Thermal sensitisation by lonidamine of human melanoma cells grown at low extracellular pH.

PURPOSE: This study tested the ability of lonidamine (LND), a clinically applicable inhibitor of monocarboxylate transporters (MCT), to thermally sensitise human melanoma cells cultured at a tumour-like extracellular pH (pHe) 6.7. MATERIALS AND METHODS: Human melanoma DB-1 cells cultured at pHe 6.7 and pHe 7.3 were exposed to 150 µM LND for 3 h, beginning 1 h prior to heating at 42 °C (2 h). Intracellular pH (pHi) was determined using 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) and whole spectrum analysis. Levels of heat shock proteins (HSPs) were determined by immunoblot analysis. Cell survival was determined by colony formation. RESULTS: Treatment with LND at pHe 6.7 reduced pHi to 6.30 ± 0.21, reduced thermal induction of HSPs, and sensitised cells growing at pHe 6.7 to 42 °C. When LND was combined with an acute acidification from pHe 6.7 to pHe 6.5, pHi was reduced to 6.09 ± 0.26, and additional sensitisation was observed. LND had negligible effects on cells cultured at pH 7.3. CONCLUSIONS: The results show that LND can reduce pHi in human melanoma cells cultured at a tumour-like low pHe so that the 42 °C induction of HSPs are abrogated and the cells are sensitised to thermal therapy. Cells cultured at a normal tissue-like pHe 7.3 were not sensitised to 42 °C by LND. These findings support the strategy that human melanoma cells growing in an acidic environment can be sensitised to thermal therapy in vivo by exposure to an MCT inhibitor such as LND.

Teaching new dogs old tricks: membrane biophysical properties in drug delivery and resistance.

"How do drugs cross the plasma membrane?" this may seem like a trivial question. This question is often overlooked to focus primarily on the different complex macro-molecular aspects involved in drug delivery or drug resistance. However, recent studies have highlighted the theme that to be fully understood, more knowledge of the underlying biology of the most complex biological processes involved in the delivery and resistance to drugs is needed. After all, why would a drug interact with a transporter then subsequently be excluded from P-glycoprotein (P-gp) expressing drug resistant cells? What are the determinants of this transition in behavior? Full consideration of the physical biology of drug delivery has allowed a better understanding of the reasons why specific membrane proteins are upregulated or overexpressed in drug resistant cells. This, in turn, allows us to identify new targets for drug chemicals. Better yet, it increases the significance of recents patents and underlines their importance in multi drug resistance.

Proton dynamics in cancer.

Cancer remains a leading cause of death in the world today. Despite decades of research to identify novel therapeutic approaches, durable regressions of metastatic disease are still scanty and survival benefits often negligible. While the current strategy is mostly converging on target-therapies aimed at selectively affecting altered molecular pathways in tumor cells, evidences are in parallel pointing to cell metabolism as a potential Achilles' heel of cancer, to be disrupted for achieving therapeutic benefit. Critical differences in the metabolism of tumor versus normal cells, which include abnormal glycolysis, high lactic acid production, protons accumulation and reversed intra-extracellular pH gradients, make tumor site a hostile microenvironment where only cancer cells can proliferate and survive. Inhibiting these pathways by blocking proton pumps and transporters may deprive cancer cells of a key mechanism of detoxification and thus represent a novel strategy for a pleiotropic and multifaceted suppression of cancer cell growth.Research groups scattered all over the world have recently started to investigate various aspects of proton dynamics in cancer cells with quite encouraging preliminary results. The intent of unifying investigators involved in this research line led to the formation of the "International Society for Proton Dynamics in Cancer" (ISPDC) in January 2010. This is the manifesto of the newly formed society where both basic and clinical investigators are called to foster translational research and stimulate interdisciplinary collaboration for the development of more specific and less toxic therapeutic strategies based on proton dynamics in tumor cell biology.

Proton transport inhibitors as potentially selective anticancer drugs.

