Cell cycle regulation by environmental pH

Tumor acidic environment directs nanoparticle impacts on cancer cells

Despite impressive progress in nanotechnology-based cancer therapy being made by in vitro research, few nanoparticles (NPs) have been translated into clinical trials. The wide gap between in vitro results and nanomedicine's clinical translation might be partly due to acidic microenvironment of cancer cells being ignored in in vitro studies. To check this hypothesis, we studied the biological impacts of two different structures of NPs on cancer cells (MDA-MB 231) at acidic (pH: 6.5) low (pH: 7) and physiological pH (pH: 7.4). We uncovered that a slight change in the pH of the cancer cell microenvironment affects the cellular uptake efficacy and toxicity mechanism of nanographene sheets and SPION@silica nanospheres. Both nanostructures exerted more substantial toxic impacts (e. g. apoptosis, necrosis, membrane disruption, and oxidative stress induction) against cancer cells at physiological pH compared to acidic niche. They also differently slowed or arrested phases of the cell cycle at different pH (S and G2/M at normal pH while G0/G1 at acidic/low pH). More specifically, cancer cells expressed higher levels of interleukins involved in cancer cell resistance at acidic pH than those incubated at physiological pH. This study revealed that a slight change in extracellular pH of cancer cells could strongly affect the therapeutic/toxic impact of nanomaterials and therefore, it should be considered in the future cancer nanomedicine research.

Maturation conditions, post-ovulatory age, medium pH, and ER stress affect [Ca2+]i oscillation patterns in mouse oocytes

Insufficiency of oocyte activation impairs the subsequent embryo development in assisted reproductive technology (ART). Intracellular Ca²⁺ concentration ([Ca²⁺]i) oscillations switch the oocytes to resume the second meiosis and initiate embryonic development. However, the [Ca²⁺]i oscillation patterns in oocytes are poorly characterized. In this study, we investigated the effects of various factors, such as the oocytes age, pH, cumulus cells, in vitro or in vivo maturation, and ER stress on [Ca²⁺]i oscillation patterns and pronuclear formation after parthenogenetic activation of mouse oocytes. Our results showed that the oocytes released to the oviduct at 17 h post-human chorionic gonadotrophin (hCG) displayed a significantly stronger [Ca²⁺]i oscillation, including higher frequency, shorter cycle, and higher peak, compared with oocytes collected at earlier or later time points. [Ca²⁺]i oscillations in acidic conditions (pH 6.4 and 6.6) were significantly weaker than those in neutral and mildly alkaline conditions (pH from 6.8 to 7.6). In vitro-matured oocytes showed reduced frequency and peak of [Ca²⁺]i oscillations compared with those matured in vivo. In vitro-matured oocytes from the cumulus-oocyte complexes (COCs) showed a significantly higher frequency, shorter cycle, and higher peak compared with the denuded oocytes (DOs). Finally, endoplasmic reticulum stress (ER stress) severely affected the parameters of [Ca²⁺]i oscillations, including elongated cycles and lower frequency. The pronuclear (PN) rate of oocytes after parthenogenetic activation was correlated with [Ca²⁺]i oscillation pattern, decreasing with oocyte aging, cumulus removal, acidic pH, and increasing ER stress. These results provide fundamental but critical information for the mechanism of how these factors affect oocyte activation.

Synchronized oscillations in growing cell populations are explained by demographic noise

Understanding synchrony in growing populations is important for applications as diverse as epidemiology and cancer treatment. Recent experiments employing fluorescent reporters in melanoma cell lines have uncovered growing subpopulations exhibiting sustained oscillations, with nearby cells appearing to synchronize their cycles. In this study, we demonstrate that the behavior observed is consistent with long-lasting transient phenomenon initiated and amplified by the finite-sample effects and demographic noise. We present a novel mathematical analysis of a multistage model of cell growth, which accurately reproduces the synchronized oscillations. As part of the analysis, we elucidate the transient and asymptotic phases of the dynamics and derive an analytical formula to quantify the effect of demographic noise in the appearance of the oscillations. The implications of these findings are broad, such as providing insight into experimental protocols that are used to study the growth of asynchronous populations and, in particular, those investigations relating to anticancer drug discovery.

Palmitic Acid Methyl Ester Induces G2/M Arrest in Human Bone Marrow-Derived Mesenchymal Stem Cells via the p53/p21 Pathway

Bone marrow-derived mesenchymal cells (BM-MSCs) are able to differentiate into adipocytes, which can secrete adipokines to affect BM-MSC proliferation and differentiation. Recent evidences indicated that adipocytes can secrete fatty acid metabolites, such as palmitic acid methyl ester (PAME), which is able to cause vasorelaxation and exerts anti-inflammatory effects. However, effects of PAME on BM-MSC proliferation remain unclear. The aim of this study was to investigate the effect of PAME on human BM-MSC (hBM-MSC) proliferation and its underlying molecular mechanisms. hBM-MSCs were treated with PAME for 48 h and then subjected to various analyses. The results from the present study show that PAME significantly reduced the levels of G₂/M phase regulatory proteins, cyclin-dependent kinase 1 (Cdk1), and cyclin B1 and inhibited proliferation in hBM-MSCs. Moreover, the level of Mdm2 protein decreased, while the levels of p21 and p53 protein increased in the PAME-treated hBM-MSCs. However, PAME treatment did not significantly affect apoptosis/necrosis, ROS generation, and the level of Cdc25C protein. PAME also induced intracellular acidosis and increased intracellular Ca²⁺ levels. Cotreatment with PAME and Na⁺/H⁺ exchanger inhibitors together further reduced the intracellular pH but did not affect the PAME-induced decreases of cell proliferation and increases of the cell population at the G₂/M phase. Cotreatment with PAME and a calcium chelator together inhibited the PAME-increased intracellular Ca²⁺ levels but did not affect the PAME-induced cell proliferation inhibition and G₂/M cell cycle arrest. Moreover, the half-life of p53 protein was prolonged in the PAME-treated hBM-MSCs. Taken together, these results suggest that PAME induced p53 stabilization, which in turn increased the levels of p53/p21 proteins and decreased the levels of Cdk1/cyclin B1 proteins, thereby preventing the activation of Cdk1, and eventually caused cell cycle arrest at the G₂/M phase. The findings from the present study might help get insight into the physiological roles of PAME in regulating hBM-MSC proliferation.

