Peptides as Pharmacological Carriers to the Brain: Promises, Shortcomings and Challenges

Central nervous system (CNS) diseases are among the most difficult to treat, mainly because the vast majority of the drugs fail to cross the blood-brain barrier (BBB) or to reach the brain at concentrations adequate to exert a pharmacological activity. The obstacle posed by the BBB has led to the in-depth study of strategies allowing the brain delivery of CNS-active drugs. Among the most promising strategies is the use of peptides addressed to the BBB. Peptides are versatile molecules that can be used to decorate nanoparticles or can be conjugated to drugs, with either a stable link or as pro-drugs. They have been used to deliver to the brain both small molecules and proteins, with applications in diverse therapeutic areas such as brain cancers, neurodegenerative diseases and imaging. Peptides can be generally classified as receptor-targeted, recognizing membrane proteins expressed by the BBB microvessels (e.g., Angiopep2, CDX, and iRGD), "cell-penetrating peptides" (CPPs; e.g. TAT_47-57, SynB1/3, and Penetratin), undergoing transcytosis through unspecific mechanisms, or those exploiting a mixed approach. The advantages of peptides have been extensively pointed out, but so far few studies have focused on the potential negative aspects. Indeed, despite having a generally good safety profile, some peptide conjugates may display toxicological characteristics distinct from those of the peptide itself, causing for instance antigenicity, cardiovascular alterations or hemolysis. Other shortcomings are the often brief lifetime in vivo, caused by the presence of peptidases, the vulnerability to endosomal/lysosomal degradation, and the frequently still insufficient attainable increase of brain drug levels, which remain below the therapeutically useful concentrations. The aim of this review is to analyze not only the successful and promising aspects of the use of peptides in brain targeting but also the problems posed by this strategy for drug delivery.

Pharmacological modulation of Kv1.3 potassium channel selectively triggers pathological B lymphocyte apoptosis in vivo in a genetic CLL model

BACKGROUND

Ion channels are emerging as promising oncological targets. The potassium channels Kv1.3 and IKCa are highly expressed in the plasma membrane and mitochondria of human chronic lymphocytic leukemia (CLL) cells, compared to healthy lymphocytes. In vitro, inhibition of mitoKv1.3 by PAPTP was shown to kill ex vivo primary human CLL cells, while targeting IKCa with TRAM-34 decreased CLL cell proliferation.

METHODS

Here we evaluated the effect of the above drugs in CLL cells from ibrutinib-resistant patients and in combination with Venetoclax, two drugs used in the clinical practice. The effects of the drugs were tested also in the Eμ-TCL1 genetic CLL murine model, characterized by a lympho-proliferative disease reminiscent of aggressive human CLL. Eμ-TCL1 mice showing overt disease state were treated with intraperitoneal injections of non-toxic 5 nmol/g PAPTP or 10 nmol/g TRAM-34 once a day and the number and percentage of pathological B cells (CD19⁺CD5⁺) in different, pathologically relevant body districts were determined.

RESULTS

We show that Kv1.3 expression correlates with sensitivity of the human and mouse neoplastic cells to PAPTP. Primary CLL cells from ibrutinib-resistant patients could be killed with PAPTP and this drug enhanced the effect of Venetoclax, by acting on mitoKv1.3 of the inner mitochondrial membrane and triggering rapid mitochondrial changes and cytochrome c release. In vivo, after 2 week- therapy of Eμ-TCL1 mice harboring distinct CLL clones, leukemia burden was reduced by more than 85%: the number and percentage of CLL B cells fall in the spleen and peritoneal cavity and in the peripheral blood, without signs of toxicity. Notably, CLL infiltration into liver and spleen and splenomegaly were also drastically reduced upon PAPTP treatment. In contrast, TRAM-34 did not exert any beneficial effect when administered in vivo to Eμ-TCL1 mice at non-toxic concentration.

CONCLUSION

Altogether, by comparing vehicle versus compound effect in different Eμ-TCL1 animals bearing unique clones similarly to CLL patients, we conclude that PAPTP significantly reduced leukemia burden in CLL-relevant districts, even in animals with advanced stage of the disease. Our results thus identify PAPTP as a very promising drug for CLL treatment, even for the chemoresistant forms of the disease.

Targeting Pancreatic Ductal Adenocarcinoma (PDAC)

Pancreatic cancers are among the most ominous, and among the most studied. Their complexities have provided ample material for a huge investigative effort, which is briefly surveyed in this review. Eradication by surgery has proven extremely difficult, and a successful chemotherapeutic approach is desperately needed. Treatment with "traditional" anticancer drugs, such as benchmark gemcitabine or the current standard-of-care FOLFIRINOX quaternary combination increase the mean overall survival by only a few months and often leads to chemoresistance. Much work is therefore currently devoted to potentiating our pharmacological weapons by accurate targeting and, in particular, by acting on the dense tumoral stroma, a distinctive feature of PDAC accounting for much of the therapeutic difficulty. We give an overview of recent developments, touching on the major aspects of PDAC physiology and biochemistry, currently-used and experimental drugs, and targeting technologies under development. A few papers are discussed in some detail to help provide a sense of how the field is moving.

