the pathogenesis of BHD, we discovered a physical interaction between folliculin (FLCN), the protein product of the BHD gene, and p0071, an armadillo repeat containing protein that localizes to the cytoplasm and to adherens junctions. Adherens junctions are one of the three cell-cell junctions that are essential to the establishment and maintenance of the cellular architecture of all epithelial tissues. Surprisingly, we found that downregulation of FLCN leads to increased cell-cell adhesion in functional cell-based assays and disruption of cell polarity in a three-dimensional lumen-forming assay, both of which are phenocopied by downregulation of p0071. These data indicate that the FLCN-p0071 protein complex is a negative regulator of cell-cell adhesion. We also found that FLCN positively regulates RhoA activity and Rho-associated kinase activity, consistent with the only known function of p0071. Finally, to examine the role of Flcn loss on cell-cell adhesion in vivo, we utilized keratin-14 cre-recombinase (K14-cre) to inactivate Flcn in the mouse epidermis. The K14-Cre-Bhd(flox/flox) mice have striking delays in eyelid opening, wavy fur, hair loss, and epidermal hyperplasia with increased levels of mammalian target of rapamycin complex 1 (mTORC1) activity. These data support a model in which dysregulation of the FLCN-p0071 interaction leads to alterations in cell adhesion, cell polarity, and RhoA signaling, with broad implications for the role of cell-cell adhesion molecules in the pathogenesis of human disease, including emphysema and renal cell carcinoma.

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.

The Birt-Hogg-Dube and tuberous sclerosis complex homologs have opposing roles in amino acid homeostasis in Schizosaccharomyces pombe.

Birt-Hogg-Dube (BHD) is a tumor suppressor gene disorder characterized by skin hamartomas, cystic lung disease, and renal cell carcinoma. The fact that hamartomas, lung cysts, and renal cell carcinoma can also occur in tuberous sclerosis complex (TSC) suggests that the BHD and TSC proteins may function within a common pathway. To evaluate this hypothesis, we deleted the BHD homolog in Schizosaccharomyces pombe. Expression profiling revealed that six permease and transporter genes, known to be down-regulated in Deltatsc1 and Deltatsc2, were up-regulated in Deltabhd, and levels of specific intracellular amino acids known to be low in Deltatsc1 and Deltatsc2 were elevated in Deltabhd. This "opposite" profile was unexpected, given the overlapping clinical phenotypes. The TSC1/2 proteins inhibit Rheb in mammals, and Tsc1/Tsc2 inhibit Rhb1 in S. pombe. Expression of a hypomorphic allele of rhb1(+) dramatically increased permease expression levels in Deltabhd but not in wild-type yeast. Loss of Bhd sensitized yeast to rapamycin-induced increases in permease expression levels, and rapamycin induced lethality in Deltabhd yeast expressing the hypomorphic Rhb1 allele. In S. pombe, it is known that Rhb1 binds Tor2, and Tor2 inhibition leads to up-regulation of permeases including those that are regulated by Bhd. Our data, therefore, suggest that Bhd activates Tor2. If the mammalian BHD protein, folliculin, similarly activates mammalian target of rapamycin, it will be of great interest to determine how mammalian target of rapamycin inhibition in BHD patients and mammalian target of rapamycin activation in TSC patients lead to overlapping clinical phenotypes.