Proton export upregulates aerobic glycolysis.

INTRODUCTION: Aggressive cancers commonly ferment glucose to lactic acid at high rates, even in the presence of oxygen. This is known as aerobic glycolysis, or the "Warburg Effect." It is widely assumed that this is a consequence of the upregulation of glycolytic enzymes. Oncogenic drivers can increase the expression of most proteins in the glycolytic pathway, including the terminal step of exporting H+ equivalents from the cytoplasm. Proton exporters maintain an alkaline cytoplasmic pH, which can enhance all glycolytic enzyme activities, even in the absence of oncogene-related expression changes. Based on this observation, we hypothesized that increased uptake and fermentative metabolism of glucose could be driven by the expulsion of H+ equivalents from the cell. RESULTS: To test this hypothesis, we stably transfected lowly glycolytic MCF-7, U2-OS, and glycolytic HEK293 cells to express proton-exporting systems: either PMA1 (plasma membrane ATPase 1, a yeast H+-ATPase) or CA-IX (carbonic anhydrase 9). The expression of either exporter in vitro enhanced aerobic glycolysis as measured by glucose consumption, lactate production, and extracellular acidification rate. This resulted in an increased intracellular pH, and metabolomic analyses indicated that this was associated with an increased flux of all glycolytic enzymes upstream of pyruvate kinase. These cells also demonstrated increased migratory and invasive phenotypes in vitro, and these were recapitulated in vivo by more aggressive behavior, whereby the acid-producing cells formed higher-grade tumors with higher rates of metastases. Neutralizing tumor acidity with oral buffers reduced the metastatic burden. CONCLUSIONS: Therefore, cancer cells which increase export of H+ equivalents subsequently increase intracellular alkalization, even without oncogenic driver mutations, and this is sufficient to alter cancer metabolism towards an upregulation of aerobic glycolysis, a Warburg phenotype. Overall, we have shown that the traditional understanding of cancer cells favoring glycolysis and the subsequent extracellular acidification is not always linear. Cells which can, independent of metabolism, acidify through proton exporter activity can sufficiently drive their metabolism towards glycolysis providing an important fitness advantage for survival.

Minimization of thermodynamic costs in cancer cell invasion.

Metastasis, the truly lethal aspect of cancer, occurs when metastatic cancer cells in a tumor break through the basement membrane and penetrate the extracellular matrix. We show that MDA-MB-231 metastatic breast cancer cells cooperatively invade a 3D collagen matrix while following a glucose gradient. The invasion front of the cells is a dynamic one, with different cells assuming the lead on a time scale of 70 h. The front cell leadership is dynamic presumably because of metabolic costs associated with a long-range strain field that precedes the invading cell front, which we have imaged using confocal imaging and marker beads imbedded in the collagen matrix. We suggest this could be a quantitative assay for an invasive phenotype tracking a glucose gradient and show that the invading cells act in a cooperative manner by exchanging leaders in the invading front.

Sweat but no gain: inhibiting proliferation of multidrug resistant cancer cells with "ersatzdroges".

ATP-binding cassette (ABC) drug transporters consuming ATPs for drug efflux is a common mechanism by which clinical cancers develop multidrug resistance (MDR). We hypothesized that MDR phenotypes could be suppressed by administration of "ersatzdroges," nonchemotherapy drugs that are, nevertheless, ABC substrates. We reasoned that, through prolonged activation of the ABC pumps, ersatzdroges will force MDR cells to divert limited resources from proliferation and invasion thus delaying disease progression. We evaluated ABC substrates as ersatzdroge by comparing their effects on proliferation and survival of MDR cell lines (MCF-7/Dox and 8226/Dox40) with the effects on the drug-sensitive parental lines (MCF-7 and 8226/s, respectively) in glucose-limited condition. The changes in glucose and energy demands were also examined in vitro and in vivo. MCF-7/Dox showed higher ATP demand and susceptibility to glucose resource limitation. Ersatzdroges significantly decreased proliferation of MCF-7/Dox when the culture media contained physiological glucose concentrations (1.0 g/L) or less, but had no effect on MCF-7. Similar evidence was obtained from 8226/Dox40 and 8226/s comparison. In vivo 18F-FDG-PET imaging demonstrated that glucose uptake was increased by systemic administration of an ersatzdroge in tumors composed of MDR. These results suggest that administration of ersatzdroges, by increasing the metabolic cost of resistance, can suppress proliferation of drug-resistance phenotypes. This provides a novel and relatively simple application model of evolution-based strategy, which can exploit the cost of resistance to delay proliferation of drug-resistant cancer phenotypes. Furthermore, suggested is the potential of ersatzdroges to identify tumors or regions of tumors that express the MDR phenotype.

