Structural Characterization, and Antioxidative and Anti-inflammatory Activities of Phylloxanthobilins in Tropaeolum majus, a Plant with Relevance in Phytomedicine.

Tropaeolum majus (garden nasturtium) is a plant with relevance in phytomedicine, appreciated not only for its pharmaceutical activities, but also for its beautiful leaves and flowers. Here, we investigated the phytochemical composition of senescent nasturtium leaves. Indeed, we identified yellow chlorophyll catabolites, also termed phylloxanthobilins, which we show to contribute to the bright yellow color of the leaves in the autumn season. Moreover, we isolated and characterized the phylloxanthobilins from T. majus, and report the identification of a pyro-phylloxanthobilin, so far only accessible by chemical synthesis. We show that the phylloxanthobilins contribute to bioactivities of T. majus by displaying strong anti-oxidative effects in vitro and in cellulo, and anti-inflammatory effects as assessed by COX-1 and COX-2 enzyme inhibition, similar to other bioactive ingredients of T. majus, isoquercitrin, and chlorogenic acid. Hence, phylloxanthobilins could play a role in the efficacy of T. majus in the treatment of urinary tract infections, an established indication of T. majus. With the results shown in this study, we aid in the completion of the phytochemical profile of T. majus by identifying additional bioactive natural products as relevant components of this medicinal plant.

Thermal Proteome Profiling Reveals Insight to Antiproliferative and Pro-Apoptotic Effects of Lagunamide A in the Modulation of DNA Damage Repair.

Lagunamide A is a biologically active natural product with a yet unidentified molecular mode of action. Cellular studies revealed that lagunamide A is a potent inhibitor of cancer cell proliferation, promotes apoptosis and causes mitochondrial dysfunction. To decipher the cellular mechanism responsible for these effects, we utilized thermal protein profiling (TPP) and identified EYA3 as a stabilized protein in cells upon lagunamide A treatment. EYA3, involved in the DNA damage repair process, was functionally investigated via siRNA based knockdown studies and corresponding effects of lagunamide A on DNA repair were confirmed. Furthermore, we showed that lagunamide A sensitized tumor cells to treatment with the drug doxorubicin highlighting a putative therapeutic strategy.

A Novel Interaction of Slug (SNAI2) and Nuclear Actin.

Actin is a protein of central importance to many cellular functions. Its localization and activity are regulated by interactions with a high number of actin-binding proteins. In a yeast two-hybrid (Y2H) screening system, snail family transcriptional repressor 2 (SNAI2 or slug) was identified as a yet unknown potential actin-binding protein. We validated this interaction using immunoprecipitation and analyzed the functional relation between slug and actin. Since both proteins have been reported to be involved in DNA double-strand break (DSB) repair, we focused on their interaction during this process after treatment with doxorubicin or UV irradiation. Confocal microscopy elicits that the overexpression of actin fused to an NLS stabilizes complexes of slug and γH2AX, an early marker of DNA damage repair.

Endolysosomal TRPML1 channel regulates cancer cell migration by altering intracellular trafficking of E-cadherin and ß1-integrin.

Metastasis still accounts for 90% of all cancer-related death cases. An increase of cellular mobility and invasive traits of cancer cells mark two crucial prerequisites of metastasis. Recent studies highlight the involvement of the endolysosomal cation channel TRPML1 in cell migration. Our results identified a widely antimigratory effect upon loss of TRPML1 function in a panel of cell lines in vitro and reduced dissemination in vivo. As mode-of-action, we established TRPML1 as a crucial regulator of cytosolic calcium levels, actin polymerization, and intracellular trafficking of two promigratory proteins: E-cadherin and ß1-integrin. Interestingly, KO of TRPML1 differentially interferes with the recycling process of E-cadherin and ß1-integrin in a cell line-dependant manner, while resulting in the same phenotype of decreased migratory and adhesive capacities in vitro. Additionally, we observed a coherence between reduction of E-cadherin levels at membrane site and phosphorylation of NF-κB in a ß-catenin/p38-mediated manner. As a result, an E-cadherin/NF-κB feedback loop is generated, regulating E-cadherin expression on a transcriptional level. Consequently, our findings highlight the role of TRPML1 as a regulator in migratory processes and suggest the ion channel as a suitable target for the inhibition of migration and invasion.

A metabolic shift toward glycolysis enables cancer cells to maintain survival upon concomitant glutamine deprivation and V-ATPase inhibition.

