Disordered-to-ordered transitions in assembly factors allow the complex II catalytic subunit to switch binding partners.

Complex II (CII) activity controls phenomena that require crosstalk between metabolism and signaling, including neurodegeneration, cancer metabolism, immune activation, and ischemia-reperfusion injury. CII activity can be regulated at the level of assembly, a process that leverages metastable assembly intermediates. The nature of these intermediates and how CII subunits transfer between metastable complexes remains unclear. In this work, we identify metastable species containing the SDHA subunit and its assembly factors, and we assign a preferred temporal sequence of appearance of these species during CII assembly. Structures of two species show that the assembly factors undergo disordered-to-ordered transitions without the appearance of significant secondary structure. The findings identify that intrinsically disordered regions are critical in regulating CII assembly, an observation that has implications for the control of assembly in other biomolecular complexes.

Germline SUCLG2 Variants in Patients With Pheochromocytoma and Paraganglioma.

BACKGROUND: Pheochromocytoma and paraganglioma (PPGL) are neuroendocrine tumors with frequent mutations in genes linked to the tricarboxylic acid cycle. However, no pathogenic variant has been found to date in succinyl-CoA ligase (SUCL), an enzyme that provides substrate for succinate dehydrogenase (SDH; mitochondrial complex II [CII]), a known tumor suppressor in PPGL. METHODS: A cohort of 352 patients with apparently sporadic PPGL underwent genetic testing using a panel of 54 genes developed at the National Institutes of Health, including the SUCLG2 subunit of SUCL. Gene deletion, succinate levels, and protein levels were assessed in tumors where possible. To confirm the possible mechanism, we used a progenitor cell line, hPheo1, derived from a human pheochromocytoma, and ablated and re-expressed SUCLG2. RESULTS: We describe 8 germline variants in the guanosine triphosphate-binding domain of SUCLG2 in 15 patients (15 of 352, 4.3%) with apparently sporadic PPGL. Analysis of SUCLG2-mutated tumors and SUCLG2-deficient hPheo1 cells revealed absence of SUCLG2 protein, decrease in the level of the SDHB subunit of SDH, and faulty assembly of the complex II, resulting in aberrant respiration and elevated succinate accumulation. CONCLUSIONS: Our study suggests SUCLG2 as a novel candidate gene in the genetic landscape of PPGL. Large-scale sequencing may uncover additional cases harboring SUCLG2 variants and provide more detailed information about their prevalence and penetrance.

SMAD4 loss limits the vulnerability of pancreatic cancer cells to complex I inhibition via promotion of mitophagy.

Pancreatic cancer is one of the deadliest forms of cancer, which is attributed to lack of effective treatment options and drug resistance. Mitochondrial inhibitors have emerged as a promising class of anticancer drugs, and several inhibitors of the electron transport chain (ETC) are being clinically evaluated. We hypothesized that resistance to ETC inhibitors from the biguanide class could be induced by inactivation of SMAD4, an important tumor suppressor involved in transforming growth factor ß (TGFß) signaling, and associated with altered mitochondrial activity. Here we show that, paradoxically, both TGFß-treatment and the loss of SMAD4, a downstream member of TGFß signaling cascade, induce resistance to biguanides, decrease mitochondrial respiration, and fragment the mitochondrial network. Mechanistically, the resistance of SMAD4-deficient cells is mediated by increased mitophagic flux driven by MAPK/ERK signaling, whereas TGFß-induced resistance is autophagy-independent and linked to epithelial-to-mesenchymal transition (EMT). Interestingly, mitochondria-targeted tamoxifen, a complex I inhibitor under clinical trial, overcomes resistance mediated by SMAD4-deficiency or TGFß signaling. Our data point to differential mechanisms underlying the resistance to treatment in PDAC arising from TGFß signaling and SMAD4 loss, respectively. The findings will help the development of mitochondria-targeted therapy for pancreatic cancer patients with SMAD4 as a plausible predictive marker.

A simple indirect colorimetric assay for measuring mitochondrial energy metabolism based on uncoupling sensitivity.

