Mor microvasculature post radiation therapy. Slightly lower MVD was observed in radiation treated tumors as compared to controls, and the 374913-63-0 web difference was not statistically significantly (14.7 vs. 12.0, Fig. 3). Long segments of the tubules formed by the MS1 cells [23] were observed in the tumor histopathologic slides but showed virtually no TUNEL or bgalactosidase staining, both in the radiation treated tumors and the controls, indicating that the observed changes were not likely influenced by radiation response of the MS1 cells. The contribution of ionizing radiation to cell apoptosis and senescence of MDA-MB-231 cells at 96 hrs post treatment was also studied in vitro. The apoptosis assay on treated and control cells demonstrated an increase in apoptosis after radiation (16.2 vs. 4.2 , Fig. 4). Similar to the tumors, a large increase in bgalactosidase positive cells were observed in treated cells as compared to control cells (64.6 vs. 4.9 , Fig. 4). The radiation treated MDA-MB-231 cells also appeared morphologically to be much larger than the controls cells, likely the result of cell senescence [38]. The average length of the cells increased significantly from 11.1 mm (stdev. = 2.7, n = 100) to 24.9 mm (stdev. = 8.2, n = 100) with radiation treatment (p,0.00001). The protein content increased five fold from 0.23 mg (stdev. = 0.035, n = 3) to 1.16 mg (stdev. = 0.125, n = 4) per 16106 cells 
post radiation (p,0.05). Changes in metabolic flux between pyruvate and lactate in the cell cultures were also investigated by 13C MRS after the cell suspensions were perfused with pre-polarized [1-13C]pyruvate. Lower lactate signal relative to the substrate signal was observed in the treated cells (36107 cells, total lactate/ pyruvate ratio = 0.11 and 0.14) as compared to controls (1.56108 cells, total lactate/pyruvate ratio = 0.27 and 0.39). The smaller number of post-treatment cells used in these experiments was chosen to keep the protein content ML-281 site constant. Western blot analysis was used to assess cell membrane monocarboxylate transport and lactate dehydrogenase levels to determine the association of these proteins with the observed decrease in metabolic flux between pyruvate and lactate. Tissue hypoxia in the tumors was also 
assessed by HIF1-a expression. In both radiation treated MDA-MB-231 tumors in vivo and cell in vitro, decreases in MCT4 expression were observed (Fig. 5. A and B) and the decrease in tumors was significant (P,0.03). An increase was found in HIF1-a expression for the treated tumors (Fig. 5. C), but the difference was not significant. Expressions of LDHA appeared unchanged between treated tumors and controls but significantly decreased LDHB expression was observed for the treated tumors (Fig. 5. D). Very little difference was found for both LDHA and LDHB expressions between the treated and control cells in vitro.DiscussionBy detecting changes in metabolic flux between key intermediates of cellular metabolism, hyperpolarized 13C metabolic imaging is a promising new tool for assessment of tumor grade and early response to therapies [6?1]. The detection of early response non-invasively may facilitate adaptive radiation therapy either alone or in conjunction with chemotherapy. With the emergence of hypofractionated and ablative radiotherapy regimens, and the advent of MR-guided linear accelerators, this technique offers the potential for functional tumor localization and delineation, and real-time tumour response assessment. In th.Mor microvasculature post radiation therapy. Slightly lower MVD was observed in radiation treated tumors as compared to controls, and the difference was not statistically significantly (14.7 vs. 12.0, Fig. 3). Long segments of the tubules formed by the MS1 cells [23] were observed in the tumor histopathologic slides but showed virtually no TUNEL or bgalactosidase staining, both in the radiation treated tumors and the controls, indicating that the observed changes were not likely influenced by radiation response of the MS1 cells. The contribution of ionizing radiation to cell apoptosis and senescence of MDA-MB-231 cells at 96 hrs post treatment was also studied in vitro. The apoptosis assay on treated and control cells demonstrated an increase in apoptosis after radiation (16.2 vs. 4.2 , Fig. 4). Similar to the tumors, a large increase in bgalactosidase positive cells were observed in treated cells as compared to control cells (64.6 vs. 4.9 , Fig. 4). The radiation treated MDA-MB-231 cells also appeared morphologically to be much larger than the controls cells, likely the result of cell senescence [38]. The average length of the cells increased significantly from 11.1 mm (stdev. = 2.7, n = 100) to 24.9 mm (stdev. = 8.2, n = 100) with radiation treatment (p,0.00001). The protein content increased five fold from 0.23 mg (stdev. = 0.035, n = 3) to 1.16 mg (stdev. = 0.125, n = 4) per 16106 cells post radiation (p,0.05). Changes in metabolic flux between pyruvate and lactate in the cell cultures were also investigated by 13C MRS after the cell suspensions were perfused with pre-polarized [1-13C]pyruvate. Lower lactate signal relative to the substrate signal was observed in the treated cells (36107 cells, total lactate/ pyruvate ratio = 0.11 and 0.14) as compared to controls (1.56108 cells, total lactate/pyruvate ratio = 0.27 and 0.39). The smaller number of post-treatment cells used in these experiments was chosen to keep the protein content constant. Western blot analysis was used to assess cell membrane monocarboxylate transport and lactate dehydrogenase levels to determine the association of these proteins with the observed decrease in metabolic flux between pyruvate and lactate. Tissue hypoxia in the tumors was also assessed by HIF1-a expression. In both radiation treated MDA-MB-231 tumors in vivo and cell in vitro, decreases in MCT4 expression were observed (Fig. 5. A and B) and the decrease in tumors was significant (P,0.03). An increase was found in HIF1-a expression for the treated tumors (Fig. 5. C), but the difference was not significant. Expressions of LDHA appeared unchanged between treated tumors and controls but significantly decreased LDHB expression was observed for the treated tumors (Fig. 5. D). Very little difference was found for both LDHA and LDHB expressions between the treated and control cells in vitro.DiscussionBy detecting changes in metabolic flux between key intermediates of cellular metabolism, hyperpolarized 13C metabolic imaging is a promising new tool for assessment of tumor grade and early response to therapies [6?1]. The detection of early response non-invasively may facilitate adaptive radiation therapy either alone or in conjunction with chemotherapy. With the emergence of hypofractionated and ablative radiotherapy regimens, and the advent of MR-guided linear accelerators, this technique offers the potential for functional tumor localization and delineation, and real-time tumour response assessment. In th.