A physiological skeletal model for radionuclide and stable element biokinetics in children and adults

The Effects of Calcium, Magnesium, Phosphorus, Fluoride, and Lead on Bone Tissue

Bones are metabolically active organs. Their reconstruction is crucial for the proper functioning of the skeletal system during bone growth and remodeling, fracture healing, and maintaining calcium-phosphorus homeostasis. The bone metabolism and tissue properties are influenced by trace elements that may act either indirectly through the regulation of macromineral metabolism, or directly by affecting osteoblast and osteoclast proliferation or activity, or through becoming part of the bone mineral matrix. This study analyzes the skeletal impact of macroelements (calcium, magnesium, phosphorus), microelements (fluorine), and heavy metals (lead), and discusses the concentration of each of these elements in the various bone tissues.

Proposed Modification to the Plutonium Systemic Model

The currently accepted biokinetic model for plutonium distribution within the human body was recommended by the International Commission on Radiological Protection in publication 67. This model was developed from human and animal studies and behavioral knowledge acquired from other known bone-seeking radionuclides. The biokinetic model provides a mathematical means of predicting the distribution, retention, and clearance of plutonium within the human body that may be used in deriving organ, tissue, and whole body dose. This work proposed a modification to the ICRP 67 systemic model for plutonium that incorporated the latest knowledge acquired from recent human injection studies with physiologically based improvements. In summary, the changes included a separation of the liver compartments, removed the intermediate soft tissue-to-bladder pathway, and added pathways from the blood compartment to both the cortical and trabecular bone volumes. The proposed model provided improved predictions for several bioassay indicators compared to the ICRP 67 model while also maintaining its basic structure. Additionally, the proposed model incorporated physiologically based improvements for the liver and skeleton and continued to ensure efficient coupling with intake biokinetic models.

Changes in bone Pb accumulation: cause and effect of altered bone turnover

This paper assesses the magnitude of Pb uptake in cortical and trabecular bones in healthy animals and animals with altered balance in bone turnover, and the impact of exposure to Pb on serum markers of bone formation and resorption. The results reported herein provide physiological evidence that Pb distributes differently in central compartments in Pb metabolism, such as cortical and trabecular bones, in healthy animals and animals with altered balance in bone turnover, and that exposure to Pb does have an impact on bone resorption resulting in OC-dependent osteopenia. These findings show that Pb may play a role in the etiology of osteoporosis and that its concentration in bones varies as a result of altered bone turnover characteristic of this disease, a long standing question in the field. In addition, data collected in this study are consistent with previous observations of increased half-life of Pb in bone at higher exposures. This evidence is relevant for the necessary revision of current physiologically based kinetic models for Pb in humans.

Revision of the biodistribution of uranyl in serum: is fetuin-A the major protein target?

Uranium is a natural actinide present as uranyl U(VI) species in aqueous environments. Its toxicity is considered to be chemical rather than radiotoxicological. Whatever the route of entry, uranyl reaches the blood, is partly eliminated via the kidneys, and accumulated in the bones. In serum, its speciation mainly involves carbonate and proteins. Direct identification of labile uranyl-protein complexes is extremely difficult because of the complexity of this matrix. Thus, until now the biodistribution of the metal in serum has not been described, and therefore, little is known about the metal transport mechanisms leading to bone accumulation. A rapid screening method based on a surface plasmon resonance (SPR) technique was used to determine the apparent affinities for U(VI) of the major serum proteins. A first biodistribution of uranyl was obtained by ranking the proteins according to the criteria of both their serum concentrations and affinities for this metal. Despite its moderate concentration in serum, fetuin-A (FETUA) was shown to exhibit an apparent affinity within the 30 nM range and to carry more than 80% of the metal. This protein involved in bone mineralization aroused interest in characterizing the U(VI) and FETUA interaction. Using complementary chromatographic and spectroscopic approaches, we demonstrated that the protein can bind 3 U(VI) at different binding sites exhibiting Kd from ∼30 nM to 10 μM. Some structural modifications and functional properties of FETUA upon uranyl complexation were also controlled. To our knowledge, this article presents the first identification of a uranyl carrier involved in bone metabolism along with the characterization of its metal binding sites.

Stem cell niches and other factors that influence the sensitivity of bone marrow to radiation-induced bone cancer and leukaemia in children and adults

PURPOSE

This paper reviews and reassesses the internationally accepted niches or 'targets' in bone marrow that are sensitive to the induction of leukaemia and primary bone cancer by radiation.

CONCLUSIONS

The hypoxic conditions of the 10 μm thick endosteal/osteoblastic niche where preleukemic stem cells and hematopoietic stem cells (HSC) reside provides a radioprotective microenvironment that is 2- to 3-fold less radiosensitive than vascular niches. This supports partitioning the whole marrow target between the low haematological cancer risk of irradiating HSC in the endosteum and the vascular niches within central marrow. There is a greater risk of induced bone cancer when irradiating a 50 μm thick peripheral marrow adjacent to the remodelling/reforming portion of the trabecular bone surface, rather than marrow next to the quiescent bone surface. This choice of partitioned bone cancer target is substantiated by the greater radiosensitivity of: (i) Bone with high remodelling rates, (ii) the young, (iii) individuals with hypermetabolic benign diseases of bone, and (iv) the epidemiology of alpha-emitting exposures. Evidence is given to show that the absence of excess bone-cancer in atomic-bomb survivors may be partially related to the extremely low prevalence among Japanese of Paget's disease of bone. Radiation-induced fibrosis and the wound healing response may be implicated in not only radiogenic bone cancers but also leukaemia. A novel biological mechanism for adaptive response, and possibility of dynamic targets, is advocated whereby stem cells migrate from vascular niches to stress-mitigated, hypoxic niches.