Mathematical models incorporating a multi-stage cell cycle replicate normally-hidden inherent synchronization in cell proliferation

We present a suite of experimental data showing that cell proliferation assays, prepared using standard methods thought to produce asynchronous cell populations, persistently exhibit inherent synchronization. Our experiments use fluorescent cell cycle indicators to reveal the normally hidden cell synchronization, by highlighting oscillatory subpopulations within the total cell population. These oscillatory subpopulations would never be observed without these cell cycle indicators. On the other hand, our experimental data show that the total cell population appears to grow exponentially, as in an asynchronous population. We reconcile these seemingly inconsistent observations by employing a multi-stage mathematical model of cell proliferation that can replicate the oscillatory subpopulations. Our study has important implications for understanding and improving experimental reproducibility. In particular, inherent synchronization may affect the experimental reproducibility of studies aiming to investigate cell cycle-dependent mechanisms, including changes in migration and drug response.

Native-mimicking in vitro microenvironment: an elusive and seductive future for tumor modeling and tissue engineering

Human connective tissues are complex physiological microenvironments favorable for optimal survival, function, growth, proliferation, differentiation, migration, and death of tissue cells. Mimicking native tissue microenvironment using various three-dimensional (3D) tissue culture systems in vitro has been explored for decades, with great advances being achieved recently at material, design and application levels. These achievements are based on improved understandings about the functionalities of various tissue cells, the biocompatibility and biodegradability of scaffolding materials, the biologically functional factors within native tissues, and the pathophysiological conditions of native tissue microenvironments. Here we discuss these continuously evolving physical aspects of tissue microenvironment important for human disease modeling, with a focus on tumors, as well as for tissue repair and regeneration. The combined information about human tissue spaces reflects the necessities of considerations when configuring spatial microenvironments in vitro with native fidelity to culture cells and regenerate tissues that are beyond the formats of 2D and 3D cultures. It is important to associate tissue-specific cells with specific tissues and microenvironments therein for a better understanding of human biology and disease conditions and for the development of novel approaches to treat human diseases.

Immobilized pH in culture reveals an optimal condition for somatic cell reprogramming and differentiation of pluripotent stem cells

Aim

One of the parameters that greatly affects homeostasis in the body is the pH. Regarding reproductive biology, germ cells, such as oocytes or sperm, are exposed to severe changes in pH, resulting in dramatic changes in their characteristics. To date, the effect of the pH has not been investigated regarding the reprogramming of somatic cells and the maintenance and differentiation of pluripotent stem cells.

Methods

In order to investigate the effects of the pH on cell culture, the methods to produce induced pluripotent stem cells (iPSCs) and to differentiate embryonic stem cells (ESCs) into mesendoderm and neuroectoderm were performed at each medium pH from 6.6 to 7.8. Using the cells of the Oct4‐GFP (green fluorescent protein) carrying mouse, the effects of pH changes were examined on the timing and colony formation at cell reprogramming and on the cell morphology and direction of the differentiation of the ESCs.

Results

The colony formation rate and timing of the reprogramming of the somatic cells varied depending on the pH of the culture medium. In addition, mesendodermal differentiation of the mouse ESCs was enhanced at the high pH level of 7.8.

Conclusion

These results suggest that the pH in the culture medium is one of the key factors in the induction of the reprogramming of somatic cells and in the differentiation of pluripotent stem cells.

Mesenchymal stem cell proliferation and mineralization but not osteogenic differentiation are strongly affected by extracellular pH

Osteomyelitis is a serious complication in oral and maxillofacial surgery affecting bone healing. Bone remodeling is not only controlled by cellular components but also by ionic and molecular composition of the extracellular fluids in which calcium phosphate salts are precipitated in a pH dependent manner.

OBJECTIVE

To determine the effect of pH on self-renewal, osteogenic differentiation and matrix mineralization of mesenchymal stem cells (MSCs).

METHODS

We selected three different pH values; acidic (6.3, 6.7), physiological (7.0-8.0) and severe alkaline (8.5). MSCs were cultured at different pH ranges, cell viability measured by WST-1, apoptosis detected by JC-1, senescence was analyzed by β-galactosidase whereas mineralization was detected by Alizarin Red and osteogenic differentiation analyzed by Real-time PCR.

RESULTS

Self-renewal was affected by pH as well as matrix mineralization in which pH other than physiologic inhibited the deposition of extracellular matrix but did not affect MSCs differentiation as osteoblast markers were upregulated. The expression of osteocalcin and alkaline phosphatase activity was upregulated whereas osteopontin was downregulated under acidic pH.

CONCLUSION

pH affected MSCs self-renewal and mineralization without influencing osteogenic differentiation. Thus, future therapies, based on shifting acid-base balance toward the alkaline direction might be beneficial for prevention or treatment of osteomyelitis.

Effects of acute hypoxia/acidosis on intracellular pH in differentiating neural progenitor cells

The response of differentiating mouse neural progenitor cells, migrating out from neurospheres, to conditions simulating ischemia (hypoxia and extracellular or intracellular acidosis) was studied. We show here, by using BCECF and single cell imaging to monitor intracellular pH (pH(i)), that two main populations can be distinguished by exposing migrating neural progenitor cells to low extracellular pH or by performing an acidifying ammonium prepulse. The cells dominating at the periphery of the neurosphere culture, which were positive for neuron specific markers MAP-2, calbindin and NeuN had lower initial resting pH(i) and could also easily be further acidified by lowering the extracellular pH. Moreover, in this population, a more profound acidification was seen when the cells were acidified using the ammonium prepulse technique. However, when the cell population was exposed to depolarizing potassium concentrations no alterations in pH(i) took place in this population. In contrast, depolarization caused an increase in pH(i) (by 0.5 pH units) in the cell population closer to the neurosphere body, which region was positive for the radial cell marker (GLAST). This cell population, having higher resting pH(i) (pH 6.9-7.1) also responded to acute hypoxia. During hypoxic treatment the resting pH(i) decreased by 0.1 pH units and recovered rapidly after reoxygenation. Our results show that migrating neural progenitor cells are highly sensitive to extracellular acidosis and that irreversible damage becomes evident at pH 6.2. Moreover, our results show that a response to acidosis clearly distinguishes two individual cell populations probably representing neuronal and radial cells.