Targeting mitochondrial ion channels for cancer therapy

Pharmacological targeting of mitochondrial ion channels is emerging as a promising approach to eliminate cancer cells; as most of these channels are differentially expressed and/or regulated in cancer cells in comparison to healthy ones, this strategy may selectively eliminate the former. Perturbation of ion fluxes across the outer and inner membranes is linked to alterations of redox state, membrane potential and bioenergetic efficiency. This leads to indirect modulation of oxidative phosphorylation, which is/may be fundamental for both cancer and cancer stem cell survival. Furthermore, given the crucial contribution of mitochondria to intrinsic apoptosis, modulation of their ion channels leading to cytochrome c release may be of great advantage in case of resistance to drugs triggering apoptotic events upstream of the mitochondrial phase. In the present review, we give an overview of the known mitochondrial ion channels and of their modulators capable of killing cancer cells. In addition, we discuss state-of-the-art strategies using mitochondriotropic drugs or peptide-based approaches allowing a more efficient and selective targeting of mitochondrial ion channel-linked events.

An Angiopep2-PAPTP Construct Overcomes the Blood-Brain Barrier. New Perspectives against Brain Tumors

A developing family of chemotherapeutics—derived from 5-(4-phenoxybutoxy)psoralen (PAP-1)—target mitochondrial potassium channel mtKv1.3 to selectively induce oxidative stress and death of diseased cells. The key to their effectiveness is the presence of a positively charged triphenylphosphonium group which drives their accumulation in the organelles. These compounds have proven their preclinical worth in murine models of cancers such as melanoma and pancreatic adenocarcinoma. In in vitro experiments they also efficiently killed glioblastoma cells, but in vivo they were powerless against orthotopic glioma because they were completely unable to overcome the blood-brain barrier. In an effort to improve brain delivery we have now coupled one of these promising compounds, PAPTP, to well-known cell-penetrating and brain-targeting peptides TAT_48–61 and Angiopep-2. Coupling has been obtained by linking one of the phenyl groups of the triphenylphosphonium to the first amino acid of the peptide via a reversible carbamate ester bond. Both TAT_48–61 and Angiopep-2 allowed the delivery of 0.3–0.4 nmoles of construct per gram of brain tissue upon intravenous (i.v.) injection of 5 µmoles/kg bw to mice. This is the first evidence of PAPTP delivery to the brain; the chemical strategy described here opens the possibility to conjugate PAPTP to small peptides in order to fine-tune tissue distribution of this interesting compound.

Synthesis and cellular effects of a mitochondria-targeted inhibitor of the two-pore potassium channel TASK-3

The two-pore potassium channel TASK-3 has been shown to localize to both the plasma membrane and the mitochondrial inner membrane. TASK-3 is highly expressed in melanoma and breast cancer cells and has been proposed to promote tumor formation. Here we investigated whether pharmacological modulation of TASK-3, and specifically of mitochondrial TASK-3 (mitoTASK-3), had any effect on cancer cell survival and mitochondrial physiology. A novel, mitochondriotropic version of the specific TASK-3 inhibitor IN-THPP has been synthesized by addition of a positively charged triphenylphosphonium moiety. While IN-THPP was unable to induce apoptosis, mitoIN-THPP decreased survival of breast cancer cells and efficiently killed melanoma lines, which we show to express mitoTASK-3. Cell death was accompanied by mitochondrial membrane depolarization and fragmentation of the mitochondrial network, suggesting a role of the channel in the maintenance of the correct function of this organelle. In accordance, cells treated with mitoIN-THPP became rapidly depleted of mitochondrial ATP which resulted in activation of the AMP-dependent kinase AMPK. Importantly, cell survival was not affected in mouse embryonic fibroblasts and the effect of mitoIN-THPP was less pronounced in human melanoma cells stably knocked down for TASK-3 expression, indicating a certain degree of selectivity of the drug both for pathological cells and for the channel. In addition, mitoIN-THPP inhibited cancer cell migration to a higher extent than IN-THPP in two melanoma cell lines. In summary, our results point to the importance of mitoTASK-3 for melanoma cell survival and migration.

Insight into the mechanism of cytotoxicity of membrane-permeant psoralenic Kv1.3 channel inhibitors by chemical dissection of a novel member of the family

The potassium channel Kv1.3, involved in several important pathologies, is the target of a family of psoralen-based drugs whose mechanism of action is not fully understood. Here we provide evidence for a physical interaction of the mitochondria-located Kv1.3 (mtKv1.3) and Complex I of the respiratory chain and show that this proximity underlies the death-inducing ability of psoralenic Kv1.3 inhibitors. The effects of PAP-1-MHEG (PAP-1, a Kv1.3 inhibitor, with six monomeric ethylene glycol units attached to the phenyl ring of PAP-1), a more soluble novel derivative of PAP-1 and of its various portions on mitochondrial physiology indicate that the psoralenic moiety of PAP-1 bound to mtKv1.3 facilitates the diversion of electrons from Complex I to molecular oxygen. The resulting massive production of toxic Reactive Oxygen Species leads to death of cancer cells expressing Kv1.3. In vivo, PAP-1-MHEG significantly decreased melanoma volume. In summary, PAP-1-MHEG offers insights into the mechanisms of cytotoxicity of this family of compounds and may represent a valuable clinical tool.

•

The mitochondrial channel mitoKv1.3 is a promising pharmacological target.

•

MitoKv1.3 interacts with Complex I of the respiratory chain.

•

Psoralenic inhibitors of Kv1.3 facilitate the diversion of e⁻ from complex I to O₂.

•

A novel psoralenic Kv1.3 inhibitor with increased solubility reduces melanoma volume.

Small-Molecule Modulators of Mitochondrial Channels as Chemotherapeutic Agents

Ion channels residing in the inner (IMM) and outer (OMM) mitochondrial membranes are emerging as noteworthy pharmacological targets in oncology. While these aspects have not been investigated for all of them, a role in cancer growth and/or metastasis and/or drug resistance has been shown at least for the IMM-residing Ca²⁺ uniporter complex and K⁺- selective mtK_V1.3, mtIK_Ca, mtSK_Ca and mtTASK-3, and for the OMM Voltage-Dependent Anion Channel (mitochondrial porin). A special case is that of the Mitochondrial Permeability Transition Pore, a large pore which forms in the IMM of severely stressed cells, and which may be exploited to precipitate the death of cancerous cells. Here we briefly discuss the oncological relevance of mitochondria and their channels, and summarize the methods that can be adopted to selectively target these intracellular organelles. We then present an updated list of known mitochondrial channels, and review the pharmacology of those with proven relevance for cancer.