Cellular modeling of cancer invasion: integration of in silico and in vitro approaches.

Cancer invasion is one of the hallmarks of cancer and a prerequisite for cancer metastasis. However, the invasive process is very complex, depending on multiple correlated intrinsic and environmental factors, and thus is difficult to study experimentally in a fully controlled way. Therefore, there is an increased demand for interdisciplinary integrated approaches combining laboratory experiments with multiscale in silico modeling. In this review, we will summarize current computational techniques applicable to model cancer invasion in silico, with a special focus on a class of individual-cell-based models developed in our laboratories. We also discuss their integration with traditional and novel in vitro experimentation, including new invasion assays whose design was inspired by computational modeling.

Mitochondrial ATP fuels ABC transporter-mediated drug efflux in cancer chemoresistance.

Chemotherapy remains the standard of care for most cancers worldwide, however development of chemoresistance due to the presence of the drug-effluxing ATP binding cassette (ABC) transporters remains a significant problem. The development of safe and effective means to overcome chemoresistance is critical for achieving durable remissions in many cancer patients. We have investigated the energetic demands of ABC transporters in the context of the metabolic adaptations of chemoresistant cancer cells. Here we show that ABC transporters use mitochondrial-derived ATP as a source of energy to efflux drugs out of cancer cells. We further demonstrate that the loss of methylation-controlled J protein (MCJ) (also named DnaJC15), an endogenous negative regulator of mitochondrial respiration, in chemoresistant cancer cells boosts their ability to produce ATP from mitochondria and fuel ABC transporters. We have developed MCJ mimetics that can attenuate mitochondrial respiration and safely overcome chemoresistance in vitro and in vivo. Administration of MCJ mimetics in combination with standard chemotherapeutic drugs could therefore become an alternative strategy for treatment of multiple cancers.

A novel circular invasion assay mimics in vivo invasive behavior of cancer cell lines and distinguishes single-cell motility in vitro.

BACKGROUND: Classical in vitro wound-healing assays and other techniques designed to study cell migration and invasion have been used for many years to elucidate the various mechanisms associated with metastasis. However, many of these methods are limited in their ability to achieve reproducible, quantitative results that translate well in vivo. Such techniques are also commonly unable to elucidate single-cell motility mechanisms, an important factor to be considered when studying dissemination. Therefore, we developed and applied a novel in vitro circular invasion assay (CIA) in order to bridge the translational gap between in vitro and in vivo findings, and to distinguish between different modes of invasion. METHOD: Our method is a modified version of a standard circular wound-healing assay with an added matrix barrier component (Matrigel), which better mimics those physiological conditions present in vivo. We examined 3 cancer cell lines (MCF-7, SCOV-3, and MDA-MB-231), each with a different established degree of aggressiveness, to test our assay's ability to detect diverse levels of invasiveness. Percent wound closure (or invasion) was measured using time-lapse microscopy and advanced image analysis techniques. We also applied the CIA technique to DLD-1 cells in the presence of lysophosphatidic acid (LPA), a bioactive lipid that was recently shown to stimulate cancer cell colony dispersal into single migratory cells, in order to validate our method's ability to detect collective and individual motility. RESULTS: CIA method was found to be highly reproducible, with negligible levels of variance measured. It successfully detected the anticipated low, moderate, and high levels of invasion that correspond to in vivo findings for cell lines tested. It also captured that DLD-1 cells exhibit individual migration upon LPA stimulation, and collective behavior in its absence. CONCLUSION: Given its ability to both determine pseudo-realistic invasive cell behavior in vitro and capture subtle differences in cell motility, we propose that our CIA method may shed some light on the cellular mechanisms underlying cancer invasion and deserves inclusion in further studies. The broad implication of this work is the development of a reproducible, quantifiable, high-resolution method that can be applied to various models, to include an unlimited number of parameters and/or agents that may influence invasion.

