Hyperpolarized in vivo pH imaging reveals grade-dependent acidification in prostate cancer - PubMed (original) (raw)
Hyperpolarized in vivo pH imaging reveals grade-dependent acidification in prostate cancer
David E Korenchan et al. Oncotarget. 2019.
Abstract
There is an unmet clinical need for new and robust imaging biomarkers to distinguish indolent from aggressive prostate cancer. Hallmarks of aggressive tumors such as a decrease in extracellular pH (pHe) can potentially be used to identify aggressive phenotypes. In this study, we employ an optimized, high signal-to-noise ratio hyperpolarized (HP) 13C pHe imaging method to discriminate between indolent and aggressive disease in a murine model of prostate cancer. Transgenic adenocarcinoma of the mouse prostate (TRAMP) mice underwent a multiparametric MR imaging exam, including HP [13C] bicarbonate MRI for pHe, with 1H apparent diffusion coefficient (ADC) mapping and HP [1-13C] pyruvate MRI to study lactate metabolism. Tumor tissue was excised for histological staining and qRT-PCR to quantify mRNA expression for relevant glycolytic enzymes and transporters. We observed good separation in pHe between low- and high-grade tumor regions, with high-grade tumors demonstrating a lower pHe. The pHe also correlated strongly with monocarboxylate transporter Mct4 gene expression across all tumors, suggesting that lactate export via MCT4 is associated with acidification in this model. Our results implicate extracellular acidification as an indicator of indolent-to-aggressive transition in prostate cancer and suggest feasibility of HP pHe imaging to detect high-grade, clinically significant disease in men as part of a multiparametric MRI examination.
Keywords: MRI; extracellular pH; hyperpolarization; metabolism; prostate cancer.
Copyright: © 2019 Korenchan et al.
Conflict of interest statement
CONFLICTS OF INTEREST The authors declare no conflicts of interest.
Figures
Figure 1. Schematic of multiparametric MR imaging protocol utilized in this work.
(A) Hyperpolarized [13C] bicarbonate was obtained by polarizing [1-13C]1,2-glycerol carbonate, which was rapidly hydrolyzed immediately prior to injection using base and heat, followed by neutralization. This approach generates a pH-neutral solution of HP [13C] bicarbonate with high signal for in vivo pHe imaging. (B) Transgenic mice with prostate lesions were subjected to 1H diffusion-weighted imaging as well as two separate hyperpolarized 13C injections of [1-13C] pyruvate and [13C] bicarbonate with MR spectroscopic imaging in a single imaging study. These imaging methods enabled measurement of tumor cellularity, glycolytic metabolism, and extracellular acidification, respectively.
Figure 2. 1H MR imaging and histological staining of TRAMP tumor tissue display differences in aggressive phenotype.
(A) Anatomical 1H MR images of TRAMP tumors during different stages of tumor progression, displaying predominantly low-grade lesions, predominantly high-grade lesions, and lesions with distinct low- and high-grade regions (“Low + High”), identified by MRI and confirmed via histology. These correspond with mice 2, 9, and 5, respectively, as listed in the Supporting Information and Methods, Supplementary Table 1. (B) 1H apparent diffusion coefficient (ADC) maps for the same mice as in part (A), demonstrating a reduction in ADC in high grade tumors. (C–E) Tumor tissue corresponding with MRIs in parts (A–B) and stained with (C) H&E (D) Ki-67 proliferation marker stain; and (E) anti-pimonidazole hypoxia stain. Images demonstrate differences in morphology, cellularity, and hypoxia between low- and high-grade tumor tissues. All microscope images were acquired with 40× magnification.
Figure 3. Low- and high-grade TRAMP lesions exhibit differences in 1H apparent diffusion coefficient (ADC) and Ki-67 proliferation staining.
(A) Scatter plots of mean 1H ADC between low- and high-grade regions, demonstrating a statistically significant difference (n = 5 low-grade, n = 7 high-grade). (B) Quantitative low- and high-grade comparison between percentage areas stained positive for Ki-67 nuclear stain (n = 5 low-grade, n = 7 high-grade). ***p < 0.005.
Figure 4. Hyperpolarized [1-13C] pyruvate imaging in TRAMP mice confirms glycolytic differences between low- and high-grade cancers.
(A) Representative overlays of hyperpolarized lactate-to-pyruvate ratio (Lac/Pyr) for the mice displayed in Figure 2. (B) Scatter plots demonstrating a significant difference in mean Lac/Pyr ratio obtained from HP 13C images between low- and high-grade regions over all tumors (n = 5 low-grade, n = 7 high-grade). (C) The mean Lac/Pyr ratio demonstrated a negative correlation with 1H ADC for each tumor that was statistically significant (p = 0.0488). *p < 0.05.
Figure 5. Extracellular pH measurements in TRAMP tumors via hyperpolarized [13C] bicarbonate imaging suggest extracellular acidification is associated with high-grade disease.
(A) Representative overlays of extracellular pH measured with hyperpolarized [13C] bicarbonate for the mice displayed in Figure 2. (B–C) Scatter plots demonstrating significant differences in pH metrics obtained from HP 13C images between low- and high-grade regions over all mice: (B) mean; and (C) regional-minimum (n = 5 low-grade, n = 7 high-grade). *p < 0.05.
Figure 6. Correlations between metabolic parameters and pHe.
(A) Spearman non-parametric regression between maximum Lac/Pyr and minimum pHe in each ROI showed a strong negative correlation (n = 12 lesions). (B) The measured Lac/Pyr ratio and pHe in each imaging voxel across all mice were negatively correlated. The pH decreased by about 0.15 unit per unit increase in Lac/Pyr ratio (n = 153 voxels). (C) Spearman non-parametric regression between mean pH and Mct4 gene expression was strongly negative and statistically significant (n = 10 lesions). Two of the twelve lesions imaged had Mct4 gene expression levels below the threshold of quantification; these lesions had average pHe values of 7.30 and 7.56. ρ and _p_-value in plot exclude the outlier; the correlation was still significant with outlier included (p = 0.00824). *** p < 0.005.
Figure 7. Summary of imaging and gene expression findings in TRAMP tumors.
High-grade disease exhibits a metabolic shift and extracellular acidification, which can both be detected through imaging with hyperpolarized 13C methods. (A). Low-grade tumors demonstrate low levels of LDH activity, low lactate production, and a normal pHe. (B) High-grade TRAMP tumors upregulate LDHA, produce increased lactate, upregulate MCT4, and have a lower pHe.
References
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