Antitumor activities of the synthetic retinoid ST1926 in two-dimensional and three-dimensional human breast cancer models.

Despite recent advances in chemotherapy, aggressive and metastatic breast cancers remain refractory to targeted therapy and the development of novel drugs is urgently needed. Retinoids are crucial regulators of cellular proliferation, differentiation, and cell death, and have shown potent chemotherapeutic and chemopreventive properties. The major drawback of the use of all-trans retinoic acid (ATRA) in cancer therapy is disease relapse. Therefore, synthetic retinoids, specifically ST1926, have emerged as potent anticancer agents. Given the importance of the microenvironment in modulating the response of cancer cells to chemotherapeutic drugs, we investigated the antitumor activities of ST1926 in two-dimensional (2D) and different three-dimensional (3D) human breast cancer models and compared them with ATRA. We have shown that in 2D cell culture models, ATRA-resistant MCF-7 and MDA-MB-231 cells were sensitive to ST1926 at submicromolar concentrations that spared the 'normal-like' breast epithelial cells. ST1926 induced apoptosis and S-phase arrest, caused DNA damage, and downregulated the Wnt/ß-catenin pathway in breast cancer cells in 2D and 3D cell culture models. ST1926-mediated growth inhibition was independent of the retinoid receptor-signaling pathway. Long-term treatments with low submicromolar ST1926 concentrations reduced the anchorage-independent growth and decreased the sphere-forming ability of breast cancer progenitor cells in the sphere formation assay. Furthermore, ST1926 potently induced cell death of breast cancer cells under 3D conditions and spared the lumen-forming ability of normal-like breast epithelial cells. In tested 3D models, ATRA had minimal effects on the growth of breast cancer cells compared with ST1926. In summary, our results highlight the therapeutic potential of ST1926 in breast cancer and warrant its further clinical development.

HSP90 inhibitors induce GPNMB cell-surface expression by modulating lysosomal positioning and sensitize breast cancer cells to glembatumumab vedotin.

Transmembrane glycoprotein NMB (GPNMB) is a prognostic marker of poor outcome in patients with triple-negative breast cancer (TNBC). Glembatumumab Vedotin, an antibody drug conjugate targeting GPNMB, exhibits variable efficacy against GPNMB-positive metastatic TNBC as a single agent. We show that GPNMB levels increase in response to standard-of-care and experimental therapies for multiple breast cancer subtypes. While these therapeutic stressors induce GPNMB expression through differential engagement of the MiTF family of transcription factors, not all are capable of increasing GPNMB cell-surface localization required for Glembatumumab Vedotin inhibition. Using a FACS-based genetic screen, we discovered that suppression of heat shock protein 90 (HSP90) concomitantly increases GPNMB expression and cell-surface localization. Mechanistically, HSP90 inhibition resulted in lysosomal dispersion towards the cell periphery and fusion with the plasma membrane, which delivers GPNMB to the cell surface. Finally, treatment with HSP90 inhibitors sensitizes breast cancers to Glembatumumab Vedotin in vivo, suggesting that combination of HSP90 inhibitors and Glembatumumab Vedotin may be a viable treatment strategy for patients with metastatic TNBC.

AMPK-dependent phosphorylation is required for transcriptional activation of TFEB and TFE3.