Different research groups have recently described a proton [H(+)]-related mechanism underlying the initiation and progression of the neoplastic process in which all cancer cells and tissues, regardless of their origin and genetic background, have a pivotal energetic and homeostatic disturbance of their metabolism that is completely different from all normal tissues: an aberrant regulation of hydrogen ion dynamics leading to a reversal of the pH gradient in cancer cells and tissues (pH(i) to pH(e)) as compared to normal tissue pH gradients. This basic specific abnormality of the relationship between the intracellular and the extracellular proton dynamics, a phenomenon that is increasingly considered to be one of the most differential hallmarks of cancer, has led to the formation of a unifying thermodynamic view of cancer research that embraces cancer fields from etiopathogenesis, cancer cell metabolism, multiple drug resistance (MDR), neovascularization and the metastatatic process to selective apoptosis, cancer chemotherapy and even the spontaneous regression of cancer (SRC). This reversed proton gradient is driven by a series of proton export mechanisms that underlie the initiation and progression of the neoplastic process. This means that therapeutic targeting of the transporters that are active in cancer cells could be selective for malignancy and is likely to open new pathways towards the development of more effective and less toxic therapeutic measures for all malignant diseases. Here we review the transporters involved in driving the reversed proton gradient and their specific inhibitors.

Targeting lactate-fueled respiration selectively kills hypoxic tumor cells in mice.

Tumors contain oxygenated and hypoxic regions, so the tumor cell population is heterogeneous. Hypoxic tumor cells primarily use glucose for glycolytic energy production and release lactic acid, creating a lactate gradient that mirrors the oxygen gradient in the tumor. By contrast, oxygenated tumor cells have been thought to primarily use glucose for oxidative energy production. Although lactate is generally considered a waste product, we now show that it is a prominent substrate that fuels the oxidative metabolism of oxygenated tumor cells. There is therefore a symbiosis in which glycolytic and oxidative tumor cells mutually regulate their access to energy metabolites. We identified monocarboxylate transporter 1 (MCT1) as the prominent path for lactate uptake by a human cervix squamous carcinoma cell line that preferentially utilized lactate for oxidative metabolism. Inhibiting MCT1 with alpha-cyano-4-hydroxycinnamate (CHC) or siRNA in these cells induced a switch from lactate-fueled respiration to glycolysis. A similar switch from lactate-fueled respiration to glycolysis by oxygenated tumor cells in both a mouse model of lung carcinoma and xenotransplanted human colorectal adenocarcinoma cells was observed after administration of CHC. This retarded tumor growth, as the hypoxic/glycolytic tumor cells died from glucose starvation, and rendered the remaining cells sensitive to irradiation. As MCT1 was found to be expressed by an array of primary human tumors, we suggest that MCT1 inhibition has clinical antitumor potential.

Angiostatin-like activity of a monoclonal antibody to the catalytic subunit of F1F0 ATP synthase.

The antiangiogenic protein angiostatin inhibits ATP synthase on the endothelial cell surface, blocking cellular proliferation. To examine the specificity of this interaction, we generated monoclonal antibodies (mAb) directed against ATP synthase. mAb directed against the beta-catalytic subunit of ATP synthase (MAb3D5AB1) inhibits the activity of the F(1) domain of ATP synthase and recognizes the catalytic beta-subunit of ATP synthase. We located the antibody recognition site of MAb3D5AB1 in domains containing the active site of the beta-subunit. MAb3D5AB1 also binds to purified Escherichia coli F(1) with an affinity 25-fold higher than the affinity of angiostatin for this protein. MAb3D5AB1 inhibits the hydrolytic activity of F(1) ATP synthase at lower concentrations than angiostatin. Like angiostatin, MAb3D5AB1 inhibits ATP generation by ATP synthase on the endothelial cell surface in acidic conditions, the typical tumor microenvironment where cell surface ATP synthase exhibits greater activity. MAb3D5AB1 disrupts tube formation and decreases intracellular pH in endothelial cells exposed to low extracellular pH. Neither angiostatin nor MAb3D5AB1 showed an antiangiogenic effect in the corneal neovascularization assay; however, both were effective in the low-pH environment of the chicken chorioallantoic membrane assay. Thus, MAb3D5AB1 shows angiostatin-like properties superior to angiostatin and may be exploited in cancer chemotherapy.

The H+-linked monocarboxylate transporter (MCT1/SLC16A1): a potential therapeutic target for high-risk neuroblastoma.

