Plausibility of trophoblastic-like regulation of cancer tissue

Background: Thus far, a well-established logical pattern of malignancy does not exist. The current approach to cancer properties is primarily descriptive with usually, for each of them, extensive analyses of the underlying associated biomolecular mechanisms. However, this remains a catalog and it would be valuable to determine the organizational chart that could account for their implementation, hierarchical links and input into tumor regulation.

Hypothesis: Striking phenotypic similarities exist between trophoblast (invasive and expanding early placenta) and cancer regarding cell functions, logistics of development, means of protection and capacity to hold sway over the host organism. The concept of cancer cell trophoblastic-like transdifferentiation appears to be a rational proposal in an attempt to explain this analogy and provide a consistent insight into how cancer cells are functioning. Should this concept be validated, it could pave the way to promising research and therapeutic perspectives given that the trophoblastic properties are vital for the tumor while they are permanently epigenetically turned off in normal cells. Specifically targeting expression of the trophoblastic master genes could thereby be envisaged to jeopardize the tumor and its metastases without, in principle, inducing adverse side effects in the healthy tissues.

Conclusion: A wide set of functional features of cancer tissue regulation, including some apparently paradoxical facts, was reviewed. Cancer cell misuse of physiological trophoblastic functions can clearly account for them, which identifies trophoblastic-like transdifferentiation as a likely key component of malignancy and makes it a potential relevant anticancer target.

Hypothesis about Transdifferentiation As Backbone of Malignancy

Background

Cancer is mainly watched through the prism of random mutations and related corruption of signaling pathways. However, it would seem puzzling to explain the tumor organization, pugnacity and steady evolution of the tumorous disease and, moreover, a systematic ascendancy over the healthy tissues, only through stochastic genomic alterations.

Malignancy specific properties

Considering the core characteristics of cancer cells, it appears that two major sets of properties are emerging, corresponding to well-identified physiological phenotypes, i.e., (1) the trophoblastic logistical functions for cell survival, protection, expansion, migration, and host-tissue conditioning for angiogenesis and immune tolerance and (2) the sexual functions for genome maintenance. To explain the resurgence of these trophoblastic and sexual phenotypes, a particular cell reprogramming, to be called “malignant transdifferentiation” in view of its key role in the precancer-to-cancer shift, appears to be a convincing hypothesis.

Perspectives

The concept of malignant transdifferentiation, in addition to oncogenic mutations, would determine a more rational approach of oncogenesis and would open so far unexplored ways of therapeutic actions. Indeed, the trophoblastic phenotype would be a good candidate for therapeutic purposes because, on the one hand, it covers numerous properties that all are vital for the tumor, and on the other hand, it can be targeted with potentially no risk of affecting the healthy tissues as it is not expressed there after birth.

The CONRAD approach to biokinetic modeling of DTPA decorporation therapy

Diethylene Triamine Pentaacetic Acid (DTPA) is used for decorporation of plutonium because it is known to be able to enhance its urinary excretion for several days after treatment by forming stable Pu-DTPA complexes. The decorporation prevents accumulation in organs and results in a dosimetric benefit, which is difficult to quantify from bioassay data using existing models. The development of a biokinetic model describing the mechanisms of actinide decorporation by administration of DTPA was initiated as a task in the European COordinated Network on RAdiation Dosimetry (CONRAD). The systemic biokinetic model from Leggett et al. and the biokinetic model for DTPA compounds of International Commission on Radiological Protection Publication 53 were the starting points. A new model for biokinetics of administered DTPA based on physiological interpretation of 14C-labeled DTPA studies from literature was proposed by the group. Plutonium and DTPA biokinetics were modeled separately. The systems were connected by means of a second order kinetics process describing the chelation process of plutonium atoms and DTPA molecules to Pu-DTPA complexes. It was assumed that chelation only occurs in the blood and in systemic compartment ST0 (representing rapid turnover soft tissues), and that Pu-DTPA complexes and administered forms of DTPA share the same biokinetic behavior. First applications of the CONRAD approach showed that the enhancement of plutonium urinary excretion after administration of DTPA was strongly influenced by the chelation rate constant. Setting it to a high value resulted in a good fit to the observed data. However, the model was not yet satisfactory since the effects of repeated DTPA administration in a short time period cannot be predicted in a realistic way. In order to introduce more physiological knowledge into the model several questions still have to be answered. Further detailed studies of human contamination cases and experimental data will be needed in order to address these issues. The work is now continued within the European Radiation Dosimetry Group, EURADOS.