Increased macroautophagy/autophagy and lysosomal activity promote tumor growth, survival and chemo-resistance. During acute starvation, autophagy is rapidly engaged by AMPK (AMP-activated protein kinase) activation and MTOR (mechanistic target of rapamycin kinase) complex 1 (MTORC1) inhibition to maintain energy homeostasis and cell survival. TFEB (transcription factor E3) and TFE3 (transcription factor binding to IGHM enhancer 3) are master transcriptional regulators of autophagy and lysosomal activity and their cytoplasm/nuclear shuttling is controlled by MTORC1-dependent multisite phosphorylation. However, it is not known whether and how the transcriptional activity of TFEB or TFE3 is regulated. We show that AMPK mediates phosphorylation of TFEB and TFE3 on three serine residues, leading to TFEB and TFE3 transcriptional activity upon nutrient starvation, FLCN (folliculin) depletion and pharmacological manipulation of MTORC1 or AMPK. Collectively, we show that MTORC1 specifically controls TFEB and TFE3 cytosolic retention, whereas AMPK is essential for TFEB and TFE3 transcriptional activity. This dual and opposing regulation of TFEB and TFE3 by MTORC1 and AMPK is reminiscent of the regulation of another critical regulator of autophagy, ULK1 (unc-51 like autophagy activating kinase 1). Surprisingly, we show that chemoresistance is mediated by AMPK-dependent activation of TFEB, which is abolished by pharmacological inhibition of AMPK or mutation of serine 466, 467 and 469 to alanine residues within TFEB. Altogether, we show that AMPK is a key regulator of TFEB and TFE3 transcriptional activity, and we validate AMPK as a promising target in cancer therapy to evade chemotherapeutic resistance.Abbreviations: ACACA: acetyl-CoA carboxylase alpha; ACTB: actin beta; AICAR: 5-aminoimidazole-4-carboxamide ribonucleotide; AMPK: AMP-activated protein kinase; AMPKi: AMPK inhibitor, SBI-0206965; CA: constitutively active; CARM1: coactivator-associated arginine methyltransferase 1; CFP: cyan fluorescent protein; CLEAR: coordinated lysosomal expression and regulation; DKO: double knock-out; DMEM: Dulbecco's modified Eagle's medium; DMSO: dimethyl sulfoxide; DQ-BSA: self-quenched BODIPY® dye conjugates of bovine serum albumin; EBSS: Earle's balanced salt solution; FLCN: folliculin; GFP: green fluorescent protein; GST: glutathione S-transferases; HD: Huntington disease; HTT: huntingtin; KO: knock-out; LAMP1: lysosomal associated membrane protein 1; MEF: mouse embryonic fibroblasts; MITF: melanocyte inducing transcription factor; MTORC1: MTOR complex 1; PolyQ: polyglutamine; RPS6: ribosomal protein S6; RT-qPCR: reverse transcription quantitative polymerase chain reaction; TCL: total cell lysates; TFE3: transcription factor binding to IGHM enhancer 3; TFEB: transcription factor EB; TKO: triple knock-out; ULK1: unc-51 like autophagy activating kinase 1.

Folliculin impairs breast tumor growth by repressing TFE3-dependent induction of the Warburg effect and angiogenesis.

Growing tumors exist in metabolically compromised environments that require activation of multiple pathways to scavenge nutrients to support accelerated rates of growth. The folliculin (FLCN) tumor suppressor complex (FLCN, FNIP1, FNIP2) is implicated in the regulation of energy homeostasis via 2 metabolic master kinases: AMPK and mTORC1. Loss-of-function mutations of the FLCN tumor suppressor complex have only been reported in renal tumors in patients with the rare Birt-Hogg-Dube syndrome. Here, we revealed that FLCN, FNIP1, and FNIP2 are downregulated in many human cancers, including poor-prognosis invasive basal-like breast carcinomas where AMPK and TFE3 targets are activated compared with the luminal, less aggressive subtypes. FLCN loss in luminal breast cancer promoted tumor growth through TFE3 activation and subsequent induction of several pathways, including autophagy, lysosomal biogenesis, aerobic glycolysis, and angiogenesis. Strikingly, induction of aerobic glycolysis and angiogenesis in FLCN-deficient cells was dictated by the activation of the PGC-1α/HIF-1α pathway, which we showed to be TFE3 dependent, directly linking TFE3 to Warburg metabolic reprogramming and angiogenesis. Conversely, FLCN overexpression in invasive basal-like breast cancer models attenuated TFE3 nuclear localization, TFE3-dependent transcriptional activity, and tumor growth. These findings support a general role of a deregulated FLCN/TFE3 tumor suppressor pathway in human cancers.

Loss of hepatic Flcn protects against fibrosis and inflammation by activating autophagy pathways.

Non-alcoholic fatty liver disease (NAFLD) is the most frequent liver disease worldwide and can progress to non-alcoholic steatohepatitis (NASH), which is characterized by triglyceride accumulation, inflammation, and fibrosis. No pharmacological agents are currently approved to treat these conditions, but it is clear now that modulation of lipid synthesis and autophagy are key biological mechanisms that could help reduce or prevent these liver diseases. The folliculin (FLCN) protein has been recently identified as a central regulatory node governing whole body energy homeostasis, and we hypothesized that FLCN regulates highly metabolic tissues like the liver. We thus generated a liver specific Flcn knockout mouse model to study its role in liver disease progression. Using the methionine- and choline-deficient diet to mimic liver fibrosis, we demonstrate that loss of Flcn reduced triglyceride accumulation, fibrosis, and inflammation in mice. In this aggressive liver disease setting, loss of Flcn led to activation of transcription factors TFEB and TFE3 to promote autophagy, promoting the degradation of intracellular lipid stores, ultimately resulting in reduced hepatocellular damage and inflammation. Hence, the activity of FLCN could be a promising target for small molecule drugs to treat liver fibrosis by specifically activating autophagy. Collectively, these results show an unexpected role for Flcn in fatty liver disease progression and highlight new potential treatment strategies.

