Mitocans Revisited: Mitochondrial Targeting as Efficient Anti-Cancer Therapy

Tunable gold nanorod/NAO conjugates for selective drug delivery in mitochondria-targeted cancer therapy

Nonyl acridine orange (NAO) is a lipophilic and positively charged molecule widely used as a mitochondrial fluorescent probe. NAO is cytotoxic at micromolar concentration and might be potentially used as a mitochondria-targeted drug for cancer therapy. However, the use of NAO under in vivo conditions would be compromised by the unspecific interactions with off-target cells and negatively charged proteins present in the bloodstream. To tackle this limitation, we have synthesized NAO analogues carrying an imidazole group for their specific binding to nitrilotriacetic (NTA) functionalized gold nanorods (AuNRs). We demonstrate that AuNRs provide 10⁴ binding sites and a controlled delivery under acidic conditions. Upon incubation with mouse embryonic fibroblasts, the endosomal acidic environment releases the NAO analogues from AuNRs, as visualized through the staining of the mitochondrial network. The addition of the monoclonal antibody Cetuximab to the conjugates enhanced their uptake within lung cancer cells and the conjugates were cytotoxic at subnanomolar concentrations (c₅₀ ≈ 0.06 nM). Moreover, the specific interactions of Cetuximab with the epidermal growth factor receptor (EGFR) provided a specific targeting of EGFR-expressing lung cancer cells. After intravenous administration in patient-derived xenografts (PDX) mouse models, the conjugates reduced the progression of EGFR-positive tumors. Overall, the NAO-AuNRs provide a promising strategy to realize membrane mitochondria-targeted conjugates for lung cancer therapy.

Мitochondrial therapy of melanoma B16/F10, pathophysiological parameters of tumor regression

The aim is to evaluate the pathophysiological parameters of the efficacy of liver mitochondrial transplantation in animals with B16/F10 melanoma. Materials and methods. In our experiment we used female and male mice of BALB/c Nude strain (n=28). Experimental groups were as follows: the reference group (n=14) with B16/ F10 melanoma; the main group (n=14) with B16/F10 melanoma + mitochondrial therapy (MC therapy). Statistical analysis of results was carried out with the Statistica 10.0 software. Results. The subcutaneous tumor in the mice of both sexes became detectable on day 5 from the time of the tumor inoculation, and the regressive effect produced by MC therapy was recorded in the males beginning with day 8 of the tumor growth. At the end of the experiment, on day 22, the difference in the average volumes of the tumor node was reported to be 3.2 times, i.e. a significant inhibition of the tumor growth in the group of the males with MC therapy was revealed. In the females on day 5 of the tumor growth, differences in the volume of the tumor focus between the reference group and the group with MC therapy were not recorded, however, a statistically significant difference was found in the sex-related comparison of the groups of the animals with MC therapy. It was determined that in the females with MC therapy, the area of the tumor spot during that period (5 days) was 1.4 times (p<0.05) less than that in the corresponding group of the males. On day 8, in the females completed MC therapy, the tumor has not yet concentrated into a solid structure, but remained as a flat tumor entity, and only by day 12 the tumor has formed from a flat structure into a volumetric tumor type. As a result, by the end of the experiment, on day 22, smaller volumes of the tumor nodes remained in the group of females treated with MC therapy, and the difference with the reference group was 2.7 times (p<0.05). Conclusion. Thus, within the framework of the experiment, it has been found that the application of mitochondrial therapy using allogeneic liver mitochondria in the BALB/c Nude mice with B16/F10 melanoma retards the tumor growth in the mice of both sexes.