Synthesis and characterization of pH sensitive carboxySNARF-1 nanoreactors

A rapid response dual wavelength emission pH sensor consisting of carboxySNARF-1 nanoreactors has been synthesized and shown to provide accurate pH measurements even in complex biological media, where the unprotected pH responsive dyes have failed. The carboxySNARF-1 nanoreactor is made of a calcium phosphate shell covering phosphatidylcholine liposomes filled with the dye. Its mean diameter is 150 nm with dynamic light scattering, the shell thickness is 5-7 nm with TEM, and it contains about 10 dyes/particle. The nanoreactor's response time to pH change nearly equals that of the dye in solution. Its pH titration curves at two different wavelengths are equivalent to those of the dye in solution and fluorescence intensity ratio dependent pH analysis is possible using the modified Henderson-Hasselbalch equation. However, the pH dependent fluorescence ratios of the dye in solution in the presence of plasma and albumin are distorted, and application of the Henderson-Hasselbalch equation is not possible. We have found that the distortions may be restored using cSNARF-1 nanoreactors and the pK(a) of the dye in the nanoreactor then equals that in solution. These results suggest that the interference to the dye for the pH analyses with the environmental molecules may be reduced or prohibited by usage of cSNARF-1 nanoreactors.

Effect of ammonium ion and extracellular pH on hybridoma cell metabolism and antibody production

The effects of NH(4)Cl addition on batch hybridoma cell growth at different external pH values (pH(e)) were investigated in a bioreactor at constant pH and dissolved oxygen concentration. In agreement with measurements in flasks, changes in pH(e) over the range 6.8-7.6 had minor effects on growth. Addition of 3 mM NH(4)Cl had little effect on cell growth while 10 mM NH(4)Cl caused a substantial growth inhibition, Measurements of the effects of pH(e) and NH(4)Cl concentration on cell metabolism gave similar results for cells grown in flasks in an incubator and in the bioreactor. As pH(e) decreases, the integral cell yield on glucose increases. There is a correlation between the effects of pH(e) on glycolysis and previous measurements of its effects on intracellular pH (pH(i)). Increases in NH(4)Cl concentration were previously determined to decrease pH(i) and are shown here to decrease the integral cell yield on glucose. At all pH(e) values in the absence of NH(4)Cl, glutamine is depleted at the time the maximum cell density is reached. Both pH(e) decreases and NH(4)Cl concentration increases lead to decreases in the integral cell yield on glutamine. Changes in pH(e) and in the NH(4)Cl concentration that cause growth inhibition have no effect on the specific antibody production rate for cells grown in flasks in an incubator or in the bioreactor. Changes in the NH(4)Cl concentration have no effect on the quality of the antibody produced, to a first level of characterization.

Hypoxia modulates cyclin and cytokine expression and inhibits peripheral mononuclear cell proliferation

Previously, we found that hypoxia can deeply affect the production of cytokines in human peripheral mononuclear cells (PBMC). Here, we demonstrated that the cycle progression of hypoxic PBMC, cultured in the presence or not of a specific T cell activator such as phytohaemagglutinin (PHA), was delayed when compared with aerobic cultures. This delay was accompanied by a decrease of the expression of specific cyclins associated to cell cycle progression phases. Ribonuclease Protection Assay (RPA) studies reveal a decrease in the expression of cyclin A and B in PHA-stimulated PBMC kept for 40 hr under hypoxic condition (2% O(2)), when compared with aerobic cultures (20% O(2)). In concomitance, a decrease of cyclin D2 expression was present after 16 hr of hypoxic treatment. However, the decrease was transient and disappeared after 40 hr of hypoxic treatment. Furthermore, cyclin C expression was not affected by hypoxia. Hypoxia-induced cyclin modulation was accompanied by an increased synthesis of interleukin (IL)-2 and IL-4, analyzed by ELISA. By evaluating these results, it appears that hypoxia induces a growth suppressive state in mitogen-activated PBMC by inhibiting the synthesis of mitotic cyclins A and B. However hypoxic PBMC maintain their viability and capability of producing stimulatory cytokines, after mitogen treatment. This should be important in local hypoxia, usually associated with necrotic areas, in inflammation, and infections, where T lymphocyte capability of producing stimulatory cytokines is desirable.

Hypoxia affects cytokine production and proliferative responses by human peripheral mononuclear cells

We have shown that hypoxia (2% O2 approximately pO2 14 mmHg) as opposed to O2 atmospheric pressure (20.9% O2 approximately pO2 140 mmHg) can deeply affect the production of cytokines in human peripheral mononuclear cells (PBMC) in the presence or absence of a specific T-cell activator such as phytohemagglutinin (PHA). In hypoxia, interleukin (IL)-2, IL-4, and interferon (IFN)-gamma production increased by 110, 70, and 50% over that of controls, respectively, in PHA-stimulated PBMC (P < 0.05). Moreover, in hypoxia, IL-6 production was significantly enhanced in both resting and PHA-stimulated PBMC by 36 and 37%, respectively (P < 0.05). However, in hypoxia, IL-10 production decreased in both resting and stimulated PBMC, being 80 and 67% of controls, respectively (P < 0.05). PBMC proliferation was not significantly affected by hypoxia, although PBMC susceptibility to PHA was about 80% of that of the control (P < 0.05) after 40 hr of treatment, whereas the cycle progression of hypoxic PBMC was delayed. From an evaluation of these results, hypoxia apparently modifies the production of cytokines by PBMC. These results have both theoretical and practical interest because local hypoxia is very common in several conditions, such as inflammation and local ischemia, and is a host-nonspecific defense against infection. Furthermore, these results suggest a differential pattern of cytokine production in vivo in hypoxic tissues.

Increased tubulin acetylation accompanies reversion to stable ploidy in vincristine-resistant CCRF-CEM cells

The T-cell leukemia line CCRF-CEM is unstable with respect to ploidy, whereas a vincristine-resistant subline, CEM/VCR R, maintains a stable pseudodiploid karyotype. Ploidy change in the parental cells requires the involvement of two cell cycle lesions. The first, in mitosis, prevents cell division after S-phase. The second, in G1, allows a cell with 4N DNA content to re-enter S-phase. We examined differences in expression of tubulin, a major component of the mitotic spindle and the cellular target for vincristine, between the two cell lines. Levels of the beta III isotype were decreased and levels of acetylated alpha-tubulin, a marker for microtubule stability, were increased in the CEM/VCR R cells relative to the parental line, which suggests that the CEM/VCR R cells have a more stable mitotic spindle. Both cell lines exhibit some level of constitutive expression of p53 and c-myc. Constitutive expression of and mutant p53 would contribute to the failure of these cells to recognise G1 checkpoints. Therefore, G1 checkpoint failure and the intrinsically less stable mitotic spindle in the CCRF-CEM cells may contribute to the observed ploidy instability. Conversely, the presence of markers of microtubule stability in the CEM/VCR R cells would predispose them to maintain their ploidy.

