Intracellular Sequestration of223Ra by the Iron-Storage Protein Ferritin

Incorporation of bone-seeking, alpha-particle-emitting, heavy-metal radionuclides dramatically increases the incidence of osteosarcoma in humans and experimental animals. The accumulation of these radionuclides within the mineral phase of the bone matrix is believed to result in local irradiation of only those proliferating cells close to the bone surface. We now present evidence for a more general pathway for the irradiation of target cells, mediated through the sequestration of heavy-metal radionuclides by the intracellular iron-storage protein ferritin. In vitro studies reveal the transfer of radionuclide from a 223Ra-transferrin complex into immunoprecipitable cytosolic ferritin. In vivo studies confirm the co-localization of incorporated 224Ra and cellular iron stores. This pathway would result in the highly localized irradiation of ferritin-containing cells. Since osteoblastic cells express large quantities of a ferritin isoform specialized in long-term metal storage, we suggest that this may represent an unrecognized source of intracellular irradiation by alpha-particle-emitting radionuclides. Such a local concentration within target cells has implications both for cellular dosimetry and for inferences of track length and target cell populations within the skeleton.

Radiation-induced systemic and local bone tumors: two types of late effects with possible different origins?

Bone sarcomas may be induced throughout the skeleton (systemic) in mice by relatively low internal alpha-particle doses that are distributed over the whole skeleton. The induction of local (periosteal) bone sarcomas after paratibial deposition of insoluble radiocolloids required much higher doses, and in addition high energies of emitted particles. Paratibial deposition of alpha-particle-emitting radiocolloids of 227Th and 228Th resulted in formation of both local and systemic bone sarcomas. The latter were most probably induced by the released radium daughters of the thorium isotopes and were distributed about the skeleton. Paratibial injections with beta-particle emitters 144Ce+ 144Pr (29 kBq per mouse) showed an incidence of local bone sarcomas of more than 80%. An estimation of the local effective doses led to values of more than 1000 Gy for the beta-particle emitter 144Ce and around 150 Gy for the thorium isotopes. Thus induction of local bone sarcomas required doses considerably greater than those needed for systemic bone sarcomas. The local induction of bone sarcomas has been reported for high-energy beta particles using similar high doses of 144Ce+ 144Pr in rats and for external 90Sr+ 90Y irradiation in mice. We conclude that the processes involved in the induction of local and systemic bone sarcomas by radiation may be quite different.

Osteosarcomagenic doses of radium (224Ra) and infectious endogenous retroviruses enhance proliferation and osteogenic differentiation of skeletal tissue differentiating in vitro

Cartilage tissue from embryonic mice which undergoes osteogenic differentiation during in vitro cultivation was used to study the effect of osteosarcomagenic doses of alpha-irradiation and bone-tumor-inducing retroviruses on proliferation and phenotypic differentiation of skeletal cells in a defined tissue culture model. Irradiated mandibular condyles showed dose-dependent enhancement of cell proliferation at day 7 of the culture and increased osteogenic differentiation at day 14. Maximal effects were found with 7.4 Bq/ml of 224Ra-labeled medium. Doses of 740 and 7400 Bq/ml of 224Ra-labeled medium induced increasing cell death. Retrovirus infection enhanced osteogenic differentiation and extended the viability of irradiated cells. After transplantation none of the treated tissues developed tumors in syngeneic mice.

Osteosarcoma risk after simultaneous incorporation of the long-lived radionuclide 227Ac and the short-lived radionuclide 227Th

The effect of injection of 1.85 kBq/kg of the long-lived radionuclide 227Ac on the induction of osteosarcomas in female NMRI mice by different dose levels (18.5, 74, and 185 kBq/kg) of the short-lived radionuclide 227Th was investigated. The highest absolute osteosarcoma incidence was observed with the highest doses of 227Th. Addition of 227Ac resulted in an additional osteosarcoma incidence only at the lowest dose of 227Th and did not affect the osteosarcoma incidence resulting from higher doses of 227Th. The longest times to tumor appearance were observed with 227Ac alone. The latent period in two different age groups (4 weeks and 10-12 weeks) appeared to be similar following injection with combined doses of 227Th and 227Ac but different after injection of each radionuclide alone.

Studies on incorporated short-lived beta-emitters with regard to the induction of late effects

The rare earth radionuclides 177 Lu and 153Sm were administered as single i.p. injections in NMRI mice. Lu was deposited principally (up to 60%) in the skeleton if the quantity of stable carrier was low. Increase of stable carrier enhanced deposition in the reticulo-endothelial system. Sm was preferentially deposited in the liver; the liver deposits were further increased by the addition of stable Sm. Liver doses of between 75 and 150 Gy, resulting from a single injection of 153Sm together with 2 mg/kg stable carrier, led to severe lesions in the liver five months after treatment. Administration of 177Lu resulting in skeletal doses of between 28 and 224 Gy was found to be osteosarcomogenic. Up to 40% osteosarcoma incidence was obtained in animals with 56 and 112 Gy doses in the skeleton. Skeletal doses of this order of magnitude are also known to be osteosarcomogenic when given as 90Sr injections. The analogous situation with alpha-emitters is discussed.