In Vitro Effects of Mitochondria-Targeted Antioxidants in a Small-Cell Carcinoma of the Ovary of Hypercalcemic Type and in Type 1 and Type 2 Endometrial Cancer

Small-cell carcinoma of the ovary of hypercalcemic type (SCCOHT) and endometrial cancer from type 1 and type 2 are gynecological tumors that affect women worldwide. The treatment encompasses the use of cytotoxic drugs that are nonspecific and inefficient. “Mitocans”, a family of drugs that specifically target tumor cells’ mitochondria, might be a solution, as they conjugate compounds, such as antioxidants, with carriers, such as lipophilic cations, that direct them to the mitochondria. In this study, caffeic acid was conjugated with triphenylphosphonium (TPP), 4-picolinium, or isoquinolinium, forming 3 new compounds (Mito6_TPP, Mito6_picol., and Mito6_isoq.) that were tested on ovarian (COV434) and endometrial (Hec50co and Ishikawa) cancer cells. The results of MTT and neutral red assays suggested a time- and concentration-dependent decrease in cell viability in all tumor cell lines. The presence of apoptosis was indicated by the Giemsa and Höechst staining and by the decrease in mitochondrial membrane potential. The measurement of intracellular reactive oxygen species demonstrated the antioxidant properties of these compounds, which might be related to cell death. Generally, Mito6_TPP was more active at lower concentrations than Mito6_picol. or Mito6_isoq., but was accompanied by more cytotoxic effects, as shown by the lactate dehydrogenase release. Non-tumorous cells (HFF-1) showed no changes after treatment. This study assessed the potential of these compounds as anticancer agents, although further investigation is needed.

Rhodamine 101 Conjugates of Triterpenoic Amides Are of Comparable Cytotoxicity as Their Rhodamine B Analogs

Pentacyclic triterpenoic acids (betulinic, oleanolic, ursolic, and platanic acid) were selected and subjected to acetylation followed by the formation of amides derived from either piperazine or homopiperazine. These amides were coupled with either rhodamine B or rhodamine 101. All of these compounds were screened for their cytotoxic activity in SRB assays. As a result, the cytotoxicity of the parent acids was low but increased slightly upon their acetylation while a significant increase in cytotoxicity was observed for piperazinyl and homopiperazinyl amides. A tremendous improvement in cytotoxicity was observed; however, for the rhodamine B and rhodamine 101 conjugates, and compound 27, an ursolic acid derived homopiperazinyl amide holding a rhodamine 101 residue showed an EC50 = 0.05 µM for A2780 ovarian cancer cells while being less cytotoxic for non-malignant fibroblasts. To date, the rhodamine 101 derivatives presented here are the first examples of triterpene derivatives holding a rhodamine residue different from rhodamine B.

Metabolic tricks of cancer cells

One of the characteristics of cancer cells important for tumorigenesis is their metabolic plasticity. Indeed, in various stress conditions, cancer cells can reshape their metabolic pathways to support the increased energy request due to continuous growth and rapid proliferation. Moreover, selective pressures in the tumor microenvironment, such as hypoxia, acidosis, and competition for resources, force cancer cells to adapt by complete reorganization of their metabolism. In this review, we highlight the characteristics of cancer metabolism and discuss its clinical significance, since overcoming metabolic plasticity of cancer cells is a key objective of modern cancer therapeutics and a better understanding of metabolic reprogramming may lead to the identification of possible targets for cancer therapy.

Intercellular Communication in the Brain through Tunneling Nanotubes

Simple Summary

Tunneling nanotubes (TNTs) are a means of cell communication which have been recently discovered. They allow the intercellular trafficking of many types of cellular compounds ranging from ions, such as Ca²⁺, to whole organelles such as mitochondria. TNTs are found in many tissues, both in physiological and pathological conditions. They are also found in the brain where they contribute to brain development and function and also to degenerative diseases and glioma.

Abstract

Intercellular communication is essential for tissue homeostasis and function. Understanding how cells interact with each other is paramount, as crosstalk between cells is often dysregulated in diseases and can contribute to their progression. Cells communicate with each other through several modalities, including paracrine secretion and specialized structures ensuring physical contact between them. Among these intercellular specialized structures, tunneling nanotubes (TNTs) are now recognized as a means of cell-to-cell communication through the exchange of cellular cargo, controlled by a variety of biological triggers, as described here. Intercellular communication is fundamental to brain function. It allows the dialogue between the many cells, including neurons, astrocytes, oligodendrocytes, glial cells, microglia, necessary for the proper development and function of the brain. We highlight here the role of TNTs in connecting these cells, for the physiological functioning of the brain and in pathologies such as stroke, neurodegenerative diseases, and gliomas. Understanding these processes could pave the way for future therapies.