Inhibition of PI-3-K and AKT Amplifies Kv1.3 Inhibitor-Induced Death of Human T Leukemia Cells

BACKGROUND/AIMS

We have previously shown that inhibition of the mitochondrial Kv1.3 channel results in an initial mitochondrial hyperpolarization and a release of oxygen radicals that mediate mitochondrial depolarization, cytochrome c release and death. Here, we investigated whether inhibition of Kv1.3 channels can also induce cellular resistance mechanisms that counteract the induction of cell death under certain conditions.

METHODS

We treated leukemic T cells with the mitochondria-targeted Kv1.3 inhibitor PCARBTP and determined the activity of different kinases associated with cell survival including ZAP70, PI-3-K, AKT, JNK and ERK by measuring the activation-associated phosphorylation of these proteins. Furthermore, we inhibited AKT and JNK and determined the effect of PCARBTP-induced tumor cell death.

RESULTS

We demonstrate that treatment of Jurkat T leukemia cells with low doses of the mitochondria-targeted inhibitor of Kv1.3 PCARBTP (0.25 μM or 1 μM) for 10 minutes induced a constitutive phosphorylation/activation of the pro-survival signaling molecules ZAP70, PI-3-K, AKT and JNK, while the phosphorylation/activation of ERK was not affected. Stimulation of Jurkat cells via the TCR/CD3 complex induced an additional activation of a similar pattern of signaling events. Higher doses of the Kv1.3 inhibitor, i.e. 10 μM PCARBTP, reduced the basal phosphorylation/activation of these signaling molecules and also impaired their activation upon stimulation via the TCR/CD3 complex. A low dose of PCARBTP, i.e. 0.25 μM PCARBTP, was almost without any effect on cell death. In contrast, concomitant inhibition of PI-3-K or AKT greatly sensitized Jurkat leukemia cells to the Kv1.3 inhibitor PCARBTP and allowed induction of cell death already at 0.25 μM PCARBTP.

CONCLUSION

These studies indicate that Jurkat leukemia cells respond to low doses of the mitochondria-targeted Kv1.3 inhibitor PCARBTP with an activation of survival signals counteracting cell death. Inhibition of these T cell survival signals sensitizes leukemia cells to death induced by mitochondria-targeted Kv1.3 inhibitors. High doses of the Kv1.3 inhibitor inactivate these signals directly permitting death.

Impaired Mitochondrial ATP Production Downregulates Wnt Signaling via ER Stress Induction

Wnt signaling affects fundamental development pathways and, if aberrantly activated, promotes the development of cancers. Wnt signaling is modulated by different factors, but whether the mitochondrial energetic state affects Wnt signaling is unknown. Here, we show that sublethal concentrations of different compounds that decrease mitochondrial ATP production specifically downregulate Wnt/β-catenin signaling in vitro in colon cancer cells and in vivo in zebrafish reporter lines. Accordingly, fibroblasts from a GRACILE syndrome patient and a generated zebrafish model lead to reduced Wnt signaling. We identify a mitochondria-Wnt signaling axis whereby a decrease in mitochondrial ATP reduces calcium uptake into the endoplasmic reticulum (ER), leading to endoplasmic reticulum stress and to impaired Wnt signaling. In turn, the recovery of the ATP level or the inhibition of endoplasmic reticulum stress restores Wnt activity. These findings reveal a mechanism that links mitochondrial energetic metabolism to the control of the Wnt pathway that may be beneficial against several pathologies.

Strategies to target bioactive molecules to subcellular compartments. Focus on natural compounds

Many potential pharmacological targets are present in multiple subcellular compartments and have different pathophysiological roles depending on location. In these cases, selective targeting of a drug to the relevant subcellular domain(s) may help to sharpen its impact by providing topological specificity, thus limiting side effects, and to concentrate the compound where needed, thus increasing its effectiveness. We review here the state of the art in precision subcellular delivery. The major approaches confer "homing" properties to the active principle via permanent or reversible (in pro-drug fashion) modifications, or through the use of special-design nanoparticles or liposomes to ferry a drug(s) cargo to its desired destination. An assortment of peptides, substituents with delocalized positive charges, custom-blended lipid mixtures, pH- or enzyme-sensitive groups provide the main tools of the trade. Mitochondria, lysosomes and the cell membrane may be mentioned as the fronts on which the most significant advances have been made. Most of the examples presented here have to do with targeting natural compounds - in particular polyphenols, known as pleiotropic agents - to one or the other subcellular compartment.

Pterostilbene Improves Cognitive Performance in Aged Rats: An in Vivo Study

BACKGROUND/AIMS

Pterostilbene (Pt; trans-3,5-dimethoxy-4'-hydroxystilbene) is a natural phenol found in blueberries and grapevines. It shows remarkable biomedical activities similar to those of resveratrol. Its high bioavailability is a major advantage for possible biomedical applications. The goal of the study was to evaluate the effects of chronic pterostilbene administration on cognitive performance in aged rats with mild cognitive impairment.