Biokinetic modelling of DTPA decorporation therapy: the CONRAD approach

Administration of diethylene triamine pentaacetic acid (DTPA) can enhance the urinary excretion rate of plutonium (Pu) for several days, but most of this Pu decorporation occurs on the first day after treatment. The development of a biokinetic model describing the mechanisms of decorporation of actinides by administration of DTPA was initiated as a task of the coordinated network for radiation dosimetry project. The modelling process was started by using the systemic biokinetic model for Pu from Leggett et al. and the biokinetic model for DTPA compounds of International Commission on Radiation Protection Publication 53. The chelation of Pu and DTPA to Pu-DTPA was treated explicitly and is assumed to follow a second-order process. It was assumed that the chelation takes place in the blood and in the rapid turnover soft tissues compartments of the Pu model, and that Pu-DTPA behaves in the same way as administered DTPA. First applications of this draft model showed that the height of the peak of urinary excretion after administration of DTPA was determined by the chelation rate. However, repetitions of DTPA administration shortly after the first one showed no effect in the application of the draft model in contrast to data from real cases. The present draft model is thus not yet realistic. Therefore several questions still have to be answered, notably about where the Pu-DTPA complexes are formed, which biological ligands of Pu are dissociated, if Pu-DTPA is stable and if the biokinetics of Pu-DTPA excretion is similar to that of DTPA. Further detailed studies of human contamination cases and experimental data about Pu-DTPA kinetics will be needed in order to address these issues. The work will now be continued within a working group of EURADOS.

Treatment of actinide exposures: a review of Ca-DTPA injections inside CEA-COGEMA plants

Calcium diethylenetriamine pentacetate (Ca-DTPA) has been used for medical treatment of plutonium and americium contaminations in the CEA and COGEMA plants from 1970 to 2003. This paper is a survey of the injections Ca-DTPA administered as a chelating molecule and it will be a part of the authorisation process for Ca-DTPA by intravenous administration. Out of 1158 injections administered to 469 persons, 548 events of possible or confirmed contamination were reported. These employees were followed by occupational physicians according to the current French regulations. These incidents took place at work, were most often minor, not requiring follow-up treatment. The authors present (1) a synthesis of the most recent findings. Due to its short biological half-time and its limited action in the blood, Ca-DTPA does not chelate with plutonium and americium as soon as these elements are deposited in the target organs. It justifies an early treatment, even in cases of suspected contamination followed by additional injections if necessary (2) data concerning these 1158 injections (route of contamination, dosage, adverse effects, etc.) The authors also investigated a study on the efficacy of the product on a group of persons having received five or more injections. These results were compared with the efficacy estimated theoretically. Dosages and therapeutic schemes were proposed based on these observations. This synthesis is the result of a collective work having mobilised the occupational medicine departments, the medical laboratories inside a working group CEA-COGEMA-SPRA.

MEDOR, a didactic tool to support interpretation of bioassay data after internal contamination by actinides

A didactic software, MEthodes DOsimètriques de REférence (MEDOR), is being developed to provide help in the interpretation of biological data. Its main purpose is to evaluate the pertinence of the application of different models. This paper describes its first version that is focused on inhalation exposure to actinide aerosols. With this tool, sensitivity analysis on different parameters of the ICRP models can be easily done for aerosol deposition, in terms of activity and particle number, actinide biokinetics and doses. The user can analyse different inhalation cases showing either that dose per unit intake cannot be applied if the aerosol contains a low number of particles or that an inhibition of the late pulmonary clearance by particle transport can occur which contributes to a 3-4 fold increase in effective dose as compared with application of default parameters. This underlines the need to estimate systematically the number of deposited particles, as well as to do chest monitoring as long as possible.