It is widely known that most cancer cells display an increased reliance on glutaminolysis to sustain proliferation and survival. Combining glutamine deprivation with additional anti-cancer therapies is an intensively investigated approach to increase therapeutic effectiveness. In this study, we examined a combination of glutamine deprivation by starvation or pharmacological tools, with the anti-cancer agent archazolid, an inhibitor of the lysosomal V-ATPase. We show that glutamine deprivation leads to lysosomal acidification and induction of pro-survival autophagy, which could be prevented by archazolid. Surprisingly, a combination of glutamine deprivation with archazolid did not lead to synergistic induction of cell death or reduction in proliferation. Investigating the underlying mechanisms revealed elevated expression and activity of amino acid transporters SLC1A5, SLC38A1 upon starvation, whereas archazolid had no additional effect. Furthermore, we found that the export of lysosomal glutamine derived from exogenous sources plays no role in the phenotype as knock-down of SLC38A7, the lysosomal glutamine exporter, could not increase V-ATPase inhibition-induced cell death or reduce proliferation. Analysis of the cellular metabolic phenotype revealed that glutamine deprivation led to a significant increase in glycolytic activity, indicated by an elevated glycolytic capacity and reserve, when V-ATPase function was inhibited concomitantly. This was confirmed by increased glutamine uptake, augmented lactate production, and an increase in hexokinase activity. Our study, therefore, provides evidence, that glutamine deprivation induces autophagy, which can be prevented by simultaneous inhibition of V-ATPase function. However, this does not lead to a therapeutic benefit, as cells are able to circumvent cell death and growth inhibition by a metabolic shift toward glycolysis.

Matrix stiffness regulates Notch signaling activity in endothelial cells.

Notch signaling is critical for many developmental and disease-related processes. It is widely accepted that Notch has a mechanotransduction module that regulates receptor cleavage. However, the role of biomechanical properties of the cellular environment in Notch signaling in general is still poorly understood. During angiogenesis, differentiation of endothelial cells into tip and stalk cells is regulated by Notch signaling, and remodeling of the extracellular matrix occurs. We investigated the influence of substrate stiffness on the Notch signaling pathway in endothelial cells. Using stiffness-tuned polydimethylsiloxane (PDMS) substrates, we show that activity of the Notch signaling pathway inversely correlates with a physiologically relevant range of substrate stiffness (i.e. increased Notch signaling activity on softer substrates). Trans-endocytosis of the Notch extracellular domain, but not the overall endocytosis, is regulated by substrate stiffness, and integrin cell-matrix connections are both stiffness dependent and influenced by Notch signaling. We conclude that mechanotransduction of Notch activation is modulated by substrate stiffness, highlighting the role of substrate rigidity as an important cue for signaling. This might have implications in pathological situations associated with stiffening of the extracellular matrix, such as tumor growth.

Targeting TPC2 sensitizes acute lymphoblastic leukemia cells to chemotherapeutics by impairing lysosomal function.

Despite novel therapy regimens and extensive research, chemoresistance remains a challenge in leukemia treatment. Of note, recent studies revealed lysosomes as regulators of cell death and chemotherapy response, suggesting this organelle is a novel target for chemosensitization. Interestingly, drug-resistant VCR-R CEM acute lymphoblastic leukemia (ALL) cells have an increased expression of the lysosomal cation channel Two-Pore-Channel 2 (TPC2) compared to drug-naïve CCRF-CEM ALL cells. Concurrently, knockout (KO) of TPC2 sensitized drug-resistant VCR-R CEM cells to treatment with cytostatics. The chemosensitizing effect could be confirmed in several cell lines as well as in heterogeneous, patient-derived xenograft ALL cells, using the pharmacological TPC2 inhibitors naringenin and tetrandrine. We reveal that a dual mechanism of action mediates chemo sensitization by loss of lysosomal TPC2 function. First, because of increased lysosomal pH, lysosomal drug sequestration is impaired, leading to an increased nuclear accumulation of doxorubicin and hence increased DNA damage. Second, lysosomes of TPC2 KO cells are more prone to lysosomal damage as a result of morphological changes and dysregulation of proteins influencing lysosomal stability. This leads to induction of lysosomal cell death (LCD), evident by increased cathepsin B levels in the cytosol, truncation of pro-apoptotic Bid, as well as the reversibility of cell death by co-treatment with the cathepsin B inhibitor CA-074Me in TPC2 KO cells. In summary, this study establishes TPC2 as a novel, promising, druggable target for combination therapy approaches in ALL to overcome chemoresistance, which could be exploited in the clinic in the future. Additionally, it unravels LCD signaling as an important death-inducing component upon loss of TPC2 function.