PURPOSE: Cancer cells rapidly adjust their balance between glycolytic and mitochondrial ATP production in response to changes in their microenvironment and to treatments like radiation and chemotherapy. Reliable, simple, high throughput assays that measure the levels of mitochondrial energy metabolism in cells are useful determinants of treatment effects. Mitochondrial metabolism is routinely determined by measuring the rate of oxygen consumption (OCR). We have previously shown that indirect inhibition of plasma membrane electron transport (PMET) by the mitochondrial uncoupler, FCCP, may also be a reliable measure of mitochondrial energy metabolism. Here, we aimed to validate these earlier findings by exploring the relationship between stimulation of oxygen consumption by FCCP and inhibition of PMET. METHODS: We measured PMET by reduction of the cell impermeable tetrazolium salt WST-1/PMS. We characterised the effect of different growth conditions on the extent of PMET inhibition by FCCP. Next, we compared FCCP-mediated PMET inhibition with FCCP-mediated stimulation of OCR using the Seahorse XF96e flux analyser, in a panel of cancer cell lines. RESULTS: We found a strong inverse correlation between stimulation of OCR and PMET inhibition by FCCP. PMET and OCR were much more severely affected by FCCP in cells that rely on mitochondrial energy production than in cells with a more glycolytic phenotype. CONCLUSION: Indirect inhibition of PMET by FCCP is a reliable, simple and inexpensive high throughput assay to determine the level of mitochondrial energy metabolism in cancer cells.

Mitochondrial DNA Affects the Expression of Nuclear Genes Involved in Immune and Stress Responses in a Breast Cancer Model.

Tumor cells without mitochondrial (mt) DNA (ρ0 cells) are auxotrophic for uridine, and their growth is supported by pyruvate. While ATP synthesis in ρ0 cells relies on glycolysis, they fail to form tumors unless they acquire mitochondria from stromal cells. Mitochondrial acquisition restores respiration that is essential for de novo pyrimidine biosynthesis and for mitochondrial ATP production. The physiological processes that underpin intercellular mitochondrial transfer to tumor cells lacking mtDNA and the metabolic remodeling and restored tumorigenic properties of cells that acquire mitochondria are not well understood. Here, we investigated the changes in mitochondrial and nuclear gene expression that accompany mtDNA deletion and acquisition in metastatic murine 4T1 breast cancer cells. Loss of mitochondrial gene expression in 4T1ρ0 cells was restored in cells recovered from subcutaneous tumors that grew from 4T1ρ0 cells following acquisition of mtDNA from host cells. In contrast, the expression of most nuclear genes that encode respiratory complex subunits and mitochondrial ribosomal subunits was not greatly affected by loss of mtDNA, indicating ineffective mitochondria-to-nucleus communication systems for these nuclear genes. Further, analysis of nuclear genes whose expression was compromised in 4T1ρ0 cells showed that immune- and stress-related genes were the most highly differentially expressed, representing over 70% of those with greater than 16-fold higher expression in 4T1 compared with 4T1ρ0 cells. The monocyte recruiting chemokine, Ccl2, and Psmb8, a subunit of the immunoproteasome that generates MHCI-binding peptides, were the most highly differentially expressed. Early monocyte/macrophage recruitment into the tumor mass was compromised in 4T1ρ0 cells but recovered before mtDNA could be detected. Taken together, our results show that mitochondrial acquisition by tumor cells without mtDNA results in bioenergetic remodeling and re-expression of genes involved in immune function and stress adaptation.

Dihydroorotate dehydrogenase in oxidative phosphorylation and cancer.

Dihydroorotate dehydrogenase (DHODH) is an enzyme of the de novo pyrimidine synthesis pathway that provides nucleotides for RNA/DNA synthesis essential for proliferation. In mammalian cells, DHODH is localized in mitochondria, linked to the respiratory chain via the coenzyme Q pool. Here we discuss the role of DHODH in the oxidative phosphorylation system and in the initiation and progression of cancer. We summarize recent findings on DHODH biology, the progress made in the development of new, specific inhibitors of DHODH intended for cancer therapy, and the mechanistic insights into the consequences of DHODH inhibition.

Replication and ribosomal stress induced by targeting pyrimidine synthesis and cellular checkpoints suppress p53-deficient tumors.

p53-mutated tumors often exhibit increased resistance to standard chemotherapy and enhanced metastatic potential. Here we demonstrate that inhibition of dihydroorotate dehydrogenase (DHODH), a key enzyme of the de novo pyrimidine synthesis pathway, effectively decreases proliferation of cancer cells via induction of replication and ribosomal stress in a p53- and checkpoint kinase 1 (Chk1)-dependent manner. Mechanistically, a block in replication and ribosomal biogenesis result in p53 activation paralleled by accumulation of replication forks that activate the ataxia telangiectasia and Rad3-related kinase/Chk1 pathway, both of which lead to cell cycle arrest. Since in the absence of functional p53 the cell cycle arrest fully depends on Chk1, combined DHODH/Chk1 inhibition in p53-dysfunctional cancer cells induces aberrant cell cycle re-entry and erroneous mitosis, resulting in massive cell death. Combined DHODH/Chk1 inhibition effectively suppresses p53-mutated tumors and their metastasis, and therefore presents a promising therapeutic strategy for p53-mutated cancers.

