A surprising link between the energetics of ovariectomy-induced weight gain and mammary tumor progression in obese rats

Abstract

Obesity increases the risk for postmenopausal breast cancer. We have modeled this metabolic context using female Wistar rats that differ in their polygenic predisposition for obesity under conditions of high-fat feeding and limited physical activity. At 52 days of age, rats were injected with 1-methyl-1-nitrosourea (MNU, 50 mg/kg) and placed in an obesogenic environment. At 19 weeks of age, the rats were separated into lean, mid-weight, and obese rats, based upon their weight gained during this time. The rats were ovariectomized (OVX) at approximately 24 weeks of age and the change in tumor multiplicity and burden, weight gain, energy intake, tumor estrogen receptor (ER) status, and humoral metabolite and cytokine profiles were examined. The survival and growth of tumors increased in obese rats in response to OVX. OVX induced a high rate of weight gain during post-OVX weeks 1-3, compared to SHAM-operated controls. During this time, feed efficiency (mg gain/kcal intake) was lower in obese rats, and this reduced storage efficiency of ingested fuels predicted the OVX-induced changes in tumor multiplicity (r = -0.64, P < 0.001) and burden (r = -0.57, P < 0.001). Tumors from obese rats contained more cells that expressed ERalpha, and post-OVX plasma from rats with the lowest feed efficiency had lower interleukin (IL)-2 and IL-4 levels. Our observations suggest a novel link between obesity and mammary tumor promotion that involves impaired fuel metabolism during OVX-induced weight gain. The metabolically inflexible state of obesity and its inability to appropriately respond to the OVX-induced energy imbalance provides a plausible explanation for this relationship and the emergence of obesity's impact on breast cancer risk after menopause.

Figure 1

Weight gain before and after OVX. (a) Body weights are shown for lean, mid-weight, obese, and SHAM groups (n = 12/group) up to the surgical intervention. (b) Post-surgical body weight was measured every few days for the first 6 weeks, and then weekly thereafter. (c) The rate of weight gain during the 2 weeks before the surgery (pre-OVX), first 3 weeks after the surgical intervention (post-OVX weeks 1–3), and after post-OVX week 4 (week 4 to END) is shown for the four groups. Groups with the same letter designation are not significantly different. OVX, ovariectomized.

Figure 2

Change in tumor multiplicity and burden in response to OVX. (a) Tumor number is shown at the time of the surgery and at the end of the study (*different from multiplicity at OVX). (b) Change in the number of progressing tumors, normalized to post-OVX time is shown. Groups with the same letter designation are not significantly different. (c) The percentage of animals in each group exhibiting an increase in tumor burden is shown. Time points marked with an asterisk indicate a significant difference between lean and obese groups. OVX, ovariectomized.

Figure 3

Intake and feed efficiency before and after surgery. (a) Daily energy intake during the three stages of the study are presented. The asterisk in lean and overweight rats indicates a significant increase in food intake during weeks 1–3 post-OVX. (b) Feed efficiency (weight gain/energy intake) during the three phases of the study is shown. At each time point, groups with the same letter designation are not significantly different. (c) The post-OVX change in tumor number is plotted against the feed efficiency during weeks 1–3 post-OVX, and the Spearman correlation coefficient for the entire cohort is shown. OVX, ovariectomized.

Figure 4

Post-OVX hormones, cytokines, and metabolites, and tumor ERα expression. (a) Estrogen levels measured at the time of surgery and 6 weeks after the surgery are presented. OVX minimized circulating estrogens levels in all three groups (P < 0.001). (b) Fifty-one randomly selected tumors were characterized for ERα expression. SHAM and OVX obese rