International Journal of Radiation Oncology*Biology*Physics
Biology contributionDifferential oxygen dynamics in two diverse Dunning prostate R3327 rat tumor sublines (MAT-Lu and HI) with respect to growth and respiratory challenge☆
Introduction
Hypoxia in solid tumors has been widely recognized as a potent factor, which leads to resistance to radiotherapy 1, 2, photodynamic therapy (3), and some anticancer drugs (1). Further, recent studies suggest that tumor hypoxia might also be associated with malignant progression in solid tumors 4, 5. Therefore, accurate measurement of tumor oxygenation, assessment of levels of hypoxia in individual tumors, and the development of effective methods to reduce the hypoxic fraction may well contribute to therapeutic outcome. Given the importance of oxygen, many techniques for monitoring oxygen tension (pO2) have been developed (6). While each method has specific attributes, many are highly invasive and impractical for longitudinal studies of specific regions of interest. Nuclear magnetic resonance (NMR) is entirely noninvasive: 31P NMR provides an indirect estimate of hypoxia based on phosphorylation potential (7), but the measured metabolic hypoxia occurs at a higher pO2 than radiobiological hypoxia, and some studies have shown a lack of correlation between high-energy phosphate metabolites and pO2 (8). Blood Oxygen Level Dependent (BOLD) contrast proton magnetic resonance imaging (MRI) provides an indication of tumor vascular oxygenation, and heterogeneity, in response to intervention, but the method does not provide pO2 values and interpretation may be complicated by flow, hence, the concept FLOOD (FLOw and Oxygenation Dependent contrast) (9).
We recently demonstrated the feasibility of measuring tumor oxygenation based on 19F NMR echo planar imaging (EPI) after direct intratumoral injection (i.t.) of hexafluorobenzene (HFB) 10, 11, for which we have chosen the acronym FREDOM (Fluorocarbon Relaxometry using Echo planar imaging for Dynamic Oxygen Mapping). This technique allows us to assess baseline pO2 at multiple locations within a tumor, and to follow dynamic changes in response to interventions. Hexafluorobenzene has many strengths as a reporter molecule; it is readily available, cheap, and nontoxic. In terms of NMR, the sixfold symmetry provides a single 19F signal offering maximum signal to noise, and the long relaxation times (T1 and T2) facilitate echo planar imaging. The spin lattice relaxation rate R1 is very sensitive to changes in pO2, but shows minimal response to variations in temperature. HFB is readily administered through a fine needle and remains at the site of administration for several hours (t1/2 typically 600 min) (12).
We have now applied the technique to investigate oxygen distribution and dynamics in two rat prostate tumor sublines exhibiting diverse characteristics. Although the baseline oxygenation of the moderately well-differentiated subline (HI) has been investigated previously using electrodes (13), we are unaware of previous investigations of oxygenation in the highly metastatic and poorly differentiated MAT-Lu subline. Furthermore, comparison of response to interventions, here respiratory challenge with oxygen and carbogen, is now established using a single technique for comparison of both sublines.
Section snippets
Methods and materials
Experiments were approved by the Institutional Animal Care and Research Advisory Committee.
Results
Histology shows distinctly different characteristics for the two sublines: the HI appears moderately well-differentiated with uniform sized tumor cells, pseudoglandular structures, and large vesicles (Fig. 1A). By comparison, the MAT-Lu appears poorly differentiated with cellular and nuclear variations in size and shape and no glandular structure (Fig. 1B). PCNA immunostaining also shows a higher proliferation rate in the MAT-Lu than the HI tumors (Fig. 1).
Hexafluorobenzene was readily
Discussion
The oxygen tension dynamics observed here demonstrate that response to gaseous intervention can be very different for sublines of a single parental tumor type. The relatively hypoxic regions of the well-differentiated HI subline responded to elevated inhaled oxygen, whereas those of the undifferentiated MAT-Lu subline did not. Tumors of a given subline behaved consistently.
In common with our previous investigations of the undifferentiated anaplastic Dunning prostate R3327-AT1 subline (VDT ∼5
Acknowledgements
We are grateful to Drs. Peter Antich and Peter Peschke for collegial support and Drs. Sophia Ran and Mark Jeffrey for technical assistance.
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This work was supported in part by NIH RO1 CA79515, the American Cancer Society (RPG-97-116-010CCE), and a postdoctoral fellowship from the DOD Prostate Cancer Initiative (DAMD 170110108) (DZ). NMR experiments were performed at the Mary Nell and Ralph B. Rogers MR Center, an NIH BRTP Facility P41-RR02584.