Rational Designs at the Forefront of Mitochondria-Targeted Gene Delivery: Recent Progress and Future Perspectives

Mitochondria play an essential role in cellular metabolism and generate energy in cells. To support these functions, several proteins are encoded in the mitochondrial DNA (mtDNA). The mutation of mtDNA causes mitochondrial dysfunction and ultimately results in a variety of inherited diseases. To date, gene delivery systems targeting mitochondria have been developed to ameliorate mtDNA mutations. However, applications of these strategies in mitochondrial gene therapy are still being explored and optimized. Thus, from this perspective, we herein highlight recent mitochondria-targeting strategies for gene therapy and discuss future directions for effective mitochondria-targeted gene delivery.

Straightforward access to novel mitochondriotropics derived from 2-arylethanol as potent and selective antiproliferative agents

The necessity for developing novel cytostatic agents with improved activities and reduced side-effects to tackle cancer prompted us to investigate mitochondria-targeted compounds, an approach that is gaining attention for the selective transportation of cytotoxic agents. We envisioned the possibility of conjugating a phenethyl alcohol motif, decorated with a series of phenol-based substituents on the aryl moiety, with a triphenyl phosphonium scaffold (a mitochondria-directed vector), through a hydrocarbon chain of different lengths. Thus, such compounds that incorporate the phenethyl skeleton can be considered as masked phenolic compounds derived from relevant natural counterparts found in olive tree (e.g. tyrosol, hydroxytyrosol). Title compounds exhibited very strong in vitro antiproliferative activities against the panel of six human tumor cell lines tested, with GI₅₀ values ranging from the nanomolar (0.026 ± 0.010 μM for 36) to the submicromolar range in most of the cases; this represents an improvement of up to 350-fold compared to classical chemotherapeutic agents, like 5-fluorouracil or cisplatin. Interestingly, decrease in the linker length led to an increase of GI₅₀ values against non-tumor cells, thus allowing a remarkable improvement of selectivity (SI up to 269). The very promising antiproliferative activities prompted us to further investigate their behaviour against multidrug resistant cell lines (MDR). The results indicated a reduced sensitivity of the multidrug resistant cells to compounds, probably due to P-gp-mediated efflux of these antiproliferative agents. Interestingly, activities were completely restored to the same levels by co-administration of tariquidar, a well-known inhibitor of P-gp. Flow cytometry analysis on sensitive cell lines revealed a decrease in the percentage of cells in G1 phase accompanied by increase in S and G2/M phases. In addition, a significant increase in subG1 area, was observed. These results are compatible with the necrotic and apoptotic cell death detected in the Annexin V assay, and with the depolarization of the mitochondria membrane. Thus, the new mitochondriotropic agents reported herein can be considered as promising antiproliferative agents, endowed with remarkable potency and selectivity, including MDR cells, upon co-administration with a pump-efflux inhibitor.

How vitamin E and its derivatives regulate tumour cells via the MAPK signalling pathway?'

In tumour cells, vitamin E and its derivatives play a critical role in the regulation of multiple signalling pathways through their oxidative and nonoxidative functions. To date, there are 8 known natural vitamin E forms and many kinds of derivatives, among which VES and α-TEA have excellent anticancer activities. The MAPK pathway consists of a complex cascade of proteins that control the proliferation, differentiation and apoptosis of tumour cells. The MAPK pathway includes four subfamilies, ERK1/2, JNK1/2, p38 MAPK, and ERK5. Most of the proteins in these subfamilies interact with each other in a complex manner. The anticancer function of vitamin E and its derivatives is closely related to the MAPK cascade. Studies have shown that in tumour cells, α-T/γ-T/γ-T3/δ-T3/VES/α-TEA regulated ERK1/2, prevent tumorigenesis, inhibit tumour cell growth and metastasis and induce cell differentiation, apoptosis, and cell cycle arrest; γ-T3/δ-T3/VES/α-TEA regulates JNK1/2, induce apoptosis, reduce ceramide synthesis and inhibit proliferation; and γ-T3/δ-T3/VES regulate p38 MAPK and induce apoptosis. This paper reviews the role of vitamin E and its derivatives in the MAPK cascade, and tumour cells are used as a model in an attempt to explore the mechanism of their interactions.