Effects of hypoxia on the antiproliferative activity of human interferons

We studied the effects of pharmacologically attainable concentrations of interferon-alpha (IFN-alpha) and gamma (IFN-gamma) on the growth of cells incubated under hypoxic conditions (2% O2; approximately 14 mm Hg partial pressure) or exposed to oxygen at atmospheric pressure (21% O2; approximately 147 mm Hg). The cells were from four IFN-sensitive lines: A-549 lung carcinoma and G-361 human melanoma cells grow better under hypoxic conditions, but the growth of Hep-2 laryngeal carcinoma and WISH amnion cells is not affected by the environmental oxygen tension. The antiproliferative effects of the IFN were assessed in terms of cell cloning efficiency and also from the number of cells, relative to controls, measured 1, 2, and 3 days after seeding. Under hypoxic conditions, the cloning efficiency of A-549 and G-361 cells was increased, and they became significantly less responsive to the antiproliferative effect of IFN, and especially of IFN-gamma. No such effects were seen with WISH or Hep-2 cells. Hypoxic conditions are found in the necrotic areas present in most solid tumors, and our results suggest that these may decrease the antiproliferative effects of IFN. They may in part explain why IFNs have so little antitumor activity in such tumors, and they also suggest methods that may increase this activity.

Laboratory scale production of monoclonal antibodies in a tumbling chamber

A simple device for laboratory scale production of monoclonal antibodies has been developed. Hybridomas were cultured in four individual dialysis tubes containing 40-50 ml medium with 10% foetal calf serum, surrounded by 1.5-2 litres supply medium without any serum supplement. Once placed on a roller the special design of the apparatus leads to an eccentric rotation, thus keeping the cells in a stable homogeneous suspension. The system is automatically gassed, and this makes long term cultivation possible. Several hybridomas were tested over a culture period of at least 3 weeks, with supply medium changes every 3-4 days. Cell densities of up to 2.5 x 10(7)/ml and antibody concentrations of 0.3-1.9 mg/ml after purification were obtained. Results with this in vitro system allow a complete renunciation of the established in vivo method. The so called 'tumbling chamber' apparatus is easy to handle and to sterilize, is economic and universally adaptable in any research laboratory.

Inhibition of potentially lethal damage recovery by altered pH, glucose utilization and proliferation in plateau growth phase human glioma cells

Recovery from potentially lethal damage (PLD) has been measured in plateau growth phase human glioblastoma cells (U-87MG) under four postirradiation medium conditions. Recovery was maximal in depleted medium at an acidic pH, conditions which inhibit cellular proliferation. Compared with this control, PLD recovery (PLDR) was increasingly inhibited by alkalization of the existing medium (to pH 7.4), exchanging the old medium with fresh medium-pH acidified (to pH 6.8), and exchanging the old medium with fresh medium-pH unaltered (pH 7.4), respectively. These three medium adjustments were made at the time of irradiation. Increased glucose utilization (glycolysis) was detected postirradiation in all three cases, while increased proliferation was detected only when fresh medium was exchanged for old medium. Thus inhibition of PLDR has been correlated with increased glycolysis and increased proliferation during the recovery period. When acting together, these two processes provided almost complete inhibition. This study was revealed that the degree of inhibition may be related to the amount of glycolysis and/or proliferation occurring during the recovery period. Examining, in vitro, the range of PLDR achieved by postirradiation manipulation of medium pH may provide some indication of the range in PLDR that may be expected in vivo. Our study demonstrates that the effect of pH on glycolysis and proliferation may be important when determining the ability of a particular cell type to recover from PLD.

Effect of lactic acid on the kinetics of growth and antibody production in a murine hybridoma: secretion patterns during the cell cycle

The effects of elevated lactic acid concentration on the cell cycle kinetics of hybridoma cell growth and antibody production in batch culture were studied using conventional methods based on population-average data analysis and using flow cytometry based on single-cell data analysis. When 33 mM lactic acid was initially present, the true specific growth rate was reduced by 37% and the cell specific antibody production rate increased by a factor of 2.6 relative to a control culture with no additional lactic acid. DNA content distribution measured during balanced exponential growth were not affected by lactic acid concentration indicating lactic acid has a uniform effect on cell growth throughout the cell cycle. There was little or no effect on single-cell distributions of intracellular antibody content measured for the total population and for each cell cycle phase. The net rate of total antibody synthesis was found to be independent of specific growth rate. This implies that the balance of the total amount of antibody synthesized is shifted from cellular accumulation towards secretion when specific growth rate decreases. Our data predict that a maximum specific secretion rate of 2.7 pg per cell per h could be achieved if the specific growth rate was reduced to zero. The rates of secretion in the G1 and S phases increased with decreasing specific growth rate, while the rate of secretion in the G2+M phase remained relatively constant. Under the assumptions that (a) at the fastest growth rate, secretion in the G1 phase is negligible and (b) the rate of synthesis increases exponentially as cells proceed from the S phase to the G2+M phase, our data predict that for the slowest growth rate, the rate of secretion in G2+M is approx. 3-times that in the G1 phase and 5-times that in the S phase.

Hypoxia enhances the antiviral activity of interferons

WISH and Hep-2 cells were incubated in an environment with atmospheric oxygen (20% O2, approximately 140 mmHg partial pressure), and under hypoxic conditions (2% O2, approximately 14 mmHg). The oxygen tension greatly affected the metabolism of the cells and their response to interferon-alpha (IFN-alpha) and IFN-gamma. Under hypoxic conditions, the cytopathogenicity of vesicular stomatitis virus (VSV) was reduced by about 50%, and the antiviral effects of the interferons (IFNs) were increased, both in terms of VSV-induced cytopathic effect (CPE), and yields of infectious virus. Local hypoxia is a nonspecific host defense against virus infection. The present results suggest that one of the mechanisms is by potentiation of the effects of the IFN produced at the sites of virus replication.