[How to consider cancer: implications for the risk due to ionizing radiation]

Cancers induced by ionizing radiation have no particular specificity nor genetic remarkable signature, excepting numerous multideletions. They should therefore be studied in the general field of cancer biology in its broad sense. A gap remains between the initial events like the rather well identified genomic damage and the subsequent emerging cellular clone with cancer characteristics. Intermediate steps are generally described as accumulation of mutations and epigenetic modifications leading at one point to the malignant phenotype. However we have no clear nor understandable model on these steps of malignant transformation till now. It is quite possible that specific causes (tobacco, alcohol, radiations, chemical toxics) which produce different initial abnormalities then lead to (or accelerate the entry in) the common and same way as that resulting of accumulations of damage due to ageing. Genomic instability is certainly an important factor involved in the cellular drift leading to malignant transformation. We postulate that only cells having both a high telomerase activity and a low apoptotic activity may become cancerous. The hypothesis is that cancer results from a genome reprogramming of these cells due to an oncogenic mitotic pressure which induces a loss of the differentiation control. We propose to name anti-apoptosis, in contrast to apoptosis which is the programmed death, the ultimate process by which a cell loses its tissue-related properties. The oncogenic activation may propagate to primordial genes of development resulting in emergence of a subnuclear with tumoral activity. Bio-molecular studies of embryonic development and of genome re-programming will probably allow us to better understand the mechanisms of cancer.

Assessment of radioactive systemic uptakes by deconvolution of individual monitoring results

The methods of interpretation presently available for evaluating individual radioactivity intakes from measured data involve difficulties connected with the adaptation of metabolic models to the situations encountered in practice. These difficulties essentially concern the definition of appropriate parameters for each encountered case, and very often- except for characterized incidents-erroneous appreciation of the time course of contamination episodes and of th routes of entry. These considerations led us to develop a simplified method of interpreting monitoring data, by considering separately the data relating to routes of entry and those concerning systemic contamination, i.e., the contamination occurring after the transfer of radionuclides to the blood. An approach to interpreting measurements of systemic contamination is proposed in this study. This method is to calculate, from these measurements, the values for the activity absorbed daily from the routes of entry into the blood using the appropriate retention and excretion functions. a day-to-day follow-up of the absorbed activity becomes possible, thus enabling its real-time evolution to be recorded and easy to consult. A few applications of the method are described, including cases of acute tritium and uranium contamination and of chronic contamination by tritium, uranium, and iodine. The conditions and constraints required to validate the proposed approach are indicated.

Radiotoxicological intercomparisons organised by the Commissariat à l'Energie Atomique group in France

People working in French nuclear plants are monitored either by whole body counting or by the measurement of biological samples. The radiochemical and radiometric procedures used have to be periodically reviewed. A working group, including practising biologists from CEA, EDF and the Armed Forces Health Service, instituted comparisons of radiotoxicological test assessments. Since 1978, about 60 intercomparisons have been made. Currently 30 European laboratories are involved in these intercomparisons. This paper provides a brief history of the intercomparison exercises, describes the logistics of sample collection and preparation, and presents the results, showing the position of each laboratory in relation to the reference and median values. Diagrams produced by radiochemical analysis, relating to plutonium in urine and in faecal samples, and to tritium, strontium and enriched uranium in urine, are analysed.

Inhaled particle deposition and clearance from the normal respiratory tract

Tracheo-bronchial and pulmonary deposited fractions of inhaled insoluble particles and their clearances rates were studied in 16 healthy non-smokers. After oral inhalation of radioactive particles (Mean Mass Aerodynamic Diameter (MMAD) = 3 microns, sigma g = 1.4 labelled with 111In) incorporated radioactivity was measured for each subject both by a gamma camera from to until the 3rd day, then from the 3rd until the 5th day first by a gamma camera and then by a low background profile scanner and from the 5th until the 35th day by a low background profile scanner alone. Clearance rates were calculated from the biological half lives of the deposited tracers. All subjects performed respiratory function tests. Experimental data were fitted to a two-compartment exponential system with two biological half lives: T1 = 76 min for 30%, T2 = 3.15 days for 40% of the deposited material. The delayed clearance phase for the remaining 30% of the deposited material approaches a constant rate. Our clearance values were compared with those of the Task Group of Lung Dynamics (T.G.L.D.) and other authors' results, especially for T1 and T2. These values are analyzed in terms of mucus velocity and mucociliary transport in distal conductive airways. Impaired transport reduces natural defenses and increases toxicological hazards. Therefore reliable techniques for detecting such impairment may be important in evaluating pulmonary involvement in environmental respiratory disease.