Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, California 94143, USA.
J Nucl Med. 2013 Jun;54(6):922-8. doi: 10.2967/jnumed.112.115402. Epub 2013 Apr 10.
Reduction and oxidation (redox) chemistry is increasingly implicated in cancer pathogenesis. To interrogate the redox status of prostate tumors noninvasively, we developed hyperpolarized [1-(13)C]dehydroascorbate ((13)C-DHA), the oxidized form of vitamin C, as an MR probe. In a model of transgenic adenocarcinoma of the mouse prostate (TRAMP), increased reduction of hyperpolarized (13)C-DHA to vitamin C was observed in tumor, as compared with normal prostate and surrounding benign tissue. We hypothesized that this difference was due to higher concentrations of glutathione and increased transport of hyperpolarized (13)C-DHA via the glucose transporters (GLUT1, GLUT3, and GLUT4) in TRAMP tumor. To test these hypotheses, hyperpolarized (13)C-DHA MR spectroscopy (MRS) and (18)F-FDG PET were applied as complementary technologies in the TRAMP model.
Late-stage TRAMP tumors (>4 cm(3)) were studied at similar time points (MR studies conducted < 24 h after PET) in fasting mice by (18)F-FDG PET and hyperpolarized (13)C-DHA MR imaging on a small-animal PET/CT scanner and a (1)H/(3)C 3-T MR scanner. PET data were processed using open-source AMIDE software to compare the standardized uptake values of tumor with those of surrounding muscle, and (13)C-DHA MRS data were processed using custom software to compare the metabolite ratios (vitamin C/[vitamin C + (13)C-DHA]). After in vivo studies, the tumor glutathione concentrations were determined using a spectrophotometric assay, and thiol staining was performed using mercury orange. Real-time polymerase chain reaction was used to evaluate the relevant transporters GLUT1, GLUT3, and GLUT4 and vitamin C transporters SVCT1 and SVCT2. GLUT1 was also evaluated by immunohistochemistry.
The average metabolite ratio was 0.28 ± 0.02 in TRAMP tumor, versus 0.11 ± 0.02 in surrounding benign tissue (n = 4), representing a 2.5-fold difference. The corresponding tumor-to-nontumor (18)F-FDG uptake ratio was 3.0. The total glutathione was 5.1 ± 0.4 mM in tumor and 1.0 ± 0.2 mM in normal prostate, whereas reduced glutathione was 2.0 ± 0.3 mM and 0.8 ± 0.3 mM, respectively, corresponding to a 2.5-fold difference. In TRAMP tumor, mercury orange staining demonstrated increased thiols. Real-time polymerase chain reaction showed no significant difference in GLUT1 messenger RNA between TRAMP tumor and normal prostate, with immunohistochemistry (anti-GLUT1) also showing comparable staining.
Both hyperpolarized (13)C-DHA and (18)F-FDG provide similar tumor contrast in the TRAMP model. Our findings suggest that the mechanism of in vivo hyperpolarized (13)C-DHA reduction and the resulting tumor contrast correlates most strongly with glutathione concentration. In the TRAMP model, GLUT1 is not significantly upregulated and is unlikely to account for the contrast obtained using hyperpolarized (13)C-DHA or (18)F-FDG.
探究前列腺肿瘤的氧化还原状态，我们开发了超极化 [1-(13)C]脱氢抗坏血酸（13C-DHA）作为 MR 探针，它是维生素 C 的氧化形式。在转染的前列腺腺癌小鼠模型（TRAMP）中，与正常前列腺和周围良性组织相比，肿瘤中观察到超极化（13）C-DHA 向维生素 C 的还原增加。我们假设这种差异是由于谷胱甘肽浓度较高以及通过葡萄糖转运蛋白（GLUT1、GLUT3 和 GLUT4）转运超极化（13）C-DHA 的增加所致。为了验证这些假设，我们在 TRAMP 模型中应用了超极化（13）C-DHA MR 光谱（MRS）和（18）F-FDG PET 作为互补技术。
在禁食小鼠中，在类似的时间点（MR 研究在 PET 后 <24 小时进行），使用小型动物 PET/CT 扫描仪和 1H/13C 3-T MR 扫描仪对晚期 TRAMP 肿瘤（>4 cm3）进行（18）F-FDG PET 和超极化（13）C-DHA MR 成像。使用开源 AMIDE 软件处理 PET 数据，以比较肿瘤与周围肌肉的标准化摄取值，使用定制软件处理（13）C-DHA MRS 数据，以比较代谢物比（维生素 C/[维生素 C + （13）C-DHA]）。在体内研究之后，使用分光光度法测定肿瘤中的谷胱甘肽浓度，并使用汞橙进行巯基染色。使用实时聚合酶链反应评估相关转运蛋白 GLUT1、GLUT3 和 GLUT4 以及维生素 C 转运蛋白 SVCT1 和 SVCT2。还通过免疫组织化学评估 GLUT1。
TRAMP 肿瘤的平均代谢物比为 0.28±0.02，周围良性组织为 0.11±0.02（n=4），代表 2.5 倍的差异。相应的肿瘤与非肿瘤（18）F-FDG 摄取比为 3.0。肿瘤中的总谷胱甘肽为 5.1±0.4 mM，正常前列腺中为 1.0±0.2 mM，而还原型谷胱甘肽分别为 2.0±0.3 mM 和 0.8±0.3 mM，对应 2.5 倍的差异。在 TRAMP 肿瘤中，汞橙染色显示增加的巯基。实时聚合酶链反应显示 TRAMP 肿瘤和正常前列腺之间 GLUT1 信使 RNA 没有显着差异，免疫组织化学（抗 GLUT1）也显示出相似的染色。
超极化（13）C-DHA 和（18）F-FDG 在 TRAMP 模型中均提供相似的肿瘤对比。我们的发现表明，体内超极化（13）C-DHA 还原的机制及其导致的肿瘤对比与谷胱甘肽浓度最密切相关。在 TRAMP 模型中，GLUT1 没有显着上调，不太可能解释使用超极化（13）C-DHA 或（18）F-FDG 获得的对比。
