DNA Turnover in Metastasis-free Organs of Tumor Bearing Mice; Influence on Whole Body Measurements with 125I-UdR

Abstract

Introduction A preceding study had shown that mice bearing sarcoma-180 incorporated 40V0 more 5- 125I-2′-deoxyuridine ( 125I-UdR) into DNA of long lived cells outside the tumor than normal controls. This was expressed by external whole body measurements of living animals over a period of 4 weeks following one single intravenous injection of 125I-UdR. This paper analyses the extent to which various tumor-free organs of the tumor bearing host contribute to the increased 125I-UdR incorporation. Material and Method Adult virgin albino NMRI mice bearing 4 days old implants of sarcoma-180 received a single i.v. injection of 125I-UdR in order to label the DNA of proliferating cells in the whole body. I-UdR not incorporated within about 1 hour after injection is rapidly catabolized and excreted when the animals are kept on 0.1% NaI drinking water in order to block the pool of inorganic iodine. At day 1, 6, 10 and 15 after application of I-UdR 10 mice were assayed in a whole body counter and sacrificed by cervical luxation; the activity was determined in liver, spleen, kidneys, lungs, femora, thymus, mesenteric lymphnodes, and skin, also in the dissected tumor. The organ activities were compared with those of normal mice. Results It was confirmed that from day 6 to day 15 after I-UdR injection the whole body activity of tumor carriers, after resection of the tumor, was about 40% higher than in controls (Fig. 3). 1 to 15 days after injection of 125I-UdR the liver remained approximately 6–7 times more active in the tumor bearing than in normal mice. In the spleen, the activity in tumor carriers at day 1 was 5–6 times higher; thereafter, the difference declined to about 2.5–3.0 and remained so until the end of the observations (Fig. 1). In kidney and lung of the tumor carriers the activity at day I was elevated by a factor of about 2.8. The difference remained nearly constant in the kidney; yet in the lungs it converged to a factor of about 1.3 at day 15. In the femora of tumor carriers the activity content was elevated by a factor of 2.3 above that of controls on day 1. Thereafter, there was no difference in activity between the two groups. No significant difference between tumor bearing and normal mice was found in thymus, mesenteric lymphnodes, and skin (Fig. 2). On the first day after application of the tracer liver and spleen of tumor carriers accounted for 50% of the activity increase in whole body measurements, kidneys and lungs for about 6% and the femora for nearly 7%. The initial contribution of the entire bone marrow was estimated to be approximately 30%. Later than day 1 the liver of the tumor carriers contributed 50–53% to the difference of whole body counts between tumor carriers and controls, the spleen about 15%, the kidneys 6–7%, and the lungs 1.5–3% (Tab. 2). There was no difference in the activity of femora between tumor carriers and controls later than day 1. After removal of the quoted organs and of the tumor the remaining bodies of tumor-bearing and normal mice showed virtually the same activity (Fig. 4). The rate of loss of activity from day 1 to day 6 greatly varied in different organs; yet it was similar in all these organs from day 10 to day 15 amounting to approximately 5–6% in tumor carriers as well as in controls. Discussion The observed data obtained with 125I-UdR are in good agreement with reports from different laboratories describing in tumor bearing rodents an increased rate of incorporation of DNA precursors in liver, spleen and kidneys; an increased DNA content in these organs and in the lungs due to an increase in DNA synthetising cells was found (Tab. 3). During the second week after injection of 125I-UdR the rate of daily loss of activity related to cells characterized by a relatively long life span was observed nearly equal in various organs and in the whole body of tumor carriers as well as of normal mice. It amounted to 5–6% per day. The calculated turnover time of about 60 days is similar to that reported for macrophages. This agrees with the data from the literature indicating an increased number of macrophages in organs of tumor carriers, including spleen, liver and precursors in cultures of bone marrow. The results suggest that the increased binding of 125I-UdR in cells characterized by a low rate of proliferation in various organs and in the whole body of tumor carriers expresses a cellular reaction to the presence of the tumor. Stimulation of proliferation of elements of the reticulo-endothelial system must be seriously considered. In conjunction with previous reports from this laboratory the present data indicate that it is possible to observe the response of the host to the tumor by non-invasive, external measurements of the living mouse.