Karyotypic analysis of CCRF-CEM and drug-resistant cell lines with stable and unstable ploidy

Karyotypic studies were performed on methotrexate (MTX)- and vincristine (VCR)-resistant cell lines derived from the human T-cell leukemia CCRF-CEM cell line. We noted karyotypic selection and additional chromosome change associated with acquisition of drug-resistance. Furthermore, we observed that both the parental and MTX-resistant sublines, CEM/MTX R1-3, had a tendency to ploidy change. Karyotypic studies of malignant cells have shown that polyploidy is frequently a consequence of a single sporadic event, followed by growth and selection of the polyploid clone [1]. In this study, however, various karyotypic clones were identified with near- and pseudotetraploid karyotypes that appeared to be derived from different near- and pseudodiploid sidelines. Polyploidy was invariably associated with loss of at least two of the four chromosomes 8 whether the pseudodiploid sideline from which it was derived carried one or both chromosomes 8. In contrast, neither polyploid clones nor loss of chromosome 8 was noted in the CEM/VCR R cells.

Calcium supplementation decreases rectal epithelial cell proliferation in subjects with sporadic adenoma

The results of three small clinical trials examining the effect of calcium carbonate supplementation on the proliferation cytokinetics of the rectal epithelium in subjects with a current history of sporadic adenoma are reported. In six subjects, a daily administration of 1500 mg of calcium carbonate for 90 days failed to significantly suppress thymidine labeling in normal-appearing mucosa of the rectum. However, a daily dose of 2000 mg of calcium significantly (P = 0.008) altered mucosal proliferation in a second set of six subjects after a 30-day trial. Finally, a placebo-controlled trial of calcium (2000 mg) was conducted in which 20 subjects were randomized to groups receiving a 4-week intervention with calcium (or placebo), followed by the alternative treatment (placebo or calcium). The results of the study show a marked suppression of rectal proliferation during the calcium phase of the study but not during the placebo phase. This study adds to accumulating evidence showing that calcium supplementation regulates the proliferative behavior of colonic epithelium in the individual at high risk for colon cancer. Longer term trials of calcium supplementation will ascertain whether a continuing benefit from increasing dietary calcium translates into inhibition of adenoma recurrence.

Role of metabolic waste products in the control of cell proliferation and antibody production by mouse hybridoma cells

In order to determine the factors limiting the proliferation and productivity of mouse hybridoma cells in batch/fed-batch cultures, we tested the influence of various environmental parameters on the growth of a model cell line VO 208. We observed that, among the major metabolic waste products, ammonium ions at concentrations superior to that present in the medium at the end of a batch culture do not exert a significant toxic effect on cell growth, whereas in contrast, lactic acid is cytotoxic at concentrations reached in cultures. Feeding of fructose instead of glucose during the stationary phase of the culture markedly prolongs the life span of the culture and enhances the antibody secretion accordingly. However, we failed to observe a satisfactory proliferation pattern in cultures grown in a glucose-free fructose-supplemented medium. We also noted that vitamin supply may be limiting in fed-batch cultures. It thus appears that thorough examination of the cell metabolic needs allows the designing of a culture regimen which significantly improves cell growth and secretion.

Keynote address: the influence of microenvironmental factors on the activity of radiation and drugs

The inherent radio- and chemosensitivity of tumor cells clearly affects their response to treatment. Accumulating evidence, however, suggests that the biochemical and physiological status of the cell during treatment is at least as important. In this review, a critique of the current evidence for, and extent of, microenvironmental heterogeneity in tumors is presented, emphasizing human tumor cells in situ. The expected consequences of those changes on cellular response to radiation and chemotherapy is then briefly reviewed. Finally, the continuing interest in developing new therapeutic strategies for which the tumor microenvironment is an asset (as opposed to a liability) is discussed in the context of the dynamic nature of tumors, and the complexity of adequately analyzing combination treatments.

Unidirectional microtubule assembly in cell-free extracts of Spisula solidissima oocytes is regulated by subtle changes in pH

Most, if not all, microtubules in vivo grow unidirectionally from a nucleation site such as the centrosome. This organized growth of microtubules can generate and maintain the radially symmetrical array of interphase microtubules as well as the bipolar mitotic apparatus. To investigate the regulation of polarized microtubule growth, we have prepared a cell-free extract from surf clam oocytes that exhibits unidirectional microtubule assembly. Immunofluorescence microscopy was used to visualize the net assembly of microtubules onto the fast (plus)- and slow (minus)- growing ends of isolated ciliary axonemes. All detectable microtubule growth in these cytoplasmic extracts occurred at the plus (+) ends and the extent of (+) end growth was regulated by subtle changes in pH. Microtubule assembly in these crude extracts was highly favored at pH 7.3, the pH of the post-fertilization cytoplasm. In contrast, when tubulin was purified from these oocyte extracts, integral components were lost, and microtubule growth became predominantly bidirectional and was favored at acidic pH. These results indicate that cytoplasmic factors may inhibit bidirectional growth in vivo and that temporal or local changes in cytoplasmic pH may influence microtubule assembly during the cell cycle.

