Energy status in the murine FSaII and MCaIV tumors under aerobic and hypoxic conditions: an in-vivo and in-vitro analysis

Measuring oxygenation in vivo with MRS/MRI--from gas exchange to the cell

Oxygen plays a major role as a substrate in metabolic processes in numerous signaling pathways, in redox metabolism, and in free radical metabolism. To study the role of oxygen in normal and pathophysiological states, methods that can be used noninvasively are required. This review examines the potential of nuclear magnetic resonance techniques to study tissue oxygenation. It is written from a systems perspective, looking at detection methods with respect to the path that oxygen takes in the mammalian system-from the lungs, through the vascular system, into the interstitial space, and finally into the cell. Methods discussed range from those that are quantifiable, such as the assessment of spin lattice relaxation time in fluorocarbon solutions, to those that are more correlative, such as assessment of lactate and high energy phosphates. Since the methods vary in their site of application, sensitivity, and specificity to the quantification of oxygen, this review provides examples of how each method has been applied. This may facilitate the reader's understanding of how to optimally apply different methods to study specific biomedical problems.

Phosphorous metabolites and steady-state energetics of transformed fibroblasts during three-dimensional growth

Rat1-T1 and MR1 spheroids represent separate transformed phenotypes originated from the same rat fibroblasts that differ in three-dimensional (3D) growth kinetics, histological structure, and oxygenation status. In the present study, (31)P-NMR spectroscopy of perfused spheroid suspensions was used to investigate cellular energetics relative to 3D growth, development of necrosis, and cell cycle distribution. Both spheroid types were characterized by a remarkably low amount of free (inorganic) phosphate (P(i)) and a low phosphocreatine peak. The ratio of nucleoside triphosphate (NTP) to P(i) ranged between 1.5 and 2.0. Intracellular pH, NTP-to-P(i) ratio, and NTP/cell remained constant throughout spheroid growth, being unaffected by the emergence of oxygen deficiency, cell quiescence, and necrosis. However, a 50% decrease in the ratio of the lipid precursors phosphorylcholine and phosphorylethanolamine (PC/PE) was observed with increasing spheroid size and was correlated with an increased G(1)/G(0) phase cell fraction. In addition, the ratio of the phospholipid degradation products glycerophosphorylcholine and glycerophosphorylethanolamine (GPC/GPE) increased with spheroid diameter in Rat1-T1 aggregates. We conclude that changes in phospholipid metabolism, rather than alterations in energy-rich phosphates, reflect cell quiescence in spheroid cultures, because cells in the inner oxygen-deficient zones seem to adapt their energy metabolism to the environmental conditions before necrotic cell destruction.

Inducible repair and the two forms of tumour hypoxia--time for a paradigm shift

Clinical experience shows that there is a therapeutic window between 60 and 70 Gy where many tumours are eradicated, but the function of the adjacent normal tissues is preserved. This implies much more cell kill in the tumour than is acceptable in the normal tissue. An SF2 of 0.5 or lower is needed to account for the eradication of all tumour cells, while an SF2 of 0.8 or higher is needed to explain why these doses are tolerated by normal tissues. No such systematic difference is known between the intrinsic sensitivity of well-oxygenated normal and tumour cells. The presence of radioresistant hypoxic cells in tumours makes it even more difficult to understand the clinical success. However, there is experimental evidence that starved cells lose their repair competence as a result of the depletion of cellular energy charge. MRS studies have shown that low ATP levels are a characteristic feature of solid tumours in rodents and man. In this paper we incorporate the concept of repair incompetence in starving, chronically hypoxic cells. The increased sensitivity of such cells has been derived from an analysis of mammalian cell lines showing inducible repair. It is proportional to the SF2 and highest in resistant cells. The distinction between acutely hypoxic radioresistant cells and chronically hypoxic radiosensitive cells provides the key to the realistic modelling of successful radiotherapy. It also opens new conceptual approaches to radiotherapy. We conclude that it is essential to distinguish between these two kinds of hypoxic cells in predictive assays and models.

Monitoring of tumor reoxygenation following irradiation by 31P magnetic resonance spectroscopy: an experimental study of human melanoma xenografts

BACKGROUND AND PURPOSE

Inadequate tumor reoxygenation during radiation therapy may cause local treatment failure. This study was aimed at investigating the potential usefulness of 31P-MRS in monitoring tumor reoxygenation following radiation treatment.