Rocaglamide promotes the infiltration and antitumor immunity of NK cells by activating cGAS-STING signaling in non-small cell lung cancer

Background: Natural killer (NK) cell-based immunotherapy is clinically limited due to insufficient tumor infiltration in solid tumors. We have previously found that the natural product rocaglamide (RocA) can enhance NK cell-mediated killing of non-small cell lung cancer (NSCLC) cells by inhibiting autophagy, and autophagic inhibition has been shown to increase NK cell tumor infiltration in melanoma. Therefore, we hypothesized that RocA could increase NK cell infiltration in NSCLC by autophagy inhibition. Methods: Flow cytometry, RNA-sequencing, real-time PCR, Western blotting analysis, and xenograft tumor model were utilized to assess the infiltration of NK cells and the underlying mechanism. Results: RocA significantly increased the infiltration of NK cells and the expressions of CCL5 and CXCL10 in NSCLC cells, which could not be reversed by the inhibitions of autophagy/ULK1, JNK and NF-κB. However, such up-regulation could be suppressed by the inhibitions of TKB1 and STING. Furthermore, RocA dramatically activated the cGAS (cyclic GMP-AMP synthase)-STING (stimulator of interferon genes) signaling pathway, and the inhibition/depletion of STING ablated the up-regulation of CCL5 and CXCL10, NK cell infiltration, and tumor regression induced by RocA. Besides, RocA damaged mitochondrial DNA (mtDNA) and promoted the cytoplasmic release of mtDNA. The mPTP inhibitor cyclosporin A could reverse RocA-induced cytoplasmic release of mtDNA. Conclusions: RocA could promote NK cell infiltration by activating cGAS-STING signaling via targeting mtDNA, but not by inhibiting autophagy. Taken together, our current findings suggested that RocA was a potent cGAS-STING agonist and had a promising potential in cancer immunotherapy, especially in NK cell-based immunotherapy.

Multiple targeted doxorubicin-lonidamine liposomes modified with p-hydroxybenzoic acid and triphenylphosphonium to synergistically treat glioma

A type of pH-sensitive multi-targeted brain tumor site-specific liposomes (Lip-CTPP) co-modified with p-hydroxybenzoic acid (p-HA) and triphenylphosphonium (TPP) were designed and prepared to co-load doxorubicin (DOX) and lonidamine (LND). Lip-CTPP are promising potential carriers to exert the anti-glioma effect of DOX and LND collaboratively given the following features: 1) Lip-CTPP have a good pharmacokinetic behavior; 2) Lip-CTPP can cross the blood-brain barrier (BBB) and recognize tumor cells through the affinity of p-HA and dopamine/sigma receptors; 3) Lip-CTPP are highly positive charged once the acid-sensitive amide bonds are cleaved in endo/lysosomes to expose TPP and protonate amine groups; 4) the positive charged Lip-CTPP escape from endo/lysosomes and accumulate in mitochondria through electrostatic adsorption; 5) DOX and LND are released and synergistically increase anti-tumor efficacy. Our in vitro and in vivo results confirmed that Lip-CTPP could greatly elevate the inhibition rate of tumor cell proliferation, migration and invasion, promote apoptosis and necrosis, and interfere with mitochondrial function. In addition, Lip-CTPP could significantly prolong the survival time of glioma bearing mice, narrow the tumor region and inhibit the infiltration and metastasis capability of glioma cells. Collectively, Lip-CTPP are promising nano formulations to enhance the synergistic effect of DOX and LND in glioma treatment.