The CPT1a inhibitor, etomoxir induces severe oxidative stress at commonly used concentrations.

Etomoxir (ETO) is a widely used small-molecule inhibitor of fatty acid oxidation (FAO) through its irreversible inhibitory effects on the carnitine palmitoyl-transferase 1a (CPT1a). We used this compound to evaluate the role of fatty acid oxidation in rapidly proliferating T cells following costimulation through the CD28 receptor. We show that ETO has a moderate effect on T cell proliferation with no observable effect on memory differentiation, but a marked effect on oxidative metabolism. We show that this oxidative metabolism is primarily dependent upon glutamine rather than FAO. Using an shRNA approach to reduce CPT1a in T cells, we further demonstrate that the inhibition of oxidative metabolism in T cells by ETO is independent of its effects on FAO at concentrations exceeding 5 µM. Concentrations of ETO above 5 µM induce acute production of ROS with associated evidence of severe oxidative stress in proliferating T cells. In aggregate, these data indicate that ETO lacks specificity for CTP1a above 5 µM, and caution should be used when employing this compound for studies in cells due to its non-specific effects on oxidative metabolism and cellular redox.

Exploiting evolutionary principles to prolong tumor control in preclinical models of breast cancer.

Conventional cancer treatment strategies assume that maximum patient benefit is achieved through maximum killing of tumor cells. However, by eliminating the therapy-sensitive population, this strategy accelerates emergence of resistant clones that proliferate unopposed by competitors-an evolutionary phenomenon termed "competitive release." We present an evolution-guided treatment strategy designed to maintain a stable population of chemosensitive cells that limit proliferation of resistant clones by exploiting the fitness cost of the resistant phenotype. We treated MDA-MB-231/luc triple-negative and MCF7 estrogen receptor-positive (ER(+)) breast cancers growing orthotopically in a mouse mammary fat pad with paclitaxel, using algorithms linked to tumor response monitored by magnetic resonance imaging. We found that initial control required more intensive therapy with regular application of drug to deflect the exponential tumor growth curve onto a plateau. Dose-skipping algorithms during this phase were less successful than variable dosing algorithms. However, once initial tumor control was achieved, it was maintained with progressively smaller drug doses. In 60 to 80% of animals, continued decline in tumor size permitted intervals as long as several weeks in which no treatment was necessary. Magnetic resonance images and histological analysis of tumors controlled by adaptive therapy demonstrated increased vascular density and less necrosis, suggesting that vascular normalization resulting from enforced stabilization of tumor volume may contribute to ongoing tumor control with lower drug doses. Our study demonstrates that an evolution-based therapeutic strategy using an available chemotherapeutic drug and conventional clinical imaging can prolong the progression-free survival in different preclinical models of breast cancer.

Role of phospholipase D1 in the regulation of mTOR activity by lysophosphatidic acid.

Mitogens activate protein translation through phosphorylation of p7S6 kinase (p70(S6K)) and eIF4E binding protein 1 (4E-BP1) mediated by the mammalian target of rapamycin (mTOR) or phosphoinositide 3-kinase (PI3K). A recent report (Science 294, 1942, 2001) has implicated phospholipase D (PLD) in mTOR signaling. We studied the role of PLD in the phosphorylation of p70(S6K) and 4E-BP1 induced by lysophosphatidic acid (LPA) and platelet-derived growth factor (PDGF) using fibroblasts deficient in PLD activity and also 1-butanol, which inhibits phosphatidic acid production by PLD. The reduction in PLD activity in both situations impaired the effect of LPA on mTOR signaling but did not inhibit the effect of PDGF. PDGF induced marked phosphorylation of Akt (a PI3K target) but this was not affected by PLD deficiency. LPA caused much less phosphorylation of Akt and this was dependent on PLD activity. Toxin B, which inactivates Rho GTPases, markedly impaired PLD1 activation and phosphorylation of Akt, p70(S6K), and 4E-BP1 induced by LPA but had a minimal or no effect on the actions of PDGF. These results support the hypothesis that LPA activates protein translation through the action of PLD1-generated PA on mTOR and the PI3K/Akt pathway whereas PDGF acts through P13K/Akt independent of PLD1.