MATERIALS AND METHODS

Tumors of two human melanoma xenograft lines (BEX-t and HUX-t) were exposed to 15.0 Gy, and then the fraction of radiobiologically hypoxic cells, measured by using the paired survival curve method, or tumor bioenergetic status, measured by 31P-MRS as the (PCr + NTPbeta)/Pi resonance ratio, was determined versus time after the radiation exposure.

RESULTS

Untreated BEX-t and HUX-t tumors showed similar fractions of radiobiologically hypoxic cells and similar bioenergetic status, whereas both parameters differed substantially between the lines in irradiated tumors. A close association was found between radiation-induced changes in tumor bioenergetic status and radiation-induced changes in the fraction of radiobiologically hypoxic cells.

CONCLUSION

31P-MRS is a potentially useful method for monitoring tumor reoxygenation following radiation treatment.

RSR13, an allosteric effector of haemoglobin, and carbogen radiosensitize FSAII and SCCVII tumours in C3H mice

Pre-clinical evaluation has demonstrated that 2-[4-(((3,5-dimethylanilino)carbonyl)methyl)phenoxy]-2-methylpropi onic acid (RSR13) acts as an allosteric effector of haemoglobin (Hb). RSR13 binding to Hb results in decreased haemoglobin-oxygen (Hb-O2) affinity, improved tumour oxygenation, and enhanced radiation-induced cell killing in several experimental tumour systems. In the present work, ex vivo clonogenic survival analyses are applied in two murine tumour systems to characterize the relationship between the magnitude of decrease in Hb-O2 affinity and radiosensitization, the influence of inspired pO2 upon this effect, and the efficacy of combining RSR13 and radiation during a course of repeated radiation exposures. For FSaII tumours in C3H mice breathing air, 100 mg kg(-1) RSR13 administered intraperitoneally produced an enhancement ratio (ER) of 1.3, but there was marked desensitization at a RSR13 dose of 300 mg kg(-1) (ER 0.6). The most likely reason for the increased radioresistance was insufficient oxygen loading of Hb in the pulmonary circulation due to reduced haemoglobin-oxygen affinity because carbogen breathing combined with 300 mg kg(-1) RSR13 reversed the effect and produced an ER of 1.8. In SCCVII tumours in C3H mice irradiated with eight fractions of 2.5 Gy over 4 days, the surviving fraction was reduced to 58-67% of control values when RSR13 was combined with radiation on days 1 and 2, days 3 and 4, or days 1-4. These results confirm that combining RSR13 and irradiation within a fractionated course of clinically relevant low-dose exposures provides significant radiosensitization. Additional preclinical experimentation is needed to define better the optimum dose-scheduling conditions for clinical applications.

Energy metabolism in human melanoma cells under hypoxic and acidic conditions in vitro

The response to treatment and the malignant progression of tumours are influenced by the ability of the tumour cells to withstand severe energy deprivation during prolonged exposure to hypoxia at normal or low extracellular pH (pHe). The objective of the present work was to demonstrate intertumour heterogeneity under conditions of microenvironment-induced energy deprivation and to investigate whether the heterogeneity can be attributed to differences in the capacity of the tumour cells to generate energy in an oxygen-deficient microenvironment. Cultures of four human melanoma cell lines (BEX-c, COX-c, SAX-c, WIX-c) were exposed to hypoxia in vitro at pHe 7.4, 7.0 or 6.6 for times up to 31 h by using the steel-chamber method. High-performance liquid chromatography was used to assess adenylate energy charge as a function of exposure time. Cellular rates of glucose uptake and lactate release were determined by using standard enzymatic test kits. The adenylate energy charge decreased with time under hypoxia in all cell lines. The decrease was most pronounced shortly after the treatment had been initiated and then tapered off. BEX-c and SAX-c showed a significantly higher adenylate energy charge under hypoxic conditions than did COX-c and WIX-c whether the pHe was 7.4, 7.0 or 6.6, showing that tumours can differ in the ability to avoid energy deprivation during microenvironmental stress. There was no correlation between the adenylate energy charge and the rates of glucose uptake and lactate release. Intertumour heterogeneity in the ability to withstand energy deprivation in an oxygen-deficient microenvironment cannot therefore be attributed mainly to differences in the capacity of the tumour cells to generate energy by anaerobic metabolism. The data presented here suggest that the heterogeneity is rather caused by differences in the capacity of the tumour cells to reduce the rate of energy consumption when exposed to hypoxia.

Is there a critical tissue oxygen tension for bioenergetic status and cellular pH regulation in solid tumors?