Lysosomal TRPML1 regulates mitochondrial function in hepatocellular carcinoma cells.

Liver cancers, including hepatocellular carcinoma (HCC), are the second leading cause of cancer death worldwide, and novel therapeutic strategies are still highly needed. Recently, the endolysosomal cation channel TRPML1 (also known as MCOLN1) has gained focus in cancer research because it represents an interesting novel target. We utilized the recently developed isoform-selective TRPML1 activator ML1-SA1 and the CRISPR/Cas9 system to generate tools for overactivation and loss-of-function studies on TRPML1 in HCC. After verification of our tools, we investigated the role of TRPML1 in HCC by studying proliferation, apoptosis and proteomic alterations. Furthermore, we analyzed mitochondrial function in detail by performing confocal and transmission electron microscopy combined with SeahorseTM and Oroboros® functional analysis. We report that TRPML1 overactivation mediated by a novel, isoform-selective small-molecule activator induces apoptosis by impairing mitochondrial function in a Ca2+-dependent manner. Additionally, TRPML1 loss-of-function deregulates mitochondrial renewal, which leads to proliferation impairment. Thus, our study reveals a novel role for TRPML1 as regulator of mitochondrial function and its modulators as promising molecules for novel therapeutic options in HCC therapy.

Using the yeast three-hybrid system for the identification of small molecule-protein interactions with the example of ethinylestradiol.

Since the first report on a yeast three-hybrid system, several approaches have successfully utilized different setups for discovering targets of small molecule drugs. Compared to broadly applied MS based target identification approaches, the yeast three-hybrid system represents a complementary method that allows for the straightforward identification of direct protein binders of selected small molecules. One major drawback of this system, however, is that the drug has to be taken up by the yeast cells in sufficient concentrations. Here, we report the establishment of a yeast three-hybrid screen in the deletion strain ABC9Δ, which is characterized by being highly permeable to small molecules. We used this system to screen for protein binding partners of ethinylestradiol, a widely used drug mainly for contraception and hormone replacement therapy. We identified procollagen-lysine 2-oxoglutarate 5-dioxygenase 2 (PLOD2 or lysyl hydroxylase, LH2) as a novel direct target and were able to confirm the interaction identified with the yeast three-hybrid system by a complementary method, affinity chromatography, to prove the validity of the hit. Furthermore, we provide evidence for an interaction between the drug and PLOD2 in vitro and in cellulo.

Disentangling cadherin-mediated cell-cell interactions in collective cancer cell migration.

Cell dispersion from a confined area is fundamental in a number of biological processes, including cancer metastasis. To date, a quantitative understanding of the interplay of single-cell motility, cell proliferation, and intercellular contacts remains elusive. In particular, the role of E- and N-cadherin junctions, central components of intercellular contacts, is still controversial. Combining theoretical modeling with in vitro observations, we investigate the collective spreading behavior of colonies of human cancer cells (T24). The spreading of these colonies is driven by stochastic single-cell migration with frequent transient cell-cell contacts. We find that inhibition of E- and N-cadherin junctions decreases colony spreading and average spreading velocities, without affecting the strength of correlations in spreading velocities of neighboring cells. Based on a biophysical simulation model for cell migration, we show that the behavioral changes upon disruption of these junctions can be explained by reduced repulsive excluded volume interactions between cells. This suggests that in cancer cell migration, cadherin-based intercellular contacts sharpen cell boundaries leading to repulsive rather than cohesive interactions between cells, thereby promoting efficient cell spreading during collective migration.

Turning the Actin Nucleating Compound Miuraenamide into Nucleation Inhibitors.

Natural compounds that either increase or decrease polymerization of actin into filaments have become indispensable tools for cell biology. However, to date, it was not possible to use them as therapeutics due to their overall cytotoxicity and their unfavorable pharmacokinetics. Furthermore, their synthesis is in general quite complicated. In an attempt to find simplified analogues of miuraenamide, an actin nucleating compound, we identified derivatives with a paradoxical inversion of the mode of action: instead of increased nucleation, they caused an inhibition. Using an extensive computational approach, we propose a binding mode and a mode of action for one of these derivatives. Based on our findings, it becomes feasible to tune actin-binding compounds to one or the other direction and to generate new synthetic actin binders with increased functional selectivity.

Tetrapyrrolic Pigments from Heme- and Chlorophyll Breakdown are Actin-Targeting Compounds.