Metal Complexes or Chelators with ROS Regulation Capacity: Promising Candidates for Cancer Treatment

Reactive oxygen species (ROS) are rapidly eliminated and reproduced in organisms, and they always play important roles in various biological functions and abnormal pathological processes. Evaluated ROS have frequently been observed in various cancers to activate multiple pro-tumorigenic signaling pathways and induce the survival and proliferation of cancer cells. Hydrogen peroxide (H₂O₂) and superoxide anion (O₂^•-) are the most important redox signaling agents in cancer cells, the homeostasis of which is maintained by dozens of growth factors, cytokines, and antioxidant enzymes. Therefore, antioxidant enzymes tend to have higher activity levels to maintain the homeostasis of ROS in cancer cells. Effective intervention in the ROS homeostasis of cancer cells by chelating agents or metal complexes has already developed into an important anti-cancer strategy. We can inhibit the activity of antioxidant enzymes using chelators or metal complexes; on the other hand, we can also use metal complexes to directly regulate the level of ROS in cancer cells via mitochondria. In this review, metal complexes or chelators with ROS regulation capacity and with anti-cancer applications are collectively and comprehensively analyzed, which is beneficial for the development of the next generation of inorganic anti-cancer drugs based on ROS regulation. We expect that this review will provide a new perspective to develop novel inorganic reagents for killing cancer cells and, further, as candidates or clinical drugs.

Activated heme synthesis regulates glycolysis and oxidative metabolism in breast and ovarian cancer cells

Heme is an essential cofactor for enzymes of the electron transport chain (ETC) and ATP synthesis in mitochondrial oxidative phosphorylation (OXPHOS). Heme also binds to and destabilizes Bach1, a transcription regulator that controls expression of several groups of genes important for glycolysis, ETC, and metastasis of cancer cells. Heme synthesis can thus affect pathways through which cells generate energy and precursors for anabolism. In addition, increased heme synthesis may trigger oxidative stress. Since many cancers are characterized by a high glycolytic rate regardless of oxygen availability, targeting glycolysis, ETC, and OXPHOS have emerged as a potential therapeutic strategy. Here, we report that enhancing heme synthesis through exogenous supplementation of heme precursor 5-aminolevulinic acid (ALA) suppresses oxidative metabolism as well as glycolysis and significantly reduces proliferation of both ovarian and breast cancer cells. ALA supplementation also destabilizes Bach1 and inhibits migration of both cell types. Our data indicate that the underlying mechanisms differ in ovarian and breast cancer cells, but involve destabilization of Bach1, AMPK activation, and induction of oxidative stress. In addition, there appears to be an inverse correlation between the activity of oxidative metabolism and ALA sensitivity. Promoting heme synthesis by ALA supplementation may thus represent a promising new anti-cancer strategy, particularly in cancers that are sensitive to altered redox signaling, or in combination with strategies that target the antioxidant systems or metabolic weaknesses of cancer cells.

Reactive Oxygen Species in Acute Lymphoblastic Leukaemia: Reducing Radicals to Refine Responses

Acute lymphoblastic leukaemia (ALL) is the most common cancer diagnosed in children and adolescents. Approximately 70% of patients survive >5-years following diagnosis, however, for those that fail upfront therapies, survival is poor. Reactive oxygen species (ROS) are elevated in a range of cancers and are emerging as significant contributors to the leukaemogenesis of ALL. ROS modulate the function of signalling proteins through oxidation of cysteine residues, as well as promote genomic instability by damaging DNA, to promote chemotherapy resistance. Current therapeutic approaches exploit the pro-oxidant intracellular environment of malignant B and T lymphoblasts to cause irreversible DNA damage and cell death, however these strategies impact normal haematopoiesis and lead to long lasting side-effects. Therapies suppressing ROS production, especially those targeting ROS producing enzymes such as the NADPH oxidases (NOXs), are emerging alternatives to treat cancers and may be exploited to improve the ALL treatment. Here, we discuss the roles that ROS play in normal haematopoiesis and in ALL. We explore the molecular mechanisms underpinning overproduction of ROS in ALL, and their roles in disease progression and drug resistance. Finally, we examine strategies to target ROS production, with a specific focus on the NOX enzymes, to improve the treatment of ALL.

Breast cancer growth and proliferation is suppressed by the mitochondrial targeted furazano[3,4-b]pyrazine BAM15

BACKGROUND

Enhanced metabolic plasticity and diversification of energy production is a hallmark of highly proliferative breast cancers. This contributes to poor pharmacotherapy efficacy, recurrence, and metastases. We have previously identified a mitochondrial-targeted furazano[3,4-b]pyrazine named BAM15 that selectively reduces bioenergetic coupling efficiency and is orally available. Here, we evaluated the antineoplastic properties of uncoupling oxidative phosphorylation from ATP production in breast cancer using BAM15.