METHODS

18-month-old animals were subjected to behavioral tests to establish the "baseline", then divided into treatment and control groups. The former were chronically fed Pt (22.5 mg/kg-day) for 20 consecutive days. At the end of this period all animals were tested again and sacrificed. The dentate gyrus, the hippocampus and the prefrontal and perirhinal cortices were then collected, and RT-qPCR and/or Western blot analyses were performed on a few transcripts/proteins involved in synaptic remodeling. Mitochondrial content was also assessed.

RESULTS

Pt administration improved performance in behavioral tests and positively affected memory consolidation. We found increased levels of REST, PSD-95 and mitochondrial porin1 in the dentate gyrus and a positive correlation between T-maze test score and levels of cAMP responsive element binding protein (CREB) phosphorylation.

CONCLUSION

These results underscore the therapeutic potential of Pt supplementation for age-related cognitive decline.

Pharmacological modulation of mitochondrial ion channels

The field of mitochondrial ion channels has undergone a rapid development during the last three decades, due to the molecular identification of some of the channels residing in the outer and inner membranes. Relevant information about the function of these channels in physiological and pathological settings was gained thanks to genetic models for a few, mitochondria-specific channels. However, many ion channels have multiple localizations within the cell, hampering a clear-cut determination of their function by pharmacological means. The present review summarizes our current knowledge about the ins and outs of mitochondrial ion channels, with special focus on the channels that have received much attention in recent years, namely, the voltage-dependent anion channels, the permeability transition pore (also called mitochondrial megachannel), the mitochondrial calcium uniporter and some of the inner membrane-located potassium channels. In addition, possible strategies to overcome the difficulties of specifically targeting mitochondrial channels versus their counterparts active in other membranes are discussed, as well as the possibilities of modulating channel function by small peptides that compete for binding with protein interacting partners. Altogether, these promising tools along with large-scale chemical screenings set up to identify new, specific channel modulators will hopefully allow us to pinpoint the actual function of most mitochondrial ion channels in the near future and to pharmacologically affect important pathologies in which they are involved, such as neurodegeneration, ischaemic damage and cancer. LINKED ARTICLES: This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

Novel Mitochondria-Targeted Furocoumarin Derivatives as Possible Anti-Cancer Agents

Targeting small molecules to appropriate subcellular compartments is a way to increase their selectivity and effectiveness while minimizing side effects. This can be accomplished either by stably incorporating specific "homing" properties into the structure of the active principle, or by attaching to it a targeting moiety via a labile linker, i.e., by producing a "targeting pro-drug." Mitochondria are a recognized therapeutic target in oncology, and blocking the population of the potassium channel Kv1.3 residing in the inner mitochondrial membrane (mtKv1.3) has been shown to cause apoptosis of cancerous cells expressing it. These concepts have led us to devise novel, mitochondria-targeted, membrane-permeant drug candidates containing the furocoumarin (psoralenic) ring system and the triphenylphosphonium (TPP) lipophilic cation. The strategy has proven effective in various cancer models, including pancreatic ductal adenocarcinoma, melanoma, and glioblastoma, stimulating us to devise further novel molecules to extend and diversify the range of available drugs of this type. New compounds were synthesized and tested in vitro; one of them-a prodrug in which the coumarinic moiety and the TPP group are linked by a bridge comprising a labile carbonate bond system-proved quite effective in in vitro cytotoxicity assays. Selective death induction is attributed to inhibition of mtKv1.3. This results in oxidative stress, which is fatal for the already-stressed malignant cells. This compound may thus be a candidate drug for the mtKv1.3-targeting therapeutic approach.

Tumor-reducing effect of the clinically used drug clofazimine in a SCID mouse model of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) represents the most common form of pancreatic cancer with rising incidence in developing countries. Unfortunately, the overall 5-year survival rate is still less than 5%. The most frequent oncogenic mutations in PDAC are loss-of function mutations in p53 and gain-of-function mutations in KRAS. Here we show that clofazimine (Lamprene), a drug already used in the clinic for autoimmune diseases and leprosy, is able to efficiently kill in vitro five different PDAC cell lines harboring p53 mutations. We provide evidence that clofazimine induces apoptosis in PDAC cells with an EC₅₀ in the μM range via its specific inhibitory action on the potassium channel Kv1.3. Intraperitoneal injection of clofazimine resulted in its accumulation in the pancreas of mice 8 hours after administration. Using an orthotopic PDAC xenotransplantation model in SCID beige mouse, we show that clofazimine significantly and strongly reduced the primary tumor weight. Thus, our work identifies clofazimine as a promising therapeutic agent against PDAC and further highlights ion channels as possible oncological targets.

LETM1-Mediated K+ and Na+ Homeostasis Regulates Mitochondrial Ca2+ Efflux

Ca²⁺ transport across the inner membrane of mitochondria (IMM) is of major importance for their functions in bioenergetics, cell death and signaling. It is therefore tightly regulated. It has been recently proposed that LETM1—an IMM protein with a crucial role in mitochondrial K⁺/H⁺ exchange and volume homeostasis—also acts as a Ca²⁺/H⁺ exchanger. Here we show for the first time that lowering LETM1 gene expression by shRNA hampers mitochondrial K⁺/H⁺ and Na⁺/H⁺ exchange. Decreased exchange activity resulted in matrix K⁺ accumulation in these mitochondria. Furthermore, LETM1 depletion selectively decreased Na⁺/Ca²⁺ exchange mediated by NCLX, as observed in the presence of ruthenium red, a blocker of the Mitochondrial Ca²⁺ Uniporter (MCU). These data confirm a key role of LETM1 in monovalent cation homeostasis, and suggest that the effects of its modulation on mitochondrial transmembrane Ca²⁺ fluxes may reflect those on Na⁺/H⁺ exchange activity.