Uncoupling of sodium chloride transport in premalignant mouse colon

Ion transport across premalignant large bowel mucosa in CF1 mice was evaluated by measuring sodium and chloride fluxes across voltage-clamped colonic segments obtained from control animals and animals treated for 4 wk with the procarcinogen 1,2-dimethylhydrazine, which induces tumor development principally in the distal colon. In control CF1 mouse colon, the net flux of sodium was 5.1 +/- 0.7 and 4.6 +/- 0.7 microEq.cm-2.h-1 and the net flux of chloride was 6.1 +/- 1.3 and 0.8 +/- 1.2 microEq.cm-2.h-1 in the distal and proximal colon, respectively. Removal of bicarbonate decreased the net flux of sodium 1.5 +/- 0.5 and 1.9 +/- 0.7 microEq.cm-2.h-1 in the distal and proximal colon, respectively, while the net flux of chloride was decreased to 1.7 +/- 1.8 microEq.cm-2.h-1 in the distal colon but was unaltered (0.8 +/- 0.1 microEq.cm-2.h-1) in the proximal colon. Addition of 25 mM bicarbonate stimulated the net flux of sodium and chloride absorption in the distal colon but increased net flux of sodium absorption alone in the proximal colon and stimulated net flux of chloride secretion. Removal of chloride decreased net flux of sodium to 3.4 +/- 1.4 and 1.8 +/- 0.8 microEq.cm-2.h-1 in the distal and proximal colon, respectively. Addition of 20 mM Cl stimulated net flux of sodium in the distal but not the proximal colon. These data suggest that sodium absorption is mediated by an electroneutral Cl-dependent, HCO3-dependent process (i.e., Na-H Cl-HCO3 dual exchange) in control distal colon and by an electroneutral HCO3-dependent process in control proximal colon. Following 1,2-dimethylhydrazine treatment, net flux of sodium in the distal colon was not stimulated by the addition of Cl or HCO3, and transport in the proximal colon was similar to that in control animals. However, 1,2-dimethylhydrazine treatment appears to uncouple Na-H Cl-HCO3 exchange in the distal colon early in the process of large bowel carcinogenesis.

Modulation of K+ currents in human lymphocytes by pH

1. Using whole-cell patch-clamp techniques, we found that the voltage-dependent K+ conductance in human peripheral blood T lymphocytes is enhanced threefold at alkaline intracellular pH (pHi) compared to acid pHi. This pH dependence can be described by a model having two strongly co-operative proton binding sites with pka 7.15. A similar pHi sensitivity exists for K+ conductance in mitogen-activated cells. 2. The reversal potential, threshold voltage for activation of the K+ conductance, and voltage dependence of steady-state inactivation are not affected by pHi. Activation and inactivation kinetics are also unchanged. 3. Single-channel measurements made in whole-cell patch-clamp mode indicate that the effect of intracellular pH on the amplitudes of single-channel events parallels, but does not wholly account for, the effect of pHi on the macroscopic currents. 4. Lowering extracellular pH (pHo) shifts the threshold for activation of the K+ current to a more depolarized voltage, consistent with a surface charge screening effect. Apparent changes in peak current and activation kinetics at acid pHo can be accounted for by this voltage shift. An additional slowing of inactivation kinetics at low pHo does occur. 5. The relevance of the pH sensitivity of the voltage-gated K+ conductance to lymphocyte mitogenesis and volume regulation is discussed.

Acidic cellular environments: activation of latent TGF-beta and sensitization of cellular responses to TGF-beta and EGF

Transient (about 2 hr) acidification to approx. pH 5.0 of agar-gelled overlayers containing untransformed NRK-49F or KiMSV-transformed NRK-49F cells in the presence of fetal calf serum or crude 49F-cell conditioned medium, as sources of latent TGF-beta, elicited EGF-dependent colony formation of 49F cells and inhibited spontaneous growth of transformed cells. Pure, active TGF-beta (porcine, type I) had the same effects on these respective cell types, suggesting that the above results were due to activation of latent TGF-beta in the transiently acidic cellular environment. Similar acidifications in the absence of a source of latent TGF-beta enhanced the positive growth response of 49F and AKR-2B cells to EGF and active TGF-beta and also the negative growth response of KiMSV-transformed 49F cells to active TGF-beta. These results are compatible with the idea that acidic cellular environments, particularly in tumor tissues, are conducive to activation of latent TGF-beta, perhaps in conjunction with other activating mechanisms, and to an enhanced response to some growth factors. However, the heterogeneity of cell populations within tumoral masses presents an obstacle to a clear understanding of the consequences of such activation.

Epidemiologic and toxicologic evidence for chronic health effects and the underlying biologic mechanisms involved in sub-lethal exposures to acidic pollutants

Since the 1880s, a disparate and extensive literature has evolved examining the biologic effects of acidification on cells. More recently, effects on the health of human and other species of acidic agents contained, for example, in pollutants have been suggested, particularly relating to long-term exposures. This paper provides a review of the epidemiologic and toxicologic evidence concerning health effects--particularly carcinogenicity--attributable to sub-lethal acid exposure. Underlying biologic mechanisms that explain adverse health outcomes include pH modulation of toxicity for a number of xenobiotics (including carcinogens, genotoxins, and teratogens), and low-pH-induced changes of cells involving, for example, alterations in mitotic and enzyme regulation. More focused research is recommended to test the relationship between long-term exposures to acidic agents (with a consequent lowered cellular pH) and various health effects.

Activation of the Na+/H+ antiport is not required for epidermal growth factor-dependent gene expression, growth inhibition or proliferation in human breast cancer cells

Mitogen interaction with specific receptors in many cell types leads to activation of the Na+/H+ antiport and a resultant cytoplasmic alkalinization. Since amiloride inhibits both Na+/H+ exchange and cell proliferation, it has been hypothesized that activation of the antiport is an obligatory requirement for mitogenesis. However, concentrations of amiloride which inhibit the antiport also inhibit other cellular processes, including protein synthesis and phosphorylation. We have used an epidermal growth factor (EGF) receptor gene-amplified human breast cancer cell line, the growth of which is inhibited by high levels of EGF in culture (MDA-468) and a variant, the growth of which is stimulated by EGF (MDA-468-S4), along with two potent amiloride analogues to examine whether activation of the Na+/H+ antiport and cytoplasmic alkalinization is necessary for both EGF-dependent effects to occur. At concentrations of the amiloride analogues which block Na+/H+ exchange in both cell types by 76-98%, the EGF-dependent alterations in [3H]thymidine incorporation or induction in c-myc or c-fos gene transcription were unaltered. These results were confirmed by a lack of effect of the amiloride analogues on both the growth-stimulatory and growth-inhibitory effects on EGF in an anchorage-independent growth assay. Similarly, in pH-altered media that prevented normal cytoplasmic alkalinization, the response of both MDA-468 and MDA-468-S4 to EGF activation was unaltered. In addition, activation of the Na+/H+ antiport alone was not sufficient to induce c-myc and c-fos transcription in either cell type. Taken together, these data suggest that neither the Na+/H+ antiport nor cytoplasmic alkalinization are necessary or sufficient for either EGF-dependent growth stimulation or growth inhibition in MDA-468 human breast cancer cells.