METHODS

The anticancer effects of BAM15 were evaluated in human triple-negative MDA-MB-231 and murine luminal B, ERα-negative EO771 cells as well as in an orthotopic allograft model of highly proliferative mammary cancer in mice fed a standard or high fat diet (HFD). Untargeted transcriptomic profiling of MDA-MB-231 cells was conducted after 16-h exposure to BAM15. Additionally, oxidative phosphorylation and electron transfer capacity was determined in permeabilized cells and excised tumor homogenates after treatment with BAM15.

RESULTS

BAM15 increased proton leak and over time, diminished cell proliferation, migration, and ATP production in both MDA-MB-231 and EO771 cells. Additionally, BAM15 decreased mitochondrial membrane potential, while inducing apoptosis and reactive oxygen species accumulation in MDA-MB-231 and EO771 cells. Untargeted transcriptomic profiling of MDA-MB-231 cells further revealed inhibition of signatures associated with cell survival and energy production by BAM15. In lean mice, BAM15 lowered body weight independent of food intake and slowed tumor progression compared to vehicle-treated controls. In HFD mice, BAM15 reduced tumor growth relative to vehicle and calorie-restricted weight-matched controls mediated in part by impaired cell proliferation, mitochondrial respiratory function, and ATP production. LC-MS/MS profiling of plasma and tissues from BAM15-treated animals revealed distribution of BAM15 in adipose, liver, and tumor tissue with low abundance in skeletal muscle.

CONCLUSIONS

Collectively, these data indicate that mitochondrial uncoupling may be an effective strategy to limit proliferation of aggressive forms of breast cancer. More broadly, these findings highlight the metabolic vulnerabilities of highly proliferative breast cancers which may be leveraged in overcoming poor responsiveness to existing therapies.

Molecular docking and simulation studies of phytocompounds derived from Centella asiatica and Andrographis paniculata against hexokinase II as mitocan agents

Hexokinase II (HK2), a glycolytic enzyme is commonly overexpressed in most cancer types. The overexpression of HK2 is reported to promote the survival of cancer cells by facilitating the constant ATP generation and protecting the cancer cell against apoptotic cell death. Hence, HK2 is considered as potential target of many mitochondria targeting anticancerous agents (referred to as mitocans). Most of the existing mitocans are synthetic and hence such compounds are observed to exhibit adverse effects, witnessed through many experimental outcomes. These limitations necessitates hunting for an alternative source of mitocans with minimum/no side effects. The need for an alternative therapy points towards the ethnomedicinal herbs, known for their minimal side effects and effectiveness. Henceforth recent studies have put forth the effort to utilize anticancer herbs in formulating naturally derived mitocans as an add-on to improve cancer therapeutics. So, our study aims to explore the HK2 targeting potential of phytocompounds from the selected anticancerous herbs Andrographis paniculata (AP) and Centella asiatica (CA). 60 phytocompounds collectively from CA and AP were docked against HK2 and drug-likeness prediction of the selected phytocompounds was performed to screen the best possible ligand for HK2. Furthermore, the docked complexes were subjected to molecular dynamics simulations (MDS) to analyse the molecular mechanism of protein-ligand interactions. The results of the study suggest that the natural compounds asiatic acid and bayogenin (from CA) and andrographolide (from AP) can bepotential natural mitocans by targeting HK2. Further experimental studies (in-vitro and in-vivo) are required to validate the results.