Reactivation of Dihydroorotate Dehydrogenase-Driven Pyrimidine Biosynthesis Restores Tumor Growth of Respiration-Deficient Cancer Cells.

Cancer cells without mitochondrial DNA (mtDNA) do not form tumors unless they reconstitute oxidative phosphorylation (OXPHOS) by mitochondria acquired from host stroma. To understand why functional respiration is crucial for tumorigenesis, we used time-resolved analysis of tumor formation by mtDNA-depleted cells and genetic manipulations of OXPHOS. We show that pyrimidine biosynthesis dependent on respiration-linked dihydroorotate dehydrogenase (DHODH) is required to overcome cell-cycle arrest, while mitochondrial ATP generation is dispensable for tumorigenesis. Latent DHODH in mtDNA-deficient cells is fully activated with restoration of complex III/IV activity and coenzyme Q redox-cycling after mitochondrial transfer, or by introduction of an alternative oxidase. Further, deletion of DHODH interferes with tumor formation in cells with fully functional OXPHOS, while disruption of mitochondrial ATP synthase has little effect. Our results show that DHODH-driven pyrimidine biosynthesis is an essential pathway linking respiration to tumorigenesis, pointing to inhibitors of DHODH as potential anti-cancer agents.

Alternative assembly of respiratory complex II connects energy stress to metabolic checkpoints.

Cell growth and survival depend on a delicate balance between energy production and synthesis of metabolites. Here, we provide evidence that an alternative mitochondrial complex II (CII) assembly, designated as CIIlow, serves as a checkpoint for metabolite biosynthesis under bioenergetic stress, with cells suppressing their energy utilization by modulating DNA synthesis and cell cycle progression. Depletion of CIIlow leads to an imbalance in energy utilization and metabolite synthesis, as evidenced by recovery of the de novo pyrimidine pathway and unlocking cell cycle arrest from the S-phase. In vitro experiments are further corroborated by analysis of paraganglioma tissues from patients with sporadic, SDHA and SDHB mutations. These findings suggest that CIIlow is a core complex inside mitochondria that provides homeostatic control of cellular metabolism depending on the availability of energy.

The human transient receptor potential vanilloid 3 channel is sensitized via the ERK pathway.

The transient receptor potential vanilloid 3 (TRPV3) channel is a Ca2+-permeable thermosensitive ion channel widely expressed in keratinocytes, where together with epidermal growth factor receptor (EGFR) forms a signaling complex regulating epidermal homeostasis. Proper signaling through this complex is achieved and maintained via several pathways in which TRPV3 activation is absolutely required. Results of recent studies have suggested that low-level constitutive activity of TRPV3 induces EGFR-dependent signaling that, in turn, amplifies TRPV3 via activation of the mitogen-activated protein kinase ERK in a positive feedback loop. Here, we explored the molecular mechanism that increases TRPV3 activity through EGFR activation. We used mutagenesis and whole-cell patch clamp experiments on TRPV3 channels endogenously expressed in an immortalized human keratinocyte cell line (HaCaT) and in transiently transfected HEK293T cells and found that the sensitizing effect of EGFR on TRPV3 is mediated by ERK. We observed that ERK-mediated phosphorylation of TRPV3 alters its responsiveness to repeated chemical stimuli. Among several putative ERK phosphorylation sites, we identified threonine 264 in the N-terminal ankyrin repeat domain as the most critical site for the ERK-dependent modulation of TRPV3 channel activity. Of note, Thr264 is in close vicinity to a structurally and functionally important TRPV3 region comprising an atypical finger 3 and oxygen-dependent hydroxylation site. In summary, our findings indicate that Thr264 in TRPV3 is a key ERK phosphorylation site mediating EGFR-induced sensitization of the channel to stimulate signaling pathways involved in regulating skin homeostasis.

Mitochondrial Targeting of Metformin Enhances Its Activity against Pancreatic Cancer.

Pancreatic cancer is one of the hardest-to-treat types of neoplastic diseases. Metformin, a widely prescribed drug against type 2 diabetes mellitus, is being trialed as an agent against pancreatic cancer, although its efficacy is low. With the idea of delivering metformin to its molecular target, the mitochondrial complex I (CI), we tagged the agent with the mitochondrial vector, triphenylphosphonium group. Mitochondrially targeted metformin (MitoMet) was found to kill a panel of pancreatic cancer cells three to four orders of magnitude more efficiently than found for the parental compound. Respiration assessment documented CI as the molecular target for MitoMet, which was corroborated by molecular modeling. MitoMet also efficiently suppressed pancreatic tumors in three mouse models. We propose that the novel mitochondrially targeted agent is clinically highly intriguing, and it has a potential to greatly improve the bleak prospects of patients with pancreatic cancer. Mol Cancer Ther; 15(12); 2875-86. ©2016 AACR.