Neuroblastomas produce high amounts of lactic acid and upregulate the H(+)-linked monocarboxylate transporter isoform 1 (MCT1/SLC16A1). We found elevated MCT1 mRNA levels in fresh neuroblastoma biopsy samples that correlated positively with risk of fatal disease and amplification of the "proto-oncogenic" transcription factor MYCN. We further investigated MCT as a potential therapeutic target in vitro. The neuroblastoma cell lines evaluated were Sk-N-SH, CHP134, IMR32, and NGP. All lines exhibited decreased intracellular pH at low tumor-like extracellular pH. Lonidamine or exogenous lactate further lowered intracellular pH. Immediate early lowering of intracellular pH with lonidamine or lactate at extracellular pH 6.5 correlated positively with diminished cell viability within 48 h. These findings indicate that MCT1 is a potential therapeutic target and that neuroblastoma therapy may be enhanced by therapeutic strategies to inhibit or overwhelm MCT. Additional experiments indicated that the mechanism of cell death by lonidamine or exogenous lactate is similar to that obtained using alpha-cyano-4-OH-cinnamate, a well established MCT inhibitor. Because lactate production is also high in melanoma and many other tumor types, MCT inhibitors may have broad application in cancer treatment. Such treatment would have selectivity by virtue of the acidic milieu surrounding tumors, because MCT is increasingly active as extracellular pH decreases below 7.0 and lactic acid production increases.

Camptothecin analogs with enhanced activity against human breast cancer cells. II. Impact of the tumor pH gradient.

Human breast tumors often exist in an acidic and hypoxic microenvironment, which can promote resistance to radiation and chemotherapies. A tumor-selective pH gradient arises in these tumors which favors uptake and retention of drugs like camptothecin that are weak acids. We evaluated the effect of alkyl substitutions at the 7 position in seven CPTs with varying groups at the 10 position on modulation by acidic extracellular pH in three human breast cancer cell lines. Growth inhibition was assessed by propidium iodide staining of nucleic acids in human breast cancer cells cultured at either extracellular pH 6.8 or 7.4 that were (1) hormone-sensitive (MCF-7/wt), (2) hormone insensitive (MDA-MB-231), or (3) alkylator-resistant (MCF-7/4-hc). Over 10-fold pH modulation was observed in 7-halomethyl analogs of methylenedioxy-CPT and in 7-alkyl analogs of 10-amino-CPT. Of 39 analogs tested, the overall pattern of activity across breast tumor cell lines was similar with some notable exceptions. For example, 7-propyl-10-amino-CPT was modulated 16- to 20-fold by acidic extracellular pH in the MCF-7 cell lines, but only 6-fold in MDA-MB-231 cells. One mechanism that can contribute to pH modulation is enhanced cellular drug uptake and retention. In MCF-7/wt cells, uptake of 10-amino-CPT increased 4-fold, while retention increased over 10-fold at acidic extracellular pH. In addition, gene expression analysis of MCF-7/wt cells indicated that expression of a number of genes changed under acidic culture conditions, including down-regulation of the CPT efflux protein pump breast cancer resistance protein (BCRP). Interestingly, expression of topoisomerase I, the molecular target of CPT, was not affected by acidic growth conditions. These results highlight the importance of maintaining key features of tumor physiology in cell culture models used to study cancer biology and to discover and develop new anticancer drugs. While several substitutions at the 7 and 10 positions enhance potency, 7-halomethyl and 10-amino CPT analogs show selective activity at the acidic pH common to the microenvironment of most solid tumors.

Angiostatin's molecular mechanism: aspects of specificity and regulation elucidated.

Tumor growth requires the development of new vessels that sprout from pre-existing normal vessels in a process known as "angiogenesis" [Folkman (1971) N Engl J Med 285:1182-1186]. These new vessels arise from local capillaries, arteries, and veins in response to the release of soluble growth factors from the tumor mass, enabling these tumors to grow beyond the diffusion-limited size of approximately 2 mm diameter. Angiostatin, a naturally occurring inhibitor of angiogenesis, was discovered based on its ability to block tumor growth in vivo by inhibiting the formation of new tumor blood vessels [O'Reilly et al. (1994a) Cold Spring Harb Symp Quant Biol 59:471-482]. Angiostatin is a proteolytically derived internal fragment of plasminogen and may contain various members of the five plasminogen "kringle" domains, depending on the exact sites of proteolysis. Different forms of angiostatin have measurably different activities, suggesting that much remains to be elucidated about angiostatin biology. A number of groups have sought to identify the native cell surface binding site(s) for angiostatin, resulting in at least five different binding sites proposed for angiostatin on the surface of endothelial cells (EC). This review will consider the data supporting all of the various reported angiostatin binding sites and will focus particular attention on the angiostatin binding protein identified by our group: F(1)F(O) ATP synthase. There have been several developments in the quest to elucidate the mechanism of action of angiostatin and the regulation of its receptor. The purpose of this review is to describe the highlights of research on the mechanism of action of angiostatin, its' interaction with ATP synthase on the EC surface, modulators of its activity, and issues that should be explored in future research related to angiostatin and other anti-angiogenic agents.

