Modulation of blood flow, hypoxia, and vascular function in orthotopic prostate tumors during exercise

Abstract

Background: Previous studies have hypothesized that tumor blood flow may be elevated or reduced during exercise, which could impact the tumor microenvironment. However, to date technical limitations have precluded the measurement of tumor blood flow during exercise. Using an orthotopic preclinical model of prostate cancer, we tested the hypotheses that during exercise tumors would experience 1) diminished vascular resistance, 2) augmented blood flow, 3) increased numbers of perfused vessels, and 4) decreased tissue hypoxia and, furthermore, that the increased perfusion would be associated with diminished vasoconstriction in prostate tumor arterioles.

Methods: Dunning R-3327 MatLyLu tumor cells were injected into the ventral prostate of male Copenhagen rats aged 4 to 6 months randomly assigned to tumor-bearing (n = 42) or vehicle control (n = 14) groups. Prostate tumor blood flow, vascular resistance, patent vessel number, and hypoxia were measured in vivo in conscious rats at rest and during treadmill exercise, and vasoconstrictor responsiveness of resistance arterioles was investigated in vitro.

Results: During exercise there was a statistically significant increase in tumor blood flow (approximately 200%) and number of patent vessels (rest mean ± standard deviation [SD] = 12.7±1.3; exercise mean ± SD = 14.3±0.6 vessels/field; Student t test two-sided P = .02) and decreased hypoxia compared with measurements made at rest. In tumor arterioles, the maximal constriction elicited by norepinephrine was blunted by approximately 95% vs control prostate vessels.

Conclusions: During exercise there is enhanced tumor perfusion and diminished tumor hypoxia due, in part, to a diminished vasoconstriction. The clinical relevance of these findings are that exercise may enhance the delivery of tumor-targeting drugs as well as attenuate the hypoxic microenvironment within a tumor and lead to a less aggressive phenotype.

Figure 1.

A) Blood flow at rest and during the steady-state of exercise in the prostate of control (n = 6) rats and prostate and prostate tumor of tumor-bearing (n = 7 per group) rats. B) Relative change in vascular resistance in the exercising vs resting condition in the prostate of control (n = 6) rats, and prostate and prostate tumor of tumor-bearing (n = 7 per group) rats. Error bars represent the standard deviation. *P ≤ .001 vs resting conditions within the same tissue. †P ≤ .001 vs prostate of control group during same condition. ‡P ≤ 0.001 vs prostate of the tumor-bearing group during same condition. All statistical tests were two-sided. A two-way analysis of variance with repeated measures was used to compare within-group (rest vs exercise) and between-group (control vs tumor-bearing) differences in blood flow. A Wilcoxon signed-rank test was used to compare percentage changes in vascular resistance from rest to exercise.

Figure 2.

Fluorescence photomicrograph of a prostate tumor cross-sectional field, obtained from a MatLyLu Copenhagen rat prostate carcinoma after intravenous injection of Hoechst-33342 (40mg/kg) at rest (A) and during acute exercise (B). The cells of the vessels perfused at the time of injection are fluoresced. C) Effect of an acute bout of exercise on patent prostatic tumor blood vessels per field (at least 30 fields per tumor were measured; see Methods for further information). Rats were injected with Hoechst-33342 at rest (n = 8) or during exercise (n = 7). Error bars represent the standard deviation. *P = .02 vs. resting condition. All statistical tests were two-sided. A Student t test was used to determine differences in perfused vessel density.

Figure 3.

Fluorescence photomicrograph of a prostate tumor cross-section, obtained from a MatLyLu Copenhagen rat prostate carcinoma after intraperitoneal injection of EF5 (30mg/kg) at rest (A) and during acute exercise (B). The EF5 binds to hypoxic regions and is fluoresced. C) Graphical representation of the fraction of tissues bound by EF5 (ie, hypoxic fraction). EF5 was injected and allowed to circulate for 1 hour during rest (n = 7) or exercise (n = 7). Error bars represent the standard deviation. *P < .001 vs resting condition. All statistical tests were two-sided. A Student t test was used to determine differences in hypoxic fraction.

Figure 4.

Dose–response relations to adrenergic receptor agonist norepinephrine in arterioles perforating the prostate from control (n = 8) rats and the prostatic tumor of tumor-bearing rats (n = 6). Error bars represent the standard deviation. *P < .001 vs prostate arteriolar constriction at a given does of norepinephrine. †P = .006 versus prostate arteriolar constriction at matched dose of norepinephrine. All statistical tests were two-sided. Dose–response curves were analyzed by two-way analysis of variance with repeated measures.

Figure 5.

A) Active and and passive diameter responses to increasing intraluminal pressure in arterioles perforating the prostate from control (n = 8) rats and the prostatic tumor of tumor-bearing rats (n = 6). Values are normalized to diameter at 90cm water (H₂O). B) Resting myogenic tone across the physiological range of pressures arteries are subjected to. Values are normalized to diameter at 90cm H₂O and expressed as a percentage change in diameter relative to passive myogenic tone. Error bars represent the standard deviation. *P < .001 vs control myogenic tone at a matched intraluminal pressure. groups. †P < .001 vs control passive at that given intraluminal pressure. ‡P = .06 between group responses for active myogenic response. All statistical tests were two-sided. Pressure–diameter curves were analyzed by two-way analysis of variance with repeated measures.