Regulation of Proliferation by a Mitochondrial Potassium Channel in Pancreatic Ductal Adenocarcinoma Cells

Previous results link the mitochondrial potassium channel Kv1.3 (mitoKv1.3) to the regulation of apoptosis. By synthesizing new, mitochondria-targeted derivatives (PAPTP and PCARBTP) of PAP-1, a specific membrane-permeant Kv1.3 inhibitor, we have recently provided evidence that both drugs acting on mitoKv1.3 are able to induce apoptosis and reduce tumor growth in vivo without affecting healthy tissues and cells. In the present article, by exploiting these new drugs, we addressed the question whether mitoKv1.3 contributes to the regulation of cell proliferation as well. When used at low concentrations, which do not compromise cell survival, both drugs slightly increased the percentage of cells in S phase while decreased the population at G0/G1 stage of cells from two different pancreatic ductal adenocarcinoma lines. Our data suggest that the observed modulation is related to ROS levels within the cells, opening the way to link mitochondrial ion channel function to downstream, ROS-related signaling events that might be important for cell cycle progression.

Mitochondrial genetic haplogroups and depressive symptoms: A large study among people in North America

BACKGROUND

A possible relationship between mitochondrial haplogroups and psychiatric diseases (e.g. schizophrenia and bipolar disorder) has been postulated, but data regarding depression is still limited. We investigated whether any mitochondrial haplogroup carried a significant higher risk of depressive symptoms in a large prospective cohort of North American people included in the Osteoarthritis Initiative.

METHODS

Cross sectional data was derived from the Osteoarthritis Initiative. The haplogroup was assigned through a combination of sequencing and PCR-RFLP techniques. All the mitochondrial haplogroups were named following this nomenclature: H, U, K, J, T, V, SuperHV, I, W, X or Others. Depression was ascertained through the 20-item Center for Epidemiologic Studies-Depression (CES-D), with ≥16 indicating depressive symptoms.

RESULTS

Overall, 3601 Caucasian participants (55.9% women), mean age of 61.7±9.3 years were included. No difference was observed in mitochondrial haplogroups frequency among those with depressive symptoms (n=285, =7.9% of the baseline population) compared to participants with no depressive symptoms (N=3316) (chi-square test=0.53). Using a logistic regression analysis, adjusted for eight potential confounders, with those having the haplogroup H as the reference group (the most common haplogroup), no significant mitochondrial haplogroup was associated with prevalent depressive symptoms. The same results were evident in secondary analysis in which we matched depressed and non-depressed participants for age and sex.

LIMITATIONS

Cross-sectional design; only CES-D for evaluating mood; participants not totally representative of general population.

CONCLUSIONS

We found no evidence of any relationship between specific mitochondrial haplogroups and depressive symptoms. Future longitudinal research is required to confirm/ refute these findings.

Resveratrol derivatives as a pharmacological tool

Prodrugs of resveratrol are under development. Among the long-term goals, still largely elusive, are (1) modulating physical properties (e.g., water-soluble derivatives bearing polyethylene glycol chains), (2) changing distribution in the body (e.g., galactosyl derivatives restricted to the intestinal lumen), (3) increasing absorption from the gastrointestinal tract (e.g., derivatives imitating the natural substrates of endogenous transporters), and (4) hindering phase II metabolism (e.g., temporarily blocking the hydroxyls), all contributing to (5) increasing bioavailability. The chemical bonds that have been tested for functionalization include carboxyester, acetal, and carbamate groups. A second approach, which can be combined with the first, seeks to reinforce or modify the biochemical activities of resveratrol by concentrating the compound at specific subcellular sites. An example is provided by mitochondria-targeted derivatives. These proved to be pro-oxidant and cytotoxic in vitro, selectively killing fast-growing and tumor cells when supplied in the low micromolar range. This suggests the possibility of anticancer applications.

Direct Pharmacological Targeting of a Mitochondrial Ion Channel Selectively Kills Tumor Cells In Vivo

The potassium channel Kv1.3 is highly expressed in the mitochondria of various cancerous cells. Here we show that direct inhibition of Kv1.3 using two mitochondria-targeted inhibitors alters mitochondrial function and leads to reactive oxygen species (ROS)-mediated death of even chemoresistant cells independently of p53 status. These inhibitors killed 98% of ex vivo primary chronic B-lymphocytic leukemia tumor cells while sparing healthy B cells. In orthotopic mouse models of melanoma and pancreatic ductal adenocarcinoma, the compounds reduced tumor size by more than 90% and 60%, respectively, while sparing immune and cardiac functions. Our work provides direct evidence that specific pharmacological targeting of a mitochondrial potassium channel can lead to ROS-mediated selective apoptosis of cancer cells in vivo, without causing significant side effects.

Now UCP(rotein), Now You Don't: UCP1 Is Not Mandatory for Thermogenesis

Adipocyte-targeted therapies could potentially combat obesity and metabolic disorders; however, our understanding of adipogenesis and the characterization of the various adipose tissues are incomplete. In this issue, Bertholet et al. (2017) report the existence of two types of heat-producing beige-like adipocytes, those with and those without the mitochondrial uncoupling protein 1 (UCP1).