Evolutionary strategy for systemic therapy of metastatic breast cancer: balancing response with suppression of resistance.

Conventional systemic therapy for disseminated breast cancer is based on the general assumption that the greatest patient benefit is achieved by killing the maximum number of tumor cells. While this strategy often achieves a significant reduction in tumor burden, most patients with metastatic breast cancer ultimately die from their disease as therapy fails because tumor cells evolve resistance. We propose that the conventional maximum dose/maximum cell kill cancer therapy, when viewed from an evolutionary vantage, is suboptimal and likely even harmful as it accelerates evolution and growth of the resistant phenotypes that ultimately cause patient death. As an alternative, we are investigating evolutionary therapeutic strategies that shift the treatment goal from killing the maximum number of cancer cells to maximizing patient survival. Here we introduce two novel approaches for systemic therapy for metastatic breast cancer, considering the evolutionary nature of tumor progression; adaptive therapy and double-bind therapy.

Evolutionary approaches to prolong progression-free survival in breast cancer.

Many cancers adapt to chemotherapeutic agents by upregulating membrane efflux pumps that export drugs from the cytoplasm, but this response comes at an energetic cost. In breast cancer patients, expression of these pumps is low in tumors before therapy but increases after treatment. While the evolution of therapeutic resistance is virtually inevitable, proliferation of resistant clones is not, suggesting strategies of adaptive therapy. Chemoresistant cells must consume excess resources to maintain resistance mechanisms, so adaptive therapy strategies explicitly aim to maintain a stable population of therapy-sensitive cells to suppress growth of resistant phenotypes through intratumoral competition. We used computational models parameterized by in vitro experiments to illustrate the efficacy of such approaches. Here, we show that low doses of verapamil and 2-deoxyglucose, to accentuate the cost of resistance and to decrease energy production, respectively, could suppress the proliferation of drug-resistant clones in vivo. Compared with standard high-dose-density treatment, the novel treatment we developed achieved a 2-fold to 10-fold increase in time to progression in tumor models. Our findings challenge the existing flawed paradigm of maximum dose treatment, a strategy that inevitably produces drug resistance that can be avoided by the adaptive therapy strategies we describe.

Lysophosphatidic Acid Upregulates Laminin-332 Expression during A431 Cell Colony Dispersal.

Lysophosphatidic acid (LPA) is a bioactive phospholipid that affects various biological functions, such as cell proliferation, migration, survival, wound healing, and tumor invasion through LPA receptors. Previously, we reported that LPA induces A431 colony dispersal, accompanied by disruption of cell-cell contacts and cell migration. However, it remains unclear how LPA affects cell migration and gene expression during A431 colony dispersal. In this paper, we performed cDNA microarray analysis to investigate this question by comparing gene expression between untreated and LPA-treated A431 cells. Interestingly, these results revealed that LPA treatment upregulates several TGF-ß1 target genes, including laminin-332 (Ln-332) components (α3, ß3, and γ2 chains). Western blot analysis also showed that LPA increased phosphorylation of Smad2, an event that is carried out by TGF-ß1 interactions. Among the genes upregulated, we further addressed the role of Ln-332. Real-time PCR analysis confirmed the transcriptional upregulation of all α3, ß3, and γ2 chains of Ln-332 by LPA, corresponding to the protein level increases revealed by western blot. Further, the addition of anti-Ln-332 antibody prevented LPA-treated A431 colonies from dispersing. Taken together, our results suggest that LPA-induced Ln-332 plays a significant role in migration of individual cells from A431 colonies.