Bioenergetic and metabolic status have been correlated with tissue oxygenation in murine fibrosarcomas (FSaII) of varying sizes (44-600 mm3). Ratios of beta-nucleoside triphosphates to inorganic phosphate (beta NTP/P) and phosphocreatine to inorganic phosphate (PCr/P(i)) ratios derived from 31P nuclear magnetic resonance spectroscopy (NMR) were positively correlated to median tissue O2 tension (pO2) values using O2-sensitive needle electrodes. pH declined during growth with intracellular acidosis being evident in tumors > 350 mm3. Whereas lactic acid formation greatly contributed to this decline in small and medium-sized tumors, adenosine triphosphate (ATP) hydrolysis and slowing down of the activities of pumps involved in cellular pH regulation seem to be major factors responsible for intracellular acidification in bulky tumors. PCr levels decreased at an early growth stage, whilst ATP concentrations dropped in bulky malignancies only, coinciding with a decrease in adenylate energy charge and a substantial rise in the levels of total P(i). On average, median pO2 values of ca. 10 mmHg represent a critical threshold for energy metabolism. At higher median O2 tensions, levels of ATP, phosphomonoester (PME) and total P(i) were relatively constant. This coincided with intracellular alkalosis or neutrality and stable adenylate ratios. On average, median pO2 values < 10 mmHg coincided with intracellular acidosis, ATP depletion, a drop in energy charge and rising P(i) levels.

Energy status parameters, hypoxia fraction and radiocurability across tumor types

Under full nutrient in vitro conditions, the cellular adenylate energy charge of six different rodent and human tumor cell types was identical, i.e., 0.94 +/- 0.01, suggesting the potential utility of this parameter as a cell (and tissue) independent marker of nutrient deprivation and hypoxia, across tumor types. The adenylate energy charge values of tumors, arising from these cells, was reduced and variable ranging from 0.72 to 0.91 for the various tumor types. However, neither the tumor adenylate energy charge, NTP/Pi, nor PCr/Pi ratios correlated with the radiobiologic hypoxic cell fractions across tumor types. The reduced adenylate energy charge in vivo suggests varying degrees of nutrient deprivation in the different tumor types, however, factors other than or in addition to hypoxia likely contribute to tumor energy status.

Determination of adenine nucleotides by fluorescence detection using high-performance liquid chromatography and post-column derivatisation with chloroacetaldehyde

A novel rapid method for the analysis of adenine nucleotides in cells and tissues using post-column derivatisation with chloroacetaldehyde (CAA) followed by fluorescence detection is described. The CAA is incorporated in the eluent, but only reacts post-column when the temperature is elevated to 100 degrees C. Samples are chromatographed following neutralisation of acid extracts. Examples are given using both trichloroacetic acid extraction for cells in culture, and perchloric acid for murine liver.

Intracellular acidosis in murine fibrosarcomas coincides with ATP depletion, hypoxia, and high levels of lactate and total Pi

Bioenergetic and metabolic status of murine FSaII tumours were evaluated using 31P MRS, acid extracts ('global' techniques) and quantitative bioluminescence ('microregional' assay). Data obtained from s.c. tumours of varying sizes (44-600 mm3) have been correlated with the oxygenation status evaluated using O2-sensitive needle electrodes. beta-NTP/Pi and phosphocreatine (PCr)/Pi ratios derived from 31P MRS were positively correlated to the median tissue pO2 values. pH declined during growth with intracellular acidosis being evident in tumours > 350 mm3. Whereas lactic acid formation greatly contributed to this decline in small- and medium-sized tumours, ATP hydrolysis and slowing down of the activities of pumps involved in pHi regulation seem to be major factors responsible for intracellular acidification in bulky tumours. PCr levels decreased at an early growth stage, whilst ATP concentrations dropped in bulky malignancies only, coinciding with a decrease in adenylate energy charge and a substantial rise in the levels of total Pi. MRS observable (mobile) Pi was consistently lower than [Pi] measured in acid extracts. On average, median pO2 values of ca 10 mmHg represent a critical threshold for energy metabolism. At higher median O2 tensions, levels of ATP, phosphomonoester and total Pi were relatively constant. This coincided with intracellular alkalosis or neutrality and stable adenylate ratios. On average, median pO2 values < 10 mmHg coincided with intracellular acidosis, ATP depletion, a drop in energy charge and rising Pi levels.

ELISA quantification of CCI-103F binding in canine tumors prior to and during irradiation

PURPOSE

The purpose of this work was to evaluate multiple injections of CCI-103F, a marker of hypoxia, as a method to quantify alterations in tumor hypoxia during irradiation.