New natural amino acid-bearing prodrugs boost pterostilbene's oral pharmacokinetic and distribution profile

The biomedical effects of the natural phenol pterostilbene are of great interest but its bioavailability is negatively affected by the phenolic group in position 4' which is an ideal target for the conjugative enzymes of phase II metabolism. We report the synthesis and characterization of prodrugs in which the hydroxyl moiety is reversibly protected as a carbamate ester linked to the N-terminus of a natural amino acid. Prodrugs comprising amino acids with hydrophobic side chains were readily absorbed after intragastric administration to rats. The Area Under the Curve for pterostilbene in blood was optimal when prodrugs with isoleucine or β-alanine were used. The prodrug incorporating isoleucine was used for further studies to map distribution into major organs. When compared to pterostilbene itself, administration of the isoleucine prodrug afforded increased absorption, reduced metabolism and higher concentrations of pterostilbene, sustained for several hours, in most of the organs examined. Experiments using Caco-2 cells as an in vitro model for human intestinal absorption suggest that the prodrug could have promising absorption profiles also in humans; its uptake is partly due to passive diffusion, and partly mediated by H⁺-dependent transporters expressed on the apical membrane of enterocytes, such as PepT1 and OATP.

The mitochondrial permeability transition pore in AD 2016: An update

Over the past 30years the mitochondrial permeability transition - the permeabilization of the inner mitochondrial membrane due to the opening of a wide pore - has progressed from being considered a curious artifact induced in isolated mitochondria by Ca(2+) and phosphate to a key cell-death-inducing process in several major pathologies. Its relevance is by now universally acknowledged and a pharmacology targeting the phenomenon is being developed. The molecular nature of the pore remains to this day uncertain, but progress has recently been made with the identification of the FOF1 ATP synthase as the probable proteic substrate. Researchers sharing this conviction are however divided into two camps: these believing that only the ATP synthase dimers or oligomers can form the pore, presumably in the contact region between monomers, and those who consider that the ring-forming c subunits in the FO sector actually constitute the walls of the pore. The latest development is the emergence of a new candidate: Spastic Paraplegia 7 (SPG7), a mitochondrial AAA-type membrane protease which forms a 6-stave barrel. This review summarizes recent developments of research on the pathophysiological relevance and on the molecular nature of the mitochondrial permeability transition pore. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Improving the efficacy of plant polyphenols

Plant polyphenols exhibit potentially useful effects in a wide variety of pathophysiological settings. They interact with proteins such as signalling kinases, transcription factors and ion channels, and modulate redox processes, such as those taking place in mitochondria. Biomedical applications of these natural compounds are however severely hindered by their low bioavailability, rapid metabolism, and often by unfavourable physico-chemical properties, e.g. a generally low water solubility. Derivatives are under development with the aim of improving their bioavailability and/or bioefficacy. Various strategies can be adopted. An increase in circulating blood levels of non-metabolized natural compound may be attainable through prodrugs. In the ideal prodrug, phenolic hydroxyls are protected by capping groups which a) help or at least do not hinder permeation of epithelia; b) prevent conjugative modifications during absorption and first-pass through the liver; c) are eliminated with opportune kinetics to regenerate the parent compound. Moreover, prodrugs may be designed with the goals of modulating physical properties of the parent compound, and/or changing its distribution in the body. A more specific action may be achieved by concentrating the compounds at specific sites of action. An example of the second approach is represented by mitochondria-targeted redox-active polyphenol derivatives, designed to intervene on radical processes in these organelles and as a tool either to protect cells from oxidative insults or to precipitate their death. Mitochondrial targeting can be achieved through conjugation with a triphenylphosphonium lipophilic cation. Quercetin and resveratrol were chosen as model polyphenols for these proof-of-concept studies. Data available at the moment show that both quercetin and resveratrol mitochondria-targeted derivatives are pro-oxidant and cytotoxic in vitro, selectively killing fast-growing and tumoural cells when supplied in the low μM range; the mechanism of ROS generation appears to differ between the two classes of compounds. These approaches are emerging as promising strategies to obtain new efficient chemopreventive and/or chemotherapeutic drugs based on polyphenols derivatives.

Mitochondrially targeted vitamin E succinate modulates expression of mitochondrial DNA transcripts and mitochondrial biogenesis

AIMS

To assess the effect of mitochondrially targeted vitamin E (VE) analogs on mitochondrial function and biogenesis.

RESULTS

Mitochondrially targeted vitamin E succinate (MitoVES) is an efficient inducer of apoptosis in cancer cells. Here, we show that unlike its untargeted counterpart α-tocopheryl succinate, MitoVES suppresses proliferation of cancer cells at sub-apoptotic doses by way of affecting the mitochondrial DNA (mtDNA) transcripts. We found that MitoVES strongly suppresses the level of the displacement loop transcript followed by those of mtDNA genes coding for subunits of mitochondrial complexes. This process is coupled to the inhibition of mitochondrial respiration, dissipation of the mitochondrial membrane potential, and generation of reactive oxygen species. In addition, exposure of cancer cells to MitoVES led to decreased expression of TFAM and diminished mitochondrial biogenesis. The inhibition of mitochondrial transcription was replicated in vivo in a mouse model of HER2(high) breast cancer, where MitoVES lowered the level of mtDNA transcripts in cancer cells but not in normal tissue.

INNOVATION

Our data show that mitochondrially targeted VE analogs represent a novel class of mitocans that not only induce apoptosis at higher concentrations but also block proliferation and suppress normal mitochondrial function and transcription at low, non-apoptogenic doses.

CONCLUSIONS

Our data indicate a novel, selective anti-cancer activity of compounds that act by targeting mitochondria of cancer cells, inducing significant alterations in mitochondrial function associated with transcription of mtDNA-coded genes. These changes subsequently result in the arrest of cell proliferation.