Flow cytometric measurement of intracellular pH in B16 tumors: intercell variance and effects of pretreatment with glucose

Flow cytometry was used to measure cytoplasmic pH (pHi) of B16 melanoma cells taken from tumor-bearing animals. We used a ratiometric method to allow measurements on an individual cell basis which were independent of cellular content of the pH indicator BCECF. In order to "freeze" any intercell variance which may have existed within the tumor mass, tumors were mechanically disaggregated in bicarbonate-free medium containing 0.5 mM amiloride at 4 degrees C and loaded with BCECF in choline chloride-based Earle's solution at 37 degrees C. Studies using cells grown in vitro showed that this protocol prevented acid load recovery during the 30-min period typically required between tumor excision and pHi measurement. A calibration curve was obtained by resuspending BCECF-stained cells in a range of buffers containing the proton ionophore nigericin. The range of values for individual cells was estimated by comparing the coefficient of variation of the test sample with that obtained when nigericin was used to reduce all cells to the pHi of the calibration buffer. The average value for mean tumor cell pH was 7.32 +/- 0.05 SD. Pretreatment of animals with intraperitoneal glucose for one hour resulted in an average for mean pHi of 7.17 +/- 0.17 SD. Mean coefficient of variation was 8.7%, and in the presence of nigericin, 8.1%. These values indicate a variance in measured pHi of approximately +/- 0.4 pH units, but most of this results from experimental error rather than true intercell pHi variance. The method used here is capable of detecting reduction in mean tumour pHi caused by ip glucose, but incapable of precise estimation of individual cell values. Despite these uncertainties, the results suggest that the range of pHi within B 16 tumors is small.

On the biophysics of transmembrane signalling

Transmembrane signalling involves a number of physical translocations, changes in proximity of membrane elements like receptor subunits, or sequestration of proteins from the membrane. The monitoring of such changes with flow cytometric energy transfer revealed a new putative subunit of the IL-2 receptor and a possible intermolecular interaction between HLA class I and class II antigens. Lateral diffusion of the components of the multi-subunit IL-2 receptor was also followed. Changes in the intracellular pH were considered as a measure of efficient signal transfer in a number of cases. An overview and critical comparison of data is presented in the paper.

Effects of changes in oxygen tension, pH, and glucose concentration on the response to CCNU of EMT6 mouse tumor monolayer cells and multicellular spheroids

We have studied the effect of individually changing medium glucose content, pH and oxygen tension upon the response to CCNU (1-(2-chloroethyl)3-cyclohexyl-1-nitrosourea) of EMT6/Ca/VJAC cells grown as early plateau phase monolayer cultures or small (200 microns diameter) spheroids. The effect of changing all three factors together has also been studied in the spheroid model. All the changes in medium conditions (except for 4 hr hypoxia) were maintained for 24 hr prior to drug exposure. Plating efficiency (PE) of monolayer cells was decreased by reduced medium pH (below 6.5) or oxygen tension while no change in PE was brought about by reduced medium glucose content. In small spheroids reduction in PE caused by low pH was similar to that seen in monolayer, there was again no effect of reduced glucose, and the effect of hypoxia was clearly less than in monolayer. Combination treatment of spheroids (pH 6.5, 120 mg/l glucose and hypoxia) reduced the PE of spheroid cells to 50% of control. Reducing medium glucose content from 920 to 0 mg/l, or oxygen tension from 20% to near zero (for either 4 or 24 hr) reduced the sensitivity of monolayer cells to CCNU. A similar pattern was seen for reducing medium pH from 7.2 to 6.1 during the 24 hr pre-incubation period and 1 hr drug exposure period. A reverse trend was, however, seen if medium pH during the drug exposure period was maintained at 7.2 following reduced pH pre-incubation. Reduced sensitivity to CCNU was seen for cells within small spheroids pre-incubated in medium at low pH (for both schedules) or under hypoxia (for either 4 or 24 hr) whereas reduced medium glucose content appeared to have no such effect. Cells in small spheroids after 24 hr combination treatment were also less sensitive to CCNU than cells from control spheroids.

Relationship between cytoplasmic pH and proliferation during exponential growth and cellular quiescence

Flow cytometry was used to measure intracellular pH (pHi) on an individual cell basis during exponential and plateau phases of growth. In all three cell lines examined a range of pHi values was associated with exponential growth. When cells from the extremes of the pHi distribution were sorted using a fluorescence-activated cell sorter and then restained for cellular DNA content, it was found that the higher pHi values were associated with enrichment of the S, G2, and M phases of the cell cycle, with a corresponding increase in the percentage of G1 cells at the lower pH1 range, suggesting cell-cycle dependence of pHi. It has been shown previously (I. W. Taylor and P. Hodson, 1984, J. Cell Physiol. 121, 517) that PMC-22 human melanoma cells are capable of entering a distinct pH-dependent quiescent state in response to the acidification of the growth medium which occurs naturally during growth to plateau phase. Simultaneous measurement of pHi and external pH showed that under these conditions pHi was maintained at control values down to an external pH of approximately 6.5, below which cytoplasmic acidification took place. This fall in pHi coincided with the onset of the transition to quiescence. Individual quiescent cells (defined by failure to incorporate bromodeoxyuridine during a 24-h exposure) could not be identified as such on the basis of a low pHi, suggesting that the probability of cell cycling is reduced by lowering pHi. Those cells which remained in cycle showed a markedly reduced rate of DNA synthesis, but a cell-cycle phase distribution similar to that in exponential growth, indicating that prolongation of all cell-cycle phases is an additional factor influencing overall population growth. The external pH at which both of these effects on cell proliferation kinetics took place in vitro is similar to that which occurs regionally within solid tumors, suggesting that pH effects could play a significant role in determining tumor cell growth in vivo.

Intracellular pH is increased after transformation of Chinese hamster embryo fibroblasts

These studies reveal that a series of tumorigenic Chinese hamster embryo fibroblast (CHEF) cell lines maintain an internal pH (pHi) that is 0.12 +/- 0.04 pH unit above that of the nontumorigenic CHEF/18 parental line. This increase of pHi in the tumorigenic CHEF cells is not due to autocrine growth factor production or to the persistent activation of pathways previously shown to modulate Na+/H+-antiporter activity present in the CHEF/18 line. These findings suggest that the defect in pHi regulation in the tumorigenic CHEF/18 derivatives lies in the Na+/H+ antiporter itself. Further studies to determine the biological significance of an increased pHi show that the external pH (pHo)-dependence curve for initiation of DNA synthesis in the tumorigenic CHEF lines is shifted by approximately 0.2 pH unit toward acidic values relative to that of the nontumorigenic CHEF/18 parent. These data show a critical role for pHi in the regulation of DNA synthesis in Chinese hamster embryo fibroblasts and demonstrate that aberrations in pHi can contribute to the acquisition of altered growth properties.