The Transcription Factors TFEB and TFE3 Link the FLCN-AMPK Signaling Axis to Innate Immune Response and Pathogen Resistance.

TFEB and TFE3 are transcriptional regulators of the innate immune response, but the mechanisms regulating their activation upon pathogen infection are poorly elucidated. Using C. elegans and mammalian models, we report that the master metabolic modulator 5'-AMP-activated protein kinase (AMPK) and its negative regulator Folliculin (FLCN) act upstream of TFEB/TFE3 in the innate immune response, independently of the mTORC1 signaling pathway. In nematodes, loss of FLCN or overexpression of AMPK confers pathogen resistance via activation of TFEB/TFE3-dependent antimicrobial genes, whereas ablation of total AMPK activity abolishes this phenotype. Similarly, in mammalian cells, loss of FLCN or pharmacological activation of AMPK induces TFEB/TFE3-dependent pro-inflammatory cytokine expression. Importantly, a rapid reduction in cellular ATP levels in murine macrophages is observed upon lipopolysaccharide (LPS) treatment accompanied by an acute AMPK activation and TFEB nuclear localization. These results uncover an ancient, highly conserved, and pharmacologically actionable mechanism coupling energy status with innate immunity.

mTOR Pathways in Cancer and Autophagy.

TOR (target of rapamycin), an evolutionarily-conserved serine/threonine kinase, acts as a central regulator of cell growth, proliferation and survival in response to nutritional status, growth factor, and stress signals. It plays a crucial role in coordinating the balance between cell growth and cell death, depending on cellular conditions and needs. As such, TOR has been identified as a key modulator of autophagy for more than a decade, and several deregulations of this pathway have been implicated in a variety of pathological disorders, including cancer. At the molecular level, autophagy regulates several survival or death signaling pathways that may decide the fate of cancer cells; however, the relationship between autophagy pathways and cancer are still nascent. In this review, we discuss the recent cellular signaling pathways regulated by TOR, their interconnections to autophagy, and the clinical implications of TOR inhibitors in cancer.

Antitumor Effect of the Atypical Retinoid ST1926 in Acute Myeloid Leukemia and Nanoparticle Formulation Prolongs Lifespan and Reduces Tumor Burden of Xenograft Mice.

Acute myeloid leukemia (AML) is one of the most frequent types of blood malignancies. It is a complex disorder of undifferentiated hematopoietic progenitor cells. The majority of patients generally respond to intensive therapy. Nevertheless, relapse is the major cause of death in AML, warranting the need for novel treatment strategies. Retinoids have demonstrated potent differentiation and growth regulatory effects in normal, transformed, and hematopoietic progenitor cells. All-trans retinoic acid (ATRA) is the paradigm of treatment in acute promyelocytic leukemia, an AML subtype. The majority of AML subtypes are, however, resistant to ATRA. Multiple synthetic retinoids such as ST1926 recently emerged as potent anticancer agents to overcome such resistance. Despite its lack of toxicity, ST1926 clinical development was restricted due to its limited bioavailability and rapid excretion. Here, we investigate the preclinical efficacy of ST1926 and polymer-stabilized ST1926 nanoparticles (ST1926-NP) in AML models. We show that sub-µmol/L concentrations of ST1926 potently and selectively inhibited the growth of ATRA-resistant AML cell lines and primary blasts. ST1926 induced-growth arrest was due to early DNA damage and massive apoptosis in AML cells. To enhance the drug's bioavailability, ST1926-NP were developed using Flash NanoPrecipitation, and displayed comparable anti-growth activities to the naked drug in AML cells. In a murine AML xenograft model, ST1926 and ST1926-NP significantly prolonged survival and reduced tumor burden. Strikingly, in vivo ST1926-NP antitumor effects were achieved at four fold lower concentrations than the naked drug. These results highlight the promising use of ST1926 in AML therapy and encourage its further development. Mol Cancer Ther; 16(10); 2047-57. ©2017 AACR.