Prodrugs of quercetin and resveratrol: a strategy under development

The biochemical activities of plant flavonoids and stilbenoids point to many health-related applications, hampered however by a low bioavailability associated with rapid metabolic modification. A possible approach to overcome this obstacle is the development of prodrugs. In this review we provide some background information and summarize the efforts made so far to obtain suitable precursors of the two best known model polyphenols belonging to the classes just mentioned, quercetin and resveratrol. Prodrug design needs to take into account two key aspects: the nature of the chemical bond linking the core molecule to the protecting substituent, and the substituent itself, which can impart desirable physico-chemical properties. Only recently a systematic study of the several possible combinations has begun. Most bond systems tested so far appear to be either too stable or too unstable under physiological conditions. A range of substituent moieties is available, allowing the modulation of properties such as water solubility and the ability to permeate biomembranes. Work so far has been largely performed in vitro, and more in vivo experiments are definitely needed for a reliable assessment of the potentialities of the classes of prodrugs produced so far and of those still awaiting creation.

Pharmacokinetics and tissue distribution of pterostilbene in the rat

SCOPE

Pterostilbene (Pt) is emerging as an important health-promoting natural compound. Pharmacokinetic studies so far have focused on plasma levels, while Pt distribution in tissues is most relevant for biological action. This study determined tissue distribution of Pt and its major metabolite, pterostilbene-4'-sulfate (Pt-S), in rats after oral administration.

METHODS AND RESULTS

Upon intravenous (iv) administration (88 μmol/kg), Pt was cleared from blood with a half-life of 1.8 ± 0.3 h. Oral administration (same dose) resulted in moderate Pt bioavailability (∼35%) and in an increased abundance of Pt-S in blood (AUC(Pt)/AUC(Pt-S) ∼0.75 and ∼0.05 after iv or oral administration, respectively). Pt-S was the major species in all organs except the brain, where intact Pt was predominant (AUC(Pt)/AUC(Pt-S) ∼5). Both Pt and Pt-S peaked in all tissues at approximately 2 h. The highest levels (∼200 nmoles/g for Pt-S and 40 nmoles/g for Pt) were measured in the liver, the lowest (≤7 nmol/g) in skeletal muscles and testes.

CONCLUSION

AUC(Pt) was ∼2- to ∼25-fold higher in tissues than in blood; this may explain its bioactivity despite barely detectable blood levels. Of particular interest is the high fraction of nonmetabolized Pt in the brain, given the reports of its activity at the level of the central nervous system.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Channel formation by yeast F-ATP synthase and the role of dimerization in the mitochondrial permeability transition

Purified F-ATP synthase dimers of yeast mitochondria display Ca(2+)-dependent channel activity with properties resembling those of the permeability transition pore (PTP) of mammals. After treatment with the Ca(2+) ionophore ETH129, which allows electrophoretic Ca(2+) uptake, isolated yeast mitochondria undergo inner membrane permeabilization due to PTP opening. Yeast mutant strains ΔTIM11 and ΔATP20 (lacking the e and g F-ATP synthase subunits, respectively, which are necessary for dimer formation) display a striking resistance to PTP opening. These results show that the yeast PTP originates from F-ATP synthase and indicate that dimerization is required for pore formation in situ.

A preliminary fastview of mitochondrial protein profile from healthy and type 2 diabetic subjects

Type 2 diabetes results from the development of insulin resistance and a concomitant impairment of insulin secretion. Mitochondrial dysfunctions are thought to be the major contributor to the development of various pathologies, including type 1 and type 2 diabetes mellitus. Mitochondrial oxidative stress has been reported in models of both type 1 and type 2 diabetes mellitus and may play a central role in mitochondrial dysfunction. In the present study, we investigated the occurrence of protein alterations, due to the presence of type 2 diabetes, in mitochondria isolated from human peripheral blood mononuclear cells (PBMCs] by matrix-assisted laser desorp- tion/ionization mass spectrometry (MALDI-MS]. PBMCs may be suitable for this investigation because they have insulin receptors that quickly respond to changes in insulin concentration, and in the presence of insulin rapidly increase their rates of glucose utiliza- tion. In the presence of insulin-resistance conditions, such as type 2 diabetes mellitus, this mechanism is altered and the glycation of cytoplasmic as well as mitochondrial proteins may plausibly appear. Therefore, PBMCs may be useful tools to verify modifications or altered expression of mitochondrial proteins. Human mitochondria were obtained from 32 subjects, 16 healthy controls and 16 type 2 diabetic patients. Two different methods for mitochondria isolation and purification were employed and compared. Some proteins have been found to be differently expressed in the two groups of subjects under investigation and can be classified into two sets: i.e. proteins related to ATP synthase [e.g. 6.8kDa mitochondrial proteolipid [MLQ]; ATP-CF6 [m/z 12,597)] and proteins related to cell proliferation and apoptosis [e.g. TIMM9 [m/z 10,378); Bcl-2-like protein 2 (m/z20,742)].

Intracellular ion channels and cancer

Several types of channels play a role in the maintenance of ion homeostasis in subcellular organelles including endoplasmatic reticulum, nucleus, lysosome, endosome, and mitochondria. Here we give a brief overview of the contribution of various mitochondrial and other organellar channels to cancer cell proliferation or death. Much attention is focused on channels involved in intracellular calcium signaling and on ion fluxes in the ATP-producing organelle mitochondria. Mitochondrial K⁺ channels (Ca²⁺-dependent BK_Ca and IK_Ca, ATP-dependent K_ATP, Kv1.3, two-pore TWIK-related Acid-Sensitive K⁺ channel-3 (TASK-3)), Ca²⁺ uniporter MCU, Mg²⁺-permeable Mrs2, anion channels (voltage-dependent chloride channel VDAC, intracellular chloride channel CLIC) and the Permeability Transition Pore (MPTP) contribute importantly to the regulation of function in this organelle. Since mitochondria play a central role in apoptosis, modulation of their ion channels by pharmacological means may lead to death of cancer cells. The nuclear potassium channel Kv10.1 and the nuclear chloride channel CLIC4 as well as the endoplasmatic reticulum (ER)-located inositol 1,4,5-trisphosphate (IP₃) receptor, the ER-located Ca²⁺ depletion sensor STIM1 (stromal interaction molecule 1), a component of the store-operated Ca²⁺ channel and the ER-resident TRPM8 are also mentioned. Furthermore, pharmacological tools affecting organellar channels and modulating cancer cell survival are discussed. The channels described in this review are summarized on Figure 1. Overall, the view is emerging that intracellular ion channels may represent a promising target for cancer treatment.

