Revisiting influences on tumor development focusing on laboratory housing

Abstract

Spontaneous tumors are reported to occur in 45% to 71% of Sprague-Dawley rats, yet few studies have considered the effect of the sedentary condition of standard laboratory cages on tumorigenesis. Tumor profiles and tumor promoting hormone prolactin were compared in female Sprague-Dawley rats (108) that were allocated into 3 groups: those housed without outside activity (SED group), with twice-weekly 1-h sessions of physical activity in large box (PA group), and with regular voluntary running-wheel exercise (EX). Compared with the EX group, SED rats had more and larger tumors throughout most of their lifespan; tumor profiles of PA rats were similar to those of the SED group. A larger percentage of animals in the SED group had tumors (54%), compared with EX rats (38%). At 64 wk, tumors in SED animals included thyroid carcinoma, malignancy, mammary fibroadenoma, cystadenoma, and granuloma, whereas benign mammary gland cysts were most common in EX. Prolactin levels were highest in SED animals at 24 and 52 wk. In conclusion, increased tumor number, increased tumor size, type of spontaneous tumor, and increased prolactin in rats were associated with standard laboratory housing, which limited physical activity, and were not primarily due to aging.

Figure 1.

Percentage of rats in the SED, PA, and EX groups with tumors during weeks 60 through 120. *, Percentage of tumor-bearing animals was greater (P < 0.05) in the SED group than among EX rats.

Figure 2.

Tumor multiplicity (mean number of tumors per animal) during weeks 60 through 120 in SED, PA, and EX rats. *, Tumor multiplicity was greater (P < 0.05) in SED rats than EX animals.

Figure 3.

Tumor size (mean ± SEM) for SED, PA, and EX rats during weeks 60 through 120. Tumors were smaller (P < 0.05) in EX rats than in PA (a) and SED (b) animals.

Figure 4.

Serum prolactin levels (mean ± SEM) of SED, PA, and EX rats at 24, 52, and 64 wk of age. *, Serum prolactin was lower (P < 0.05) in EX rats than in either PA or SED animals.