Cytostatic Effect of a Novel Mitochondria-Targeted Pyrroline Nitroxide in Human Breast Cancer Lines

Mitochondria have emerged as a prospective target to overcome drug resistance that limits triple-negative breast cancer therapy. A novel mitochondria-targeted compound, HO-5114, demonstrated higher cytotoxicity against human breast cancer lines than its component-derivative, Mito-CP. In this study, we examined HO-5114′s anti-neoplastic properties and its effects on mitochondrial functions in MCF7 and MDA-MB-231 human breast cancer cell lines. At a 10 µM concentration and within 24 h, the drug markedly reduced viability and elevated apoptosis in both cell lines. After seven days of exposure, even at a 75 nM concentration, HO-5114 significantly reduced invasive growth and colony formation. A 4 h treatment with 2.5 µM HO-5114 caused a massive loss of mitochondrial membrane potential, a decrease in basal and maximal respiration, and mitochondrial and glycolytic ATP production. However, reactive oxygen species production was only moderately elevated by HO-5114, indicating that oxidative stress did not significantly contribute to the drug’s anti-neoplastic effect. These data indicate that HO-5114 may have potential for use in the therapy of triple-negative breast cancer; however, the in vivo toxicity and anti-neoplastic effectiveness of the drug must be determined to confirm its potential.

Mitochondria-targeted nanoparticles (mitoNANO): An emerging therapeutic shortcut for cancer

The early understanding of mitochondria posited that they were 'innocent organelles' solely devoted to energy production and utilisation. Intriguingly, recent findings have outlined in detail the 'modern-day' view that mitochondria are an important but underappreciated drug target. Mitochondria have been implicated in the pathophysiology of many human diseases, ranging from neurodegenerative disorders and cardiovascular diseases to infections and cancer. It is now clear that normal mitochondrial function involves the building blocks of a cell to generate lipids, proteins and nucleic acids thereby facilitating cell growth. On the other hand, mitochondrial dysfunction reprograms crucial cellular functions into pathological pathways, and is considered as an integral hallmark of cancer. Therefore, strategies to target mitochondria can provide a wealth of new therapeutic approaches in the fight against cancer, by overcoming a number of problems associated with conventional pharmaceutical drugs, including low solubility, poor bioavailability and non-selective biodistribution. The combination of nanoparticles with 'classical' chemotherapeutic drugs to create biocompatible, multifunctional mitochondria-targeted nanoplatforms has been recently studied. This approach is now rapidly expanding for targeted drug delivery systems, and for hybrid nanostructures that can be activated with light (photodynamic and/or photothermal therapy). The selective delivery of nanoparticles to mitochondria is an elegant shortcut to more selective, targeted, and safer cancer treatment. We propose that the use of nanoparticles to target mitochondria be termed "mitoNANO". The present minireview sheds light on the design and application of mitoNANO as advanced cancer therapeutics, that may overcome drug resistance and show fewer side effects.

Failure to Guard: Mitochondrial Protein Quality Control in Cancer

Mitochondria are energetic and dynamic organelles with a crucial role in bioenergetics, metabolism, and signaling. Mitochondrial proteins, encoded by both nuclear and mitochondrial DNA, must be properly regulated to ensure proteostasis. Mitochondrial protein quality control (MPQC) serves as a critical surveillance system, employing different pathways and regulators as cellular guardians to ensure mitochondrial protein quality and quantity. In this review, we describe key pathways and players in MPQC, such as mitochondrial protein translocation-associated degradation, mitochondrial stress responses, chaperones, and proteases, and how they work together to safeguard mitochondrial health and integrity. Deregulated MPQC leads to proteotoxicity and dysfunctional mitochondria, which contributes to numerous human diseases, including cancer. We discuss how alterations in MPQC components are linked to tumorigenesis, whether they act as drivers, suppressors, or both. Finally, we summarize recent advances that seek to target these alterations for the development of anti-cancer drugs.

Type and position of linkage govern the cytotoxicity of oleanolic acid rhodamine B hybrids

Oleanolic acid/rhodamine B hybrids exhibit different cytotoxicity depending on the way these two structural elements are linked. While a hybrid holding a piperazinyl spacer at C-28 proved to be cytotoxic in the nano-molar concentration range, hybrids with a direct linkage of the Rho B residue to C-3 of the triterpenoid skeleton are cytotoxic only in the low micro-molar concentration range without any selectivity. This once again underlines the importance of selecting the right spacer and the most appropriate position on the skeleton of the triterpene to achieve the most cytotoxic hybrids possible.