Ectopic localization of mitochondrial ATP synthase: a target for anti-angiogenesis intervention?

A receptor for angiostatin was identified on the surface of endothelial cells as F(1)-F(0) ATP synthase (Moser et al., 1999). Proc. Natl. Acad. Sci. U.S.A. 96, 2811-2816. This ectopic ATP synthase catalyzes ATP synthesis and is inhibited by angiostatin over a wide pH range. Endothelial cells grown at normal pH suffer no ill effects from this angiostatin-mediated inhibition of ATP synthase, whereas endothelial cells grown at low, tumor-like extracellular pH cannot maintain a normal intracellular pH and die. Angiostatin inhibits both ATP synthesis and ATP hydrolysis (Moser et al., 2001) and interferes with intracellular pH regulation (Wahl and Grant, 2002; Wahl et al., 2002). Although angiostatin administered intravenously is cleared from the circulation in a matter of minutes, angiostatin-mimetics that are more stable have potential for clinical application. An angiostatin-mimetic activity has recently been observed using a polyclonal antibody against the beta catalytic subunit of ATP synthase. In order to explore the mechanism of action of angiostatin and its mimetics, further work needs to be done to evaluate clinical applicability, specificity, and contraindications for this class of therapeutics.

An Inhibitor of the F1 subunit of ATP synthase (IF1) modulates the activity of angiostatin on the endothelial cell surface.

Angiostatin binds to endothelial cell (EC) surface F(1)-F(0) ATP synthase, leading to inhibition of EC migration and proliferation during tumor angiogenesis. This has led to a search for angiostatin mimetics specific for this enzyme. A naturally occurring protein that binds to the F1 subunit of ATP synthase and blocks ATP hydrolysis in mitochondria is inhibitor of F1 (IF1). The present study explores the effect of IF1 on cell surface ATP synthase. IF1 protein bound to purified F(1) ATP synthase and inhibited F(1)-dependent ATP hydrolysis consistent with its reported activity in studies of mitochondria. Although exogenous IF1 did not inhibit ATP production on the surface of EC, it did conserve ATP on the cell surface, particularly at low extracellular pH. IF1 inhibited ATP hydrolysis but not ATP synthesis, in contrast to angiostatin, which inhibited both. In cell-based assays used to model angiogenesis in vitro, IF1 did not inhibit EC differentiation to form tubes and only slightly inhibited cell proliferation compared with angiostatin. From these data, we conclude that inhibition of ATP synthesis is necessary for an anti-angiogenic outcome in cell-based assays. We propose that IF1 is not an angiostatin mimetic, but it can serve a protective role for EC in the tumor microenvironment. This protection may be overridden in a concentration-dependent manner by angiostatin. In support of this hypothesis, we demonstrate that angiostatin blocks IF1 binding to ATP synthase and abolishes its ability to conserve ATP. These data suggest that there is a relationship between the binding sites of IF1 and angiostatin on ATP synthase and that IF1 could be employed to modulate angiogenesis.

Angiostatin and anti-angiogenic therapy in human disease.

Many diseases have abnormal quality and/or quantity of vascularization as a characteristic feature. Cancer cells elicit the growth of new capillaries during neovascularization in a process termed angiogenesis. In diabetics, pathologic angiogenesis in various tissues is a clinical feature of many common complications. Therefore, the diabetic cancer patient warrants special consideration and extra care in the design of anti-angiogenic treatments without adverse side effects. Some treatment regimens that look promising in vitro, in animal models, or in early clinical trials may be contra-indicated for diabetics. This chapter will review the common complications of diabetes, with emphasis on the angiogenic pathology. Recent research related to the mechanism of action and basis for specificity of the anti-angiogenic peptide, angiostatin, will be the focus. The aim is to shed light on areas in which more research is needed with respect to angiostatin and other anti-angiogenic agents and the microenvironmental conditions that affect their activities, in order to develop improved therapeutic strategies for diabetic cancer patients.

Intracellular acidification abrogates the heat shock response and compromises survival of human melanoma cells.