Flow cytometric measurement of cytoplasmic pH: a critical evaluation of available fluorochromes

Three pH-sensitive fluorochromes-4-methyl-umbelliferone(4MU),2, 3-dicyano-hydroquinone (DCH), and 2',7'-bis(carboxyethyl)-5,6-carboxy fluorescein (BCECF)--were evaluated for their resolution, range, and stability of cellular fluorescence. Flow cytometric techniques for determining cytoplasmic pH (pHi) have been fully described for 4MU and DCH; BCECF has previously been used for fluorimetric estimation of pHi, and was adapted to flow cytometry. For each fluorochrome, the ratio of fluorescence intensity at two wavelengths gives a measure of pHi, which may be calibrated by obtaining the fluorescence ratios for cells suspended in buffers of varying pH in the presence of a proton ionophore. Reliable calibration proved difficult using 4MU, partly because of poor retention within cells. Both DCH and BCECF could be calibrated using a fluorescence ratio and had resolutions of 0.2 and 0.4 pH units, respectively. The fluorescence of DCH is so strongly pH dependent that there were practical difficulties in its use over a wide pH range; however, pHi measurements are possible between pH 6.0 and pH 7.5 using either DCH or BCECF. Substantial dye leakage was found for 4MU and, to a lesser extent, DCH, while BCECF was retained by cells for up to 2 hours. Despite its lower resolution BCECF had a usable range of more than 1.5 pH units and this coupled with its stable fluorescence and excitation at 488 nm rather than UV suggests a wide application.

Sublethal pH decrease may cause genetic damage to eukaryotic cell: a study on sea urchins and Salmonella typhimurium

Further evidence is reported here of genetic and developmental damage that can be induced by a sublethal pH decrease. The effects of three inorganic acids (HCl, H2SO4, and H3PO4) on embryos and sperm from the sea urchins Sphaerechinus granularis and Paracentrotus lividus were evaluated. In addition, acidification of the medium was tested for spontaneous reversion to His+ prototrophy in Salmonella typhimurium (strains TA97, TA98, TA100, TA102, TA1535) up to toxic levels, by both liquid incubation and agar plate incorporation. The induction of developmental and mitotic abnormalities in S. granularis confirmed our previous observations on P. lividus. Embryotoxicity was exerted in S. granularis more severely by H3PO4 than by HCl or H2SO4 (pH 7 to 6), while the induction of mitotic abnormalities appeared at a pH of less than or equal to 6.5 irrespective of the acids used. By suspending S. granularis or P. lividus sperm in acidified filtered seawater (fsw) and then inseminating the eggs in natural fsw (pH = 8.0), the offspring showed developmental and mitotic abnormalities. Low-pH-induced spermiotoxicity was ruled out in our experiments, since fertilization success of acid-exposed sperm was actually enhanced, as compared to sperm suspended in untreated fsw. The exposure of S. typhimurium to different pH's (ranging from 4 to 9) invariably failed to induce any changes in reversion rates, regardless of the acids (or alkali) being used. These results suggest that extracellular acidification may cause sublethal damage that in turn leads to an impairment of mitotic activity and cell differentiation.

Estimation of nonproliferating cells in the neural retina of embryonic chicks by flow cytometry

Nonproliferating cells (N cells) in the neural retina of embryonic chicks were estimated after isolating them in the 2C peak of a DNA distribution by exposure to the cell-cycle inhibitor ICRF 159. ICRF 159 inhibits cell division but not DNA synthesis, so proliferating cells can leave the 2C peak but not reenter it. Cells left in the 2C peak after exposure to ICRF 159 were assumed to be N cells. The effectiveness of ICRF 159 in inhibiting cell division but not inhibiting DNA synthesis was demonstrated in neural retinae from stage 17 embryos which showed no evidence of a 2C peak after 6-h exposure to ICRF 159 and which were thus shown to have no N cells. A test to detect escape from the cell division block in older embryos with an N cell population in the neural retina showed some escape after longer exposures to ICRF 159. The escape was suppressed by a second dose of ICRF 159, given some hours after the first.

Modulation of transferrin receptor expression by inhibitors of nucleic acid synthesis

We investigated the effects of the iron chelator desferrioxamine on the expression of transferrin receptors (TfR) by CCRF-CEM human T-cell leukaemia and B16 mouse melanoma cells growing in tissue culture. Desferrioxamine (DFOA) enhanced TfR expression when added in the dose range of 10(-5)-10(-4) to CCRF-CEM cells, but was toxic to these cells, the lower concentrations producing a slowing of cell growth with a build up in S-phase, while higher concentrations caused cell death with a block at the G1/S-phase interface. These toxic effects are compatible with its previously reported inhibition of the non-haem iron containing (M2) subunit of ribonucleotide reductase. In marked contrast, DFOA caused the growth of B16 melanoma cells to arrest in G1, without loss of cloning efficiency, and resulted in a fall in TfR expression to approximately 50% of control values. These results suggested that the effects of DFOA on TfR expression were linked to DNA synthesis rather than to a more generalised inhibition of iron-dependent cellular processes. It was subsequently found that inhibition of the M2 subunit of ribonucleotide reductase in CCRF-CEM cells with 5 X 10(-5) M hydroxyurea, which is not an iron chelator, also enhanced TfR expression, as did thymidine and cytosine arabinoside, which have different enzyme targets. By measuring cellular DNA and RNA content simultaneously it was shown that all of these agents caused unbalanced growth, i.e., inhibited DNA synthesis more than RNA synthesis. In contrast, 6-thioguanine was more inhibitory to RNA synthesis, and treatment with this drug caused a fall in TfR expression. Thus, although CCRF-CEM cells treated with DFOA show enhanced TfR expression, similar effects are also seen with other inhibitors of DNA synthesis, provided that RNA synthesis is allowed to continue. These results provide further evidence that the regulation of TfR expression by proliferating cells is specifically linked to DNA synthesis rather than to the iron requirements of other cellular processes.