Chlorophyll and heme are among the "pigments of life", tetrapyrrolic structures, without which life on Earth would not be possible. Their catabolites, the phyllobilins and the bilins, respectively, share not only structural features, but also a similar story: Long considered waste products of detoxification processes, important bioactivities for both classes have now been demonstrated. For phyllobilins, however, research on physiological roles is sparse. Here, we introduce actin, the major component of the cytoskeleton, as the first discovered target of phyllobilins and as a novel target of bilins. We demonstrate the inhibition of actin dynamics in vitro and effects on actin and related processes in cancer cells. A direct interaction with G-actin is shown by in silico studies and confirmed by affinity chromatography. Our findings open a new chapter in bioactivities of tetrapyrroles-especially phyllobilins-for which they form the basis for broad implications in plant science, ecology, and physiology.

Phyllobilins as a challenging diverse natural product class: Exploration of pharmacological activities.

Phyllobilins are a group of chlorophyll-derived bilin-type linear tetrapyrroles, generated in the process of chlorophyll breakdown. Since the first phyllobilin was isolated and characterized in 1991, more and more structures of these chlorophyll catabolites were identified alongside the biochemical players involved in chlorophyll breakdown. In the meantime, phyllobilins are known to occur in a large natural structural variety, and new modifications are still being discovered. Phyllobilins have been regarded as products of chlorophyll detoxification for a very long time, hence they have been completely overlooked as a natural product class in terms of their biological role or pharmacological activity. A change of this paradigm, however, is long overdue. Here, we review the current knowledge of the pharmacological activities of phyllobilins and give an overview of the diverse structural modifications, laying the groundwork for analyzing their role(s) as active components in medicinal plants.

Small molecule inhibitors of the mitochondrial ClpXP protease possess cytostatic potential and re-sensitize chemo-resistant cancers.

The human mitochondrial ClpXP protease complex (HsClpXP) has recently attracted major attention as a target for novel anti-cancer therapies. Despite its important role in disease progression, the cellular role of HsClpXP is poorly characterized and only few small molecule inhibitors have been reported. Herein, we screened previously established S. aureus ClpXP inhibitors against the related human protease complex and identified potent small molecules against human ClpXP. The hit compounds showed anti-cancer activity in a panoply of leukemia, liver and breast cancer cell lines. We found that the bacterial ClpXP inhibitor 334 impairs the electron transport chain (ETC), enhances the production of mitochondrial reactive oxygen species (mtROS) and thereby promotes protein carbonylation, aberrant proteostasis and apoptosis. In addition, 334 induces cell death in re-isolated patient-derived xenograft (PDX) leukemia cells, potentiates the effect of DNA-damaging cytostatics and re-sensitizes resistant cancers to chemotherapy in non-apoptotic doses.

A yellow chlorophyll catabolite in leaves of Urtica dioica L.: An overlooked phytochemical that contributes to health benefits of stinging nettle.

Stinging nettle is appreciated for its antioxidant and anti-inflammatory properties, which renders the plant a popular ingredient in a healthy diet in form of salads or smoothies. The most common use, presumably, is of dried leaves as ingredient in tea mixtures. The plant's health benefits are attributed primarily to phenolic phytochemicals. Here we describe the characterization and quantification of a phylloxanthobilin (PxB), a yellow chlorophyll catabolite, in nettle tea. Despite their abundance in the plant kingdom, chlorophyll catabolites have been overlooked as phytochemicals and as part of human nutrition. Our investigations of tea reveal that one cup of nettle tea contains about 50 µg of PxB with large variations depending on the supplier. When investigating the bioactivities of PxB, our observations show that PxB has antioxidative and anti-inflammatory activities comparable to known bioactive small molecules found in nettle, indicating the phylloxanthobilin to be an overlooked ingredient of nettle tea.

Flavonoids increase melanin production and reduce proliferation, migration and invasion of melanoma cells by blocking endolysosomal/melanosomal TPC2.

Two-pore channel 2 (TPC2) resides in endolysosomal membranes but also in lysosome-related organelles such as the melanin producing melanosomes. Gain-of-function polymorphisms in hTPC2 are associated with decreased melanin production and blond hair color. Vice versa genetic ablation of TPC2 increases melanin production. We show here an inverse correlation between melanin production and melanoma proliferation, migration, and invasion due to the dual activity of TPC2 in endolysosomes and melanosomes. Our results are supported by both genetic ablation and pharmacological inhibition of TPC2. Mechanistically, our data show that loss/block of TPC2 results in reduced protein levels of MITF, a major regulator of melanoma progression, but an increased activity of the melanin-generating enzyme tyrosinase. TPC2 inhibition thus provides a twofold benefit in melanoma prevention and treatment by increasing, through interference with tyrosinase activity, the synthesis of UV blocking melanin in melanosomes and by decreasing MITF-driven melanoma progression by increased GSK3ß-mediated MITF degradation.

