NASA Human Research Program Investigators' Workshop (2012)
4090.pdf
IONIZING RADIATION CAUSES ACTIVE DEGRADATION AND REDUCES NEW MATRIX
SYNTHESIS IN HUMAN ARTICULAR CARTILAGE
J.S. Willey1 , D.L. Long2, and R.F. Loeser2
1
Wake Forest School of Medicine Department of Radiation Oncology, 2 Wake Forest School of Medicine Section
of Molecular Medicine
PURPOSE: Ionizing radiation can cause a persistent reduction in the structural integrity of skeletal tissue,
particularly bone. However, little is known regarding radiation effects on articular (joint) cartilage. Though the
chondrocytes within articular cartilage are radioresistent in terms of viability, progressive deterioration of cartilage
has been reported from various joints exposed to radiation following cancer treatment or occupational exposures.
New preclinical evidence indicates a loss of compressive stiffness of cartilage after a single 2 Gy dose of photons.
The spaceflight environment, with characteristic microgravity and radiation, may therefore represent a hazard for
astronaut joint health as limb disuse on earth has been shown to cause cartilage thinning. IGF-1 is an important
growth factor for articular cartilage matrix production, stimulating proteoglycan (PG) and collagen synthesis
through activation of the PI3-kinase-Akt pathway. We examined the effects of 2 and 10 Gy single dose (137) Cs
gamma-ray exposure on cartilage degradation, PG synthesis, and IGF-1 signaling in human primary chondrocytes
and explants.
METHODS: Chondrocytes and explants were isolated from the normal (non-osteoarthritic) ankles of human
donors; tissue was digested for monolayer assays. Confluent cultures were irradiated, then stimulated with IGF-1
after 1 hour. Immunoblotting of cell lysates at 1 day after IGF-1 stimulation was used to measure phospho- and total
Akt and IGF-1 Receptor (IGF-1R) yields. Proteoglycan synthesis via 35S uptake was quantified from cells 24 hours
after IGF stimulation. Whole explants were also cultured. PG content from the conditioned media of explants was
determined at day 2 and 4 after exposure. Immunoblotting of media measured matrix degrading enzymes
ADAMTS5 (Aggrecanase), MMP-13, and MMP-1 produced from cells at day 1.
RESULTS: Cell viability was unchanged after irradiation. Doses of 2 and 10 Gy stimulated increased ADAMTS5,
MMP-1, and MMP-13 production (day 1) as well as PG released (day 2) into media, suggesting early and active
degradation. While IGF-1 stimulation increased Akt phosphorylation at Thr 308 and Ser 473 and increased PG
synthesis by control chondrocytes, 2 and 10 Gy exposures inhibited both IGF-induced Akt activation and PG
synthesis. Total and phosphorylated IGF-1R were lower following irradiation, both with and without IGF-1
stimulation.
CONCLUSION: Radiation appears to induce an active degradation of cartilage matrix, reduces PG synthesis, and
impairs normal IGF-1 signaling in chondrocytes. Reduced IGF-1 receptor signaling may account for lowered
activation of the PI3-kinase-Akt pathway, resulting in lowerd matrix synthesis. Loss of cartilage matrix coupled with
lower matrix production could decrease stiffness. Radiation may ultimately cause a functional decline of cartilage
health in joints after exposure, and when combined with unloading during spaceflight could further increase the risk
for athropathy in astronauts during long duration missions.