This study tests the hypothesis that lowering intracellular pH (pHi) in melanoma cells grown at low extracellular pH (pHe) selectively abrogates 42 degrees C-induced heat shock protein (HSP) expression and reduces survival. Cells were acidified by a combination of a 0.2-pH-unit decrease in pHe coupled with the lactate/H+ transport inhibitor alpha-cyano-4-hydroxy-cinnamic acid (CNCn). A mild acute extracellular acidification was used to mimic the acute extracellular acidification observed in tumors that can be induced in vivo by oral glucose administration. CNCn blocks the activity of H(+)-linked monocarboxylate transporters (MCTs), particularly MCT isoform 1 (MCT-1). This transporter removes lactic acid from cells and has a high activity in DB-1 melanoma cells grown at low pHe. The effect of extracellular acidification combined with CNCn on pHi was measured in cells grown at pHe 6.7 and pHe 7.3. Cells grown at pHe 6.7 serve as an in vitro model for cells in an acidic tumor microenvironment. When cells were grown at pHe 6.7 and incubated with CNCn at pHe 6.5, the pHi decreased from 6.9 to below 6.5, and the 42 degrees C induction of HSP70 and HSP27 was blocked. The abrogation of HSP induction correlated positively with decreased clonogenic survival. In contrast, when cells growing at pHe 7.3 were acidified by a 0.2-pH unit to pHe 7.1, the inhibitor had less effect on pHi, which remained above 7.0. Under these conditions, the 42 degrees C-induction of HSPs was not inhibited, and cytotoxicity was not enhanced. These results indicate that a significant decrease in the pHi of melanoma cells can selectively sensitize the cells to 42 degrees C hyperthermia, possibly through the inhibition of HSP expression. This strategy could result in a therapeutic gain, because normal tissues, existing at a pHe above 7.0, would not be sensitized.

Tumor oxygenation and acidification are increased in melanoma xenografts after exposure to hyperglycemia and meta-iodo-benzylguanidine.

Tumor oxygen tension and extracellular pH (pH(e)) are physiological parameters that can be manipulated to improve current cancer therapies. Many human tumors consist of cells that are chronically exposed to low pH(e). Exposure of tumor cells in culture to glucose decreases oxygen consumption (oxygen sparing or Crabtree effect), and while this effect is absent in low pH-adapted tumor cells, it can be restored by combining the respiratory inhibitor meta-iodo-benzylguanidine (MIBG) with glucose (Burd et al., Cancer Res. 61, 5630-5635, 2001). The effects of hyperglycemia and MIBG on tumor oxygen tension and on pH(e) were investigated in human melanoma xenografts in SCID mice. An oral gavage of 1 M glucose (2 g/kg) increased the average blood glucose concentration from <140 mg/dl to approximately 400 mg/dl. Although tumor pH(e) decreased from pH 6.7 to pH 6.5 (P < 0.01) after about 60 min, no change in tumor oxygen tension was observed. However, when oral glucose and MIBG (15 mg/kg) were administered together, oxygen tension increased from 2.8 mmHg to approximately 17 mmHg, and tumor pH(e) decreased from pH 6.7 to pH 6.3 (P < 0.01) after about 115 min. In conclusion, administration of glucose together with MIBG increases tumor oxygen tension and also increases the magnitude and duration of acidification. Hyperglycemia plus MIBG has the potential to improve response to radiation therapy as well as to hyperthermia and some chemotherapies.

Regulation of intracellular pH in human melanoma: potential therapeutic implications.