Cadherin-bound beta-catenin feeds into the Wnt pathway upon adherens junctions dissociation: evidence for an intersection between beta-catenin pools.

Beta-catenin is an essential component of two cellular systems: cadherin-based adherens junctions (AJ) and the Wnt signaling pathway. A functional or physical connection between these beta-catenin pools has been suggested in previous studies, but not conclusively demonstrated to date. To further examine this intersection, we treated A431 cell colonies with lysophosphatidic acid (LPA), which forces rapid and synchronized dissociation of AJ. A combination of immunostaining, time-lapse microscopy using photoactivatable-GFP-tagged beta-catenin, and image analyses indicate that the cadherin-bound pool of beta-catenin, internalized together with E-cadherin, accumulates at the perinuclear endocytic recycling compartment (ERC) upon AJ dissociation, and can be translocated into the cell nucleus upon Wnt pathway activation. These results suggest that the ERC may be a site of residence for beta-catenin destined to enter the nucleus, and that dissociation of AJ may influence beta-catenin levels in the ERC, effectively affecting beta-catenin substrate levels available downstream for the Wnt pathway. This intersection provides a mechanism for integrating cell-cell adhesion with Wnt signaling and could be critical in developmental and cancer processes that rely on beta-catenin-dependent gene expression.

Nest expansion assay: a cancer systems biology approach to in vitro invasion measurements.

BACKGROUND: Traditional in vitro cell invasion assays focus on measuring one cell parameter at a time and are often less than ideal in terms of reproducibility and quantification. Further, many techniques are not suitable for quantifying the advancing margin of collectively migrating cells, arguably the most important area of activity during tumor invasion. We have developed and applied a highly quantitative, standardized, reproducible Nest Expansion Assay (NEA) to measure cancer cell invasion in vitro, which builds upon established wound-healing techniques. This assay involves creating uniform circular "nests" of cells within a monolayer of cells using a stabilized, silicone-tipped drill press, and quantifying the margin expansion into an overlaid extracellular matrix (ECM)-like component using computer-assisted applications. FINDINGS: The NEA was applied to two human-derived breast cell lines, MCF10A and MCF10A-CA1d, which exhibit opposite degrees of tumorigenicity and invasion in vivo. Assays were performed to incorporate various microenvironmental conditions, in order to test their influence on cell behavior and measures. Two types of computer-driven image analysis were performed using Java's freely available ImageJ software and its FracLac plugin to capture nest expansion and fractal dimension, respectively - which are both taken as indicators of invasiveness. Both analyses confirmed that the NEA is highly reproducible, and that the ECM component is key in defining invasive cell behavior. Interestingly, both analyses also detected significant differences between non-invasive and invasive cell lines, across various microenvironments, and over time. CONCLUSION: The spatial nature of the NEA makes its outcome susceptible to the global influence of many cellular parameters at once (e.g., motility, protease secretion, cell-cell adhesion). We propose the NEA as a mid-throughput technique for screening and simultaneous examination of factors contributing to cancer cell invasion, particularly suitable for parameterizing and validating Cancer Systems Biology approaches such as mathematical modeling.

Dispersal of epithelial cancer cell colonies by lysophosphatidic acid (LPA).

We describe a model system in which cancer cell colonies disperse into single, highly migratory cells in response to lysophosphatidic acid (LPA). Though LPA is known to stimulate chemotaxis and chemokinesis, a colony dispersal effect has not been reported, to our knowledge. Cancer colony dispersal by LPA is comprised of an ordered sequence of events: (1) stimulation of membrane ruffling and formation of lamellipodia, (2) dissolution of adherens junctions, (3) single cell migration in a mesenchymal-like morphology we term "ginkgo-leaf." The net result is dispersal of carcinoma cells from a compact colony. We analyzed these three steps using live-cell imaging and computer-assisted quantification and measured the following parameters: onset of lamellipodia formation, lamellipodia velocity, colony dispersal, trans-epithelial resistance, migrating cell number and speed. Because hepatocyte growth factor (HGF) was described as an epithelial scatter factor, we compared it to LPA in our system and found that HGF has no epithelial colony dispersal properties and that this effect is strictly related to LPA. Given its striking similarity to tumor cell budding observed in patients, we propose that LPA-colony dispersal may provide a cellular mechanism underlying cancer invasion and as such deserves further studies.