Acetal derivatives as prodrugs of resveratrol

The pharmacological exploitation of resveratrol is hindered by rapid phase-II conjugative metabolism in enterocytes and hepatocytes. One approach to the solution of this problem relies on prodrugs. We report the synthesis and characterization as well as the assessment of in vivo absorption and metabolism of a set of prodrugs of resveratrol in which the OH groups are engaged in the formal (-OCH2OR) or the more labile acetal (-OCH(CH3)OR) linkages. As carrier group (R) of the prodrug, we have used short ethyleneglycol oligomers (OEG) capped by a terminal methoxy group: -O-(CH2CH2O)n-CH3 (n = 0, 1, 2, 3, 4, 6). These moieties are expected to exhibit, to a degree, the favorable properties of longer polyethyleneglycol (PEG) chains, while their relatively small size makes for a more favorable drug loading capacity. After administration of formal-based prodrugs to rats by oral gavage, significant concentrations of derivatives were measured in blood samples over several hours, in all cases except for n = 0. Absorption was maximal for n = 4. Complete deprotection to give resveratrol and its metabolites was however too slow to be of practical use. Administration of the acetal prodrug carrying tetrameric OEG chains resulted instead in the protracted presence of resveratrol metabolites in blood, consistent with a progressive regeneration of the parent molecule from the prodrug after its absorption. The results suggest that prodrugs of polyphenols based on the acetal bond and short ethyleneglycol oligomers of homogeneous size may be a convenient tool for the systemic delivery of the unconjugated parent compound.

Resveratrol and health: the starting point

Cascade of youth? Resveratrol, the celebrated phytoalexin of red wine, was known to activate AMPK indirectly, but how this happened was unclear. In a paper recently published in Cell, S.-J. Park, J. H. Chung and co-workers identify the signalling cascade, which begins with the inhibition of phosphodiesterases, in particular PDE4. But questions remain, even while new perspectives open up.

Inhibitors of mitochondrial Kv1.3 channels induce Bax/Bak-independent death of cancer cells

Overcoming the resistance of tumours to chemotherapy, often due to downregulation of Bax and Bak, represents a significant clinical challenge. It is therefore important to identify novel apoptosis inducers that bypass Bax and Bak. Potassium channels are emerging as oncological targets and a crucial role of mitochondrial Kv1.3 in apoptosis has been demonstrated. Here we report for the first time that Psora-4, PAP-1 and clofazimine, three distinct membrane-permeant inhibitors of Kv1.3, induce death by directly targeting the mitochondrial channel in multiple human and mouse cancer cell lines. Importantly, these drugs activated the intrinsic apoptotic pathway also in the absence of Bax and Bak, a result in agreement with the current mechanistic model for mitochondrial Kv1.3 action. Genetic deficiency or short interfering RNA (siRNA)-mediated downregulation of Kv1.3 abrogated the effects of the drugs. Intraperitoneal injection of clofazimine reduced tumour size by 90% in an orthotopic melanoma B16F10 mouse model in vivo, while no adverse effects were observed in several healthy tissues. The study indicates that inhibition of mitochondrial Kv1.3 might be a novel therapeutic option for the induction of cancer cell death independent of Bax and Bak.

Polyphenols and human health: a prospectus

The lay press often heralds polyphenols as panacea for all sorts of diseases. The rationale is that their antioxidant activity would prevent free radical damage to macromolecules. However, basic and clinical science is showing that the reality is much more complex than this and that several issues, notably content in foodstuff, bioavailability, or in vivo antioxidant activity are yet to be resolved. We summarize the recent findings concerning the effects of polyphenols on human health, analyze the current limitations at pitfalls, and propose future directions for research.

Mitochondrial effects of plant-made compounds

SIGNIFICANCE

Plants produce many small molecules with biomedical potential. Their absorption from foods, metabolism, their effects on physiological and pathological processes, and the mechanisms of action are intensely investigated. Many are known to affect multiple cellular functions. Mitochondria are coming to be recognized as a major target for these compounds, especially redox-active ones, but the mechanisms involved still need clarification. At the same time, frontline research is uncovering the importance of processes involving these organelles for the cell and for an array of physiological and pathological processes. We review the major functions and possible dysfunctions of mitochondria, identify signaling pathways through which plant-derived molecules have an impact, and show how this may be relevant for major pathologies.

RECENT ADVANCES

Antioxidant, protective effects may arise as a reaction to a low-level pro-oxidant activity, largely taking place at mitochondria. Some plant-derived molecules can activate AMP-dependent kinase, with a consequent upregulation of mitochondrial biogenesis and a potential favorable impact on aging, pathologies like diabetes and neurodegeneration, and on ischemic damage.

CRITICAL ISSUES

The extrapolation of in vitro results and the verification of paradigms in vivo is a key issue for current research on both plant-derived compounds and mitochondria. The low bioavailability of many of these molecules poses a problem for both the study of their activities and their utilization.

FUTURE DIRECTIONS

The further clarification of the role of mitochondria in the activities of plant dietary compounds and their metabolites, mitochondrial targeting, the development of analogs and pro-drugs are all topics for promising research.