Amplified cold transduction in native nociceptors by M-channel inhibition.

Topically applied camphor elicits a sensation of cool, but nothing is known about how it affects cold temperature sensing. We found that camphor sensitizes a subpopulation of menthol-sensitive native cutaneous nociceptors in the mouse to cold, but desensitizes and partially blocks heterologously expressed TRPM8 (transient receptor potential cation channel subfamily M member 8). In contrast, camphor reduces potassium outward currents in cultured sensory neurons and, in cold nociceptors, the cold-sensitizing effects of camphor and menthol are additive. Using a membrane potential dye-based screening assay and heterologously expressed potassium channels, we found that the effects of camphor are mediated by inhibition of Kv7.2/3 channels subtypes that generate the M-current in neurons. In line with this finding, the specific M-current blocker XE991 reproduced the cold-sensitizing effect of camphor in nociceptors. However, the M-channel blocking effects of XE991 and camphor are not sufficient to initiate cold transduction but require a cold-activated inward current generated by TRPM8. The cold-sensitizing effects of XE991 and camphor are largest in high-threshold cold nociceptors. Low-threshold corneal cold thermoreceptors that express high levels of TRPM8 and lack potassium channels are not affected by camphor. We also found that menthol--like camphor--potently inhibits Kv7.2/3 channels. The apparent functional synergism arising from TRPM8 activation and M-current block can improve the effectiveness of topical coolants and cooling lotions, and may also enhance TRPM8-mediated analgesia.

Protons stabilize the closed conformation of gain-of-function mutants of the TRPV1 channel.

The vanilloid transient receptor potential channel TRPV1 is a molecular integrator of noxious stimuli, including capsaicin, heat and protons. Despite clear similarities between the overall architecture of TRPV1 and voltage-dependent potassium (Kv) channels, the extent of conservation in the molecular logic for gating is unknown. In Kv channels, a small contact surface between S1 and the pore-helix is required for channel functioning. To explore the function of S1 in TRPV1, we used tryptophan-scanning mutagenesis and characterized the responses to capsaicin and protons. Wild-type-like currents were generated in 9 out of 17 mutants; three mutants (M445W, A452W, R455W) were non-functional. The conservative mutation R455K in the extracellular extent of S1 slowed down capsaicin-induced activation and prevented normal channel closure. This mutant was neither activated nor potentiated by protons, on the contrary, protons promoted a rapid deactivation of its currents. Similar phenotypes were found in two other gain-of-function mutants and also in the pore-helix mutant T633A, known to uncouple proton activation. We propose that the S1 domain contains a functionally important region that may be specifically involved in TRPV1 channel gating, and thus be important for the energetic coupling between S1-S4 sensor activation and gate opening. Analogous to Kv channels, the S1-pore interface might serve to stabilize conformations associated with TRPV1 channel gating.

Conserved residues within the putative S4-S5 region serve distinct functions among thermosensitive vanilloid transient receptor potential (TRPV) channels.

The vanilloid transient receptor potential channel TRPV1 is a tetrameric six-transmembrane segment (S1-S6) channel that can be synergistically activated by various proalgesic agents such as capsaicin, protons, heat, or highly depolarizing voltages, and also by 2-aminoethoxydiphenyl borate (2-APB), a common activator of the related thermally gated vanilloid TRP channels TRPV1, TRPV2, and TRPV3. In these channels, the conserved charged residues in the intracellular S4-S5 region have been proposed to constitute part of a voltage sensor that acts in concert with other stimuli to regulate channel activation. The molecular basis of this gating event is poorly understood. We mutated charged residues all along the S4 and the S4-S5 linker of TRPV1 and identified four potential voltage-sensing residues (Arg(557), Glu(570), Asp(576), and Arg(579)) that, when specifically mutated, altered the functionality of the channel with respect to voltage, capsaicin, heat, 2-APB, and/or their interactions in different ways. The nonfunctional charge-reversing mutations R557E and R579E were partially rescued by the charge-swapping mutations R557E/E570R and D576R/R579E, indicating that electrostatic interactions contribute to allosteric coupling between the voltage-, temperature- and capsaicin-dependent activation mechanisms. The mutant K571E was normal in all aspects of TRPV1 activation except for 2-APB, revealing the specific role of Lys(571) in chemical sensitivity. Surprisingly, substitutions at homologous residues in TRPV2 or TRPV3 had no effect on temperature- and 2-APB-induced activity. Thus, the charged residues in S4 and the S4-S5 linker contribute to voltage sensing in TRPV1 and, despite their highly conserved nature, regulate the temperature and chemical gating in the various TRPV channels in different ways.