Rapamycin Plus Doxycycline Combination Affects Growth Arrest and Selective Autophagy-Dependent Cell Death in Breast Cancer Cells

Metabolic alteration is characteristic during tumour growth and therapy; however, targeting metabolic rewiring could overcome therapy resistance. mTOR hyperactivity, autophagy and other metabolic processes, including mitochondrial functions, could be targeted in breast cancer progression. We investigated the growth inhibitory mechanism of rapamycin + doxycycline treatment in human breast cancer model systems. Cell cycle and cell viability, including apoptotic and necrotic cell death, were analysed using flow cytometry, caspase activity measurements and caspase-3 immunostainings. mTOR-, autophagy-, necroptosis-related proteins and treatment-induced morphological alterations were analysed by Wes^TM, Western blot, immunostainings and transmission electron microscopy. The rapamycin + doxycycline combination decreased tumour proliferation in about 2/3rd of the investigated cell lines. The continuous treatment reduced tumour growth significantly both in vivo and in vitro. The effect after short-term treatment was reversible; however, autophagic vacuoles and degrading mitochondria were detected simultaneously, and the presence of mitophagy was also observed after the long-term rapamycin + doxycycline combination treatment. The rapamycin + doxycycline combination did not cause apoptosis or necrosis/necroptosis, but the alterations in autophagy- and mitochondria-related protein levels (LC3-B-II/I, p62, MitoTracker, TOM20 and certain co-stainings) were correlated to autophagy induction and mitophagy, without mitochondria repopulation. Based on these results, we suggest considering inducing metabolic stress and targeting mTOR hyperactivity and mitochondrial functions in combined anti-cancer treatments.

Mitochondrial Metabolism in Carcinogenesis and Cancer Therapy

Simple Summary

Reprogramming metabolism is a hallmark of cancer. Warburg’s effect, defined as increased aerobic glycolysis at the expense of mitochondrial respiration in cancer cells, opened new avenues of research in the field of cancer. Later findings, however, have revealed that mitochondria remain functional and that they actively contribute to metabolic plasticity of cancer cells. Understanding the mechanisms by which mitochondrial metabolism controls tumor initiation and progression is necessary to better characterize the onset of carcinogenesis. These studies may ultimately lead to the design of novel anti-cancer strategies targeting mitochondrial functions.

Abstract

Carcinogenesis is a multi-step process that refers to transformation of a normal cell into a tumoral neoplastic cell. The mechanisms that promote tumor initiation, promotion and progression are varied, complex and remain to be understood. Studies have highlighted the involvement of oncogenic mutations, genomic instability and epigenetic alterations as well as metabolic reprogramming, in different processes of oncogenesis. However, the underlying mechanisms still have to be clarified. Mitochondria are central organelles at the crossroad of various energetic metabolisms. In addition to their pivotal roles in bioenergetic metabolism, they control redox homeostasis, biosynthesis of macromolecules and apoptotic signals, all of which are linked to carcinogenesis. In the present review, we discuss how mitochondria contribute to the initiation of carcinogenesis through gene mutations and production of oncometabolites, and how they promote tumor progression through the control of metabolic reprogramming and mitochondrial dynamics. Finally, we present mitochondrial metabolism as a promising target for the development of novel therapeutic strategies.

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.

Mitochondrial K+ channels and their implications for disease mechanisms

The field of mitochondrial ion channels underwent a rapid development during the last decade, thanks to the molecular identification of some of the nuclear-encoded organelle channels and to advances in strategies allowing specific pharmacological targeting of these proteins. Thereby, genetic tools and specific drugs aided definition of the relevance of several mitochondrial channels both in physiological as well as pathological conditions. Unfortunately, in the case of mitochondrial K⁺ channels, efforts of genetic manipulation provided only limited results, due to their dual localization to mitochondria and to plasma membrane in most cases. Although the impact of mitochondrial K⁺ channels on human diseases is still far from being genuinely understood, pre-clinical data strongly argue for their substantial role in the context of several pathologies, including cardiovascular and neurodegenerative diseases as well as cancer. Importantly, these channels are druggable targets, and their in-depth investigation could thus pave the way to the development of innovative small molecules with huge therapeutic potential. In the present review we summarize the available experimental evidence that mechanistically link mitochondrial potassium channels to the above pathologies and underline the possibility of exploiting them for therapy.

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.