Requirement of phospholipase D1 activity in H-RasV12-induced transformation.

The ability of the Ras oncogene to transform normal cells has been well established. One downstream effector of Ras is the lipid hydrolyzing enzyme phospholipase D. Recent evidence has emerged indicating a role for phospholipase D in cell proliferation, membrane trafficking, and migration. To study the potential importance of phospholipase D in the oncogenic ability of Ras, we used Rat-2 fibroblasts with reduced phospholipase D1 activity (Rat-2V25). Here, we show that H-Ras transformation of Rat-2 fibroblasts requires normal phospholipase D1 activity. WT Rat-2 fibroblasts transfected with the H-RasV12 oncogene grew colonies in soft agar and tumors in nude mice. However, Rat-2V25 cells when transfected with the H-RasV12 oncogene did not form colonies in soft agar or produce tumors when xenografted onto nude mice. Interestingly, in the presence of phosphatidic acid, the product of phospholipase D, growth in soft agar and tumor formation was restored. We also observed a dramatic increase in the expression of phospholipase D1 in colorectal tumors when compared with adjacent normal mucosa. Our studies identify phospholipase D1 as a critical downstream mediator of H-Ras-induced tumor formation.

Role of phospholipase D in the activation of protein kinase D by lysophosphatidic acid.

Protein kinase D was auto-phosphorylated at Ser916 and trans-phosphorylated at Ser744/Ser748 in Rat-2 fibroblasts treated with lysophosphatidic acid. Both phosphorylations were inhibited by 1-butanol, which blocks phosphatidic acid formation by phospholipase D. The phosphorylations were also reduced in Rat-2 clones with decreased phospholipase D activity. Platelet-derived growth factor-induced protein kinase D phosphorylation showed a similar requirement for phospholipase D, but that induced by 4beta-phorbol 12 myristate 13-acetate did not. Propranolol an inhibitor of diacylglycerol formation from phosphatidic acid blocked the phosphorylation of protein kinase D, whereas dioctanoylglycerol induced it. The temporal pattern of auto-phosphorylation of protein kinase D closely resembled that of phospholipase D activation and preceded the trans-phosphorylation by protein kinase C. These results suggest that protein kinase D is activated by lysophosphatidic acid through sequential phosphorylation and that diacylglycerol produced by PLD via phosphatidic acid is required for the autophosphorylation that occurs prior to protein kinase C-mediated phosphorylation.

Dimerization of phospholipase d isozymes.

Two mammalian phospholipase D (PLD) isozymes (PLD1 and PLD2) have been reported. In this study, we differentially tagged these isozymes with enhanced green fluorescent protein (EGFP-rPLD1 and EGFP-rPLD2) or Xpress peptide epitope (Xpress-rPLD1 and Xpress-rPLD2) to examine the association between these isozymes. Overexpressed EGFP-rPLD1 coimmunoprecipitated with Xpress-rPLD1 using anti-Xpress antibody. However, the coimmunoprecipitation was independent of the activity of rPLD1. Xpress-rPLD2 also bound to EGFP-rPLD1 although the binding was less efficient than observed with Xpress-rPLD1. The association between rPLD2 and rPLD1 was confirmed by coimmunoprecipitation of EGFP-rPLD2 with Xpress-rPLD1. EGFP-rPLD2 also bound to Xpress-rPLD2 as shown by coimmunoprecipitation. Immunofluorescence staining of COS-7 cells coexpressing EGFP-rPLDs and Xpress-rPLDs showed that the PLD isozymes colocalized in the perinuclear and plasma membrane regions, suggesting that they could associate in a cellular setting. These results suggest that rPLD1 and rPLD2 can exist as homodimers and can form heterodimers.