METHODS AND MATERIALS

Twelve dogs with spontaneous solid tumors were given intravenous CCI-103F, and tumor biopsies were taken at various times after injection. Two tumor samples were taken at each biopsy procedure. CCI-103F antigen concentration was quantified by ELISA. Four of the dogs were given one injection of CCI-103F, and the other eight received two injections. In dogs receiving two injections, CCI-103F was administered before irradiation and 7 days later, following a total dose of 15.0 Gy. Plasma CCI-103F pharmacokinetics were assessed in dogs receiving two injections.

RESULTS

CCI-103F antigen was detectable in the initial biopsy in each of the four dogs receiving one injection, and the amount of detectable antigen decreased in subsequent biopsies with an initial half life of approximately 19 h. This suggests that multiple injections of CCI-103F could be used in the same subject to monitor tumor hypoxia as a function of time or during a course of treatment. In the eight dogs receiving two injections of CCI-103F, the CCI-103F antigen concentration in the 24 h samples ranged from 4.66-151.9 mumol CCI-103F antigen/kg tumor, a difference of a factor of approximately 33. The ratio of maximum to minimum concentration of CCI-103F antigen in 51 paired biopsy samples ranged from 1.01-4.07, with a mean (+/- s.d.) of 1.67 +/- 0.67. Seventy-five percent of the ratios were < or = 2.02. There was no apparent relationship between the magnitude of the ratio, i.e., intratumoral variation, and tumor volume or the absolute tumor concentration of CCI-103F antigen. Absolute radiobiologic hypoxic fraction was not known but the pattern of change in amount of intratumoral CCI-103F antigen in dogs given two injections of CCI-103F was consistent with little change in pretreatment oxygen status in six dogs, and an increase in tumor oxygenation in two dogs.

CONCLUSION

It appears possible to obtain an estimate of the change in tumor hypoxia in an individual tumor over time by assaying biopsy samples for CCI-103F antigen concentration.

In vivo 31P MRS of experimental tumours

More than 50% of cancers fail to respond to any individual treatment and tumour follow-up after treatment plays a major role in routine therapy planning and pharmacological research. Today, MRS is the only technological approach providing non-invasive access to tumour biochemistry. Ten years ago, expectations were raised concerning 31P MRS as an exciting and promising technical approach to the study of tumours. However the expectations have not always come to fruition. How close are we now to seeing routine 31P NMR in clinical oncology? This review of the 127 published papers shows spectroscopy results in more than 150 experimental animal tumour models. These tumour/host/treatment systems provide us with a useful basis to evaluate the current state of the art, summarize the basic knowledge presently available, determine the key points underlying the present disappointment of some clinical oncologists and stimulate new basic research. The information collected concerns the discussion of the reliability of experimental models in oncology, the technical improvement of magnetic resonance technology and the monitoring of bioenergetic status, pH regulation and phospholipid metabolism in treated and untreated tumours. Recent advances (two-thirds of the papers have been published in the last 5 years) seem to provide more optimistic perspectives than those generally accepted a few years ago, in the depressing period following early pioneering work.

Role of oxygen vs. glucose in energy metabolism in a mammary carcinoma perfused ex vivo: direct measurement by 31P NMR

The role of glycolysis vs. respiration in tumor energy metabolism has been studied, to date, primarily in vitro by using single cells, multicellular spheroids, or tissue slices. With the advent of in vivo NMR spectroscopy, several investigators have shown that tumor energy status depends on its blood flow. Since manipulation of blood flow alters both oxygen and glucose delivery to a solid tumor, these studies have not been able to separate the relative contribution of oxygen vs. glucose in energy metabolism in vivo. In the present study, we have overcome this problem by combining two methods: the tissue-isolated R3230AC mammary adenocarcinoma perfused ex vivo and 31P NMR spectroscopy. The isolated tumor permits one to control the perfusion pressure as well as the metabolite concentrations in the perfusate. NMR spectroscopy permits one to measure the ratio of nucleoside triphosphate to inorganic phosphate (NTP/Pi) and pH. Our results show that (i) the NTP/Pi ratio ex vivo is similar to that observed in vivo prior to surgery, (ii) the NTP/Pi ratio is insensitive to flow changes at high flow rates but is proportional to flow rate at flows comparable to those found in vivo, (iii) the NTP/Pi ratio of these tumors is resistant to hypoxia and is not maintained when glucose is removed or replaced with glutamine, and (iv) although both O2 and glucose are consumed by these tumors, the effect of perfusate flow rate appears to be mediated largely through glucose delivery. The current approach not only provides information about the role of glycolysis vs. respiration in a rodent tumor but also is general and versatile enough to provide similar data in human tumors perfused ex vivo.