The therapeutic index of treating cancer with ionizing radiation (IR) can be increased by minimizing normal tissue toxicity. Unfortunately, the therapies that have been developed to date have had limited efficacy. Therefore, identifying novel targets to protect normal tissue is essential during treatment.
Colin E. Champ, MD, Lianjin Jin, MD, PhD, Nicole L. Simone, MD; Department of Radiation Oncology, University of Pittsburgh Cancer Institute; UPMC CancerCenter, UPMC St. Margaret; Kimmel Cancer Center and Jefferson Medical College of Thomas Jefferson University
Purpose and Objectives: The therapeutic index of treating cancer with ionizing radiation (IR) can be increased by minimizing normal tissue toxicity. Unfortunately, the therapies that have been developed to date have had limited efficacy. Therefore, identifying novel targets to protect normal tissue is essential during treatment. Advanced glycation end products (AGEs), such as NÎµ-(carboxymethyl)-lysine (CML) and pentoside, may be one such class of molecules. AGEs occur when glucose is oxidized and forms irreversible crosslinks with collagen. This allows for sustained/chronic damage. AGEs are thought to be responsible for the end-organ damage noted in diabetics with chronic hyperglycemia, such as nephropathy, cataracts, and retinopathy, when ultraviolet irradiation (UVIR) interacts with the optical structures, inducing collagen crosslinks. Since UVIR induces most of its damage by free radicals, AGEs may be responsible in part for IR treatment–associated normal tissue toxicity. Therefore, we sought to determine if AGEs are induced by IR and if they are sustained over an extended period of time.
Materials and Methods: HEK293 cells were exposed to sham irradiation or 6 Gy of ionizing radiation (PanTek X-RAD 320 irradiator) and collected at 0.5, 3, and 24 hours after radiation exposure. Cells were collected, lysed with RIPA buffer (50 mM Tris-Cl, 150 mM NaCl, 1% NP40, 0.25% Na-deoxycholate, 1 × Mini protease inhibitor cocktail and phosphatase inhibitor cocktail), and quantified with BCA protein assay kit (Thermo Scientific, IL). AGE levels were then measured using a CML enzyme-linked immunosorbent assay (ELISA test) in our samples and compared with the four-parameter standard curve for the CML standards (Echelon Bioscience, UT).
Results: Twenty-four hours after irradiation, AGE production was elevated to 101.8 ng/dL vs 74.5 ng/dL in the control arm. The time course revealed that AGEs were induced and maintained at a level above 100 ng/dL at all time points, including 0.5, 3, and 24 hours. At the 0.5- and 24-hour time points, AGE levels were elevated to 103.9 vs 65.6 ng/dL in the control and 109.4 vs 86.1 ng/dL in the control arm, respectively. This increase was significant at all time points, with a P value < .001.
Conclusions: AGE formation may be a potential cause of acute and chronic toxicity from radiation. We have demonstrated for the first time that AGEs are induced by IR and may be a potential target to reduce toxicity. AGE inhibitors, such as carnosine, have already been developed and may potentially be used therapeutically to inhibit AGE formation and reduce radiation toxicity.