Spatio-selective activation of nuclear translocation of YAP with light directs invasion of cancer cell spheroids.

The mechanical properties of the extracellular matrix strongly influence tumor progression and invasion. Yes-associated protein (YAP) has been shown to be a key regulator of this process translating mechanical cues from the extracellular matrix into intracellular signals. Despite its apparent role in tumor progression and metastasis, it is not clear yet, whether YAP activation can actively trigger the onset of invasion. To address this question, we designed a photo-activatable YAP (optoYAP), which allows for spatiotemporal control of its activation. The activation mechanism of optoYAP is based on optically triggered nuclear translocation of the protein. Activation of optoYAP induces downstream signaling for several hours and leads to increased proliferation in two- and three-dimensional cultures. Applied to cancer spheroids, optoYAP activation induces invasion. Site-selective activation of optoYAP in cancer spheroids strikingly directs invasion into the activated direction. Thus, nuclear translocation of YAP may be enough to trigger the onset of invasion.

Gene editing and synthetically accessible inhibitors reveal role for TPC2 in HCC cell proliferation and tumor growth.

The role of two-pore channel 2 (TPC2), one of the few cation channels localized on endolysosomal membranes, in cancer remains poorly understood. Here, we report that TPC2 knockout reduces proliferation of cancer cells in vitro, affects their energy metabolism, and successfully abrogates tumor growth in vivo. Concurrently, we have developed simplified analogs of the alkaloid tetrandrine as potent TPC2 inhibitors by screening a library of synthesized benzyltetrahydroisoquinoline derivatives. Removal of dispensable substructures of the lead molecule tetrandrine increases antiproliferative properties against cancer cells and impairs proangiogenic signaling of endothelial cells to a greater extent than tetrandrine. Simultaneously, toxic effects on non-cancerous cells are reduced, allowing in vivo administration and revealing a TPC2 inhibitor with antitumor efficacy in mice. Hence, our study unveils TPC2 as valid target for cancer therapy and provides easily accessible tetrandrine analogs as a promising option for effective pharmacological interference.

Cell-Based Strain Remodeling of a Nonfibrous Matrix as an Organizing Principle for Vasculogenesis.

Endothelial tube formation on a reconstituted basement membrane (Matrigel) is a well-established in vitro model for studying the processes of angiogenesis and vasculogenesis. However, to date, the organizing principles that underlie the morphogenesis of this network and that shape the initial process of cells' finding one another remain elusive. Here, we identify a mechanism that allows cells to form networks by mechanically reorganizing and stiffening their extracellular matrix, independent of chemical guidance cues. Interestingly, we find that this cellular self-organization strongly depends on the connectivity, plasticity, and topology of the surrounding matrix; cell contractility; and cell density. Cells rearrange the matrix and form bridges of matrix material that are stiffer than their surroundings, thus creating a durotactic track for the initiation of cell protrusions and cell-cell contacts. This contractility-based communication via strain stiffening and matrix rearrangement might be a general organizing principle during tissue development or regeneration.

Targeting Lysosomes in Cancer as Promising Strategy to Overcome Chemoresistance-A Mini Review.

To date, cancer remains a worldwide leading cause of death, with a still rising incidence. This is essentially caused by the fact, that despite the abundance of therapeutic targets and treatment strategies, insufficient response and multidrug resistance frequently occur. Underlying mechanisms are multifaceted and extensively studied. In recent research, it became evident, that the lysosome is of importance in drug resistance phenotypes. While it has long been considered just as cellular waste bag, it is now widely known that lysosomes play an important role in important cellular signaling processes and are in the focus of cancer research. In that regard lysosomes are now considered as so-called "drug safe-houses" in which chemotherapeutics are trapped passively by diffusion or actively by lysosomal P-glycoprotein activity, which prevents them from reaching their intracellular targets. Furthermore, alterations in lysosome to nucleus signaling by the transcription factor EB (TFEB)-mTORC1 axis are implicated in development of chemoresistance. The identification of lysosomes as essential players in drug resistance has introduced novel strategies to overcome chemoresistance and led to innovate therapeutic approaches. This mini review gives an overview of the current state of research on the role of lysosomes in chemoresistance, summarizing underlying mechanisms and treatment strategies and critically discussing open questions and drawbacks.