Early impact of social isolation and breast tumor progression in mice

Abstract

Evidence from cancer patients and animal models of cancer indicates that exposure to psychosocial stress can promote tumor growth and metastasis, but the pathways underlying stress-induced cancer pathogenesis are not fully understood. Social isolation has been shown to promote tumor progression. We examined the impact of social isolation on breast cancer pathogenesis in adult female severe combined immunodeficiency (SCID) mice using the human breast cancer cell line, MDA-MB-231, a high β-adrenergic receptor (AR) expressing line. When group-adapted mice were transferred into single housing (social isolation) one week prior to MB-231 tumor cell injection into a mammary fat pad (orthotopic), no alterations in tumor growth or metastasis were detected compared to group-housed mice. When social isolation was delayed until tumors were palpable, tumor growth was transiently increased in singly-housed mice. To determine if sympathetic nervous system activation was associated with increased tumor growth, spleen and tumor norepinephrine (NE) was measured after social isolation, in conjunction with tumor-promoting macrophage populations. Three days after transfer to single housing, spleen weight was transiently increased in tumor-bearing and non-tumor-bearing mice in conjunction with reduced splenic NE concentration and elevated CD11b+Gr-1+ macrophages. At day 10 after social isolation, no changes in spleen CD11b+ populations or NE were detected in singly-housed mice. In the tumors, social isolation increased CD11b+Gr-1+, CD11b+Gr-1-, and F4/80+ macrophage populations, with no change in tumor NE. The results indicate that a psychological stressor, social isolation, elicits dynamic but transient effects on macrophage populations that may facilitate tumor growth. The transiency of the changes in peripheral NE suggest that homeostatic mechanisms may mitigate the impact of social isolation over time. Studies are underway to define the neuroendocrine mechanisms underlying the tumor-promoting effects of social isolation, and to determine the contributions of increased tumor macrophages to tumor pathogenesis.

Fig. 1

MB-231 Tumor Growth is Not Altered by Social Isolation Prior to Tumor Cell Injection. SCID female mice were singly housed seven days prior to orthotopic injection of MB-231 cells. Tumor diameter was measured with calipers on the days indicated. Tumor volume is expressed as mean ± SEM, n=8-10 mice per group.

Fig. 2

MB-231 Tumor Growth is Transiently Increased by Social Isolation After Tumor Injection. SCID female were injected with MB-231 cells, and when all mice had palpable tumors, half of the mice were transferred from group to single housing. A. Tumor volume over time. NE (B) and human IL-6 (C) were measured by ELISA in tumors harvested at day 34 post separation. Results are expressed as mean ± SEM, n=8-9 mice per group. See text for statistical analysis of tumor growth. For tumor NE and human IL-6, no significant effects based on the non-parametric Mann-Whitney test, p = 0.1.

Fig. 3

Social Isolation Reciprocally Alters Spleen NE (A) and Spleen Weight (B) Early After Social Isolation. A subset of mice described in Fig. 2 was sacrificed at day 3 post-separation. Results are expressed as mean ± SEM. n=5 mice per group. Asterisk indicates different versus group-housed at the corresponding time point by Newman-Keuls post-hoc analysis.

Fig. 4

Early Effects of Social Isolation on Spleen NE and Spleen Weight is Independent of Tumor Growth. Three days after transfer to single housing, SCID female mice were sacrificed and spleen NE (A), weight (B), and macrophage populations (C-E) were determined. Asterisk indicates significant difference by Student’s t-test, p<0.05. Results are expressed as mean ± SEM, n= 5 mice per group.

Fig. 5

Social Isolation Alters Tumor and Spleen Macrophage Populations. SCID female mice were injected with MB-231 in the mammary fat pad. When tumors were palpable, one-half the mice were transferred from group-housing to single housing. After 10 days, mice were sacrificed and tumor (A,B) and spleen (C,D) weight and NE concentration were determined, and macrophage populations were analyzed by flow cytometry in tumors (E-G) and spleens (H-J). Asterisk indicates significance based on the non-parametric Mann-Whitney test (A, B) or by student’s t-test (D), p<0.05. In (C), p = 0.067. The results are expressed as mean ± SEM of 9-10 mice per group.