Melanoma cells in vivo maintain intracellular pH (pHi) in a viable range despite an extracellular tumor pH (pHe) that is typically below 7.0. In general, three families of transporters are capable of removing metabolic protons, but the specific transporters responsible for the maintenance of pHi at low pHe in melanomas have not been identified. Although the transporters exist in most cells, an inhibitor would be predicted to have selectivity for cells located in an acidic tumor bed because cells in that environment would be expected to have transporters chronically activated. In this report, the levels and extent of expression of the Na+/H+ exchanger (NHE-1) and two of the H+-linked monocarboxylate transporters (MCTs) were evaluated in three melanoma cell lines. The effects of inhibitors of each transporter were tested at an extracellular pH (pHe) of 7.3, 6.7, or 6.5 in melanoma cells that were grown at pHe 7.3 or 6.7. The activity of MCT isoform 1 (MCT-1) was up-regulated in three melanoma cell lines at low pHe, but that of NHE-1 was not. Furthermore, NHE-1 activity was lower in the melanomas than in other normal and malignant cell lines that were tested. Reverse transcription-PCR using primers specific for MCT-1, MCT-4, and NHE-1 showed that expression of none of these transporters was reproducibly up-regulated at the level of transcription when cells were grown at pHe 6.7 instead of pHe 7.3. Ex vivo experiments using DB-1 human melanoma xenografts grown in severe combined immunodeficient mice found that MCT-1 and not NHE-1 was a major determinant of DB-1 tumor cell pHi. Taken together, the data indicate that MCTs are major determinants of pH regulation in melanoma. In contrast, keratinocytes and melanocytes under low pHe conditions relied on NHE-1. Inhibitors of MCTs thus have great potential to improve the effectiveness of chemotherapeutic drugs that work best at low pHi, such as alkylating agents and platinum-containing compounds, and they should be selective for cells in an acidic tumor bed. In most tissues, it is proposed that the NHE-1 could compensate for an inhibited MCT to prevent acidification, but in melanoma cells this did not occur. Therefore, MCT inhibitors may be particularly effective against malignant melanoma.

Angiostatin induces intracellular acidosis and anoikis in endothelial cells at a tumor-like low pH.

Angiostatin inhibits angiogenesis by binding to endothelial cells (ECs) lining the vasculature of growing tumors. These cells are in a dynamic state during angiogenesis and are thus not firmly attached to the extracellular matrix. This makes them more vulnerable to anoikis, a process resulting in cell death initiated by or promoted by loss of attachment. Another potential source of EC vulnerability during tumor angiogenesis is that tumor extracellular pH is typically lower than in normal tissues. This presents an additional challenge to ECs in terms of maintaining ionic homeostasis. We report here that the lethality of angiostatin is significantly enhanced both by reduced matrix attachment during exposure and lowered extracellular pH (pH(e)). Another effect of angiostatin at reduced pH(e) is a decreased intracellular pH (pH(i)). These effects were observed in three model systems: aortic ring sprouts, ECs during tube formation, and ECs in a scratch/migration assay. In these three dynamic assays, angiostatin-induced cell death and intracellular acidification were clearly seen when pH(e) was reduced to 6.7. The intracellular acidification was far greater than that induced by pH(e) reduction alone. In contrast, the effect of angiostatin on pH(i) and on viability were not observed in a subconfluent monolayer in which the cells were allowed to attach to substrate for 48 h prior to exposure to angiostatin. These data suggest that low pH(e) and reduced adhesion to matrix play a role in the specificity of angiostatin for tumor neovasculature in contrast to wound healing and other normal angiogenic processes. The results also implicate roles for both pH(e) and pH(i) regulation in the mechanism of angiostatin action.

The mechanism of action of angiostatin: can you teach an old dog new tricks?

What is angiostatin? In 1994, Folkman and colleagues published a landmark paper describing anti-tumor effects in mice with a purified fragment of plasminogen they named angiostatin (1). Although many papers have been published describing activities of cryptic polypeptides derived from plasminogen fragments, this was the first report which associated plasminogen kringles 1-4 as a suppressor of metastasis development. This review will describe what is known about the mechanism of action of angiostatin from the current literature.

Abnormal acidification of melanoma cells induces tyrosinase retention in the early secretory pathway.

In tyrosinase-positive amelanotic melanoma cells, inactive tyrosinase accumulates in the endoplasmic reticulum. Based on studies described here, we propose that aberrant vacuolar proton ATPase (V-ATPase)-mediated proton transport in melanoma cells disrupts tyrosinase trafficking through the secretory pathway. Amelanotic but not melanotic melanoma cells or normal melanocytes display elevated proton export as observed by the acidification of the extracellular medium and their ability to maintain neutral intracellular pH. Tyrosinase activity and transit through the Golgi were restored by either maintaining the melanoma cells in alkaline medium (pH 7.4-7.7) or by restricting glucose uptake. The translocation of tyrosinase out of the endoplasmic reticulum and the induction of cell pigmentation in the presence of the ionophore monensin or the specific V-ATPase inhibitors concanamycin A and bafilomycin A1 supported a role for V-ATPases in this process. Because it was previously shown that V-ATPase activity is increased in solid tumors in response to an acidified environment, the appearance of hypopigmented cells in tyrosinase-positive melanoma tumors may indicate the onset of enhanced glycolysis and extracellular acidification, conditions known to favor metastatic spread and resistance to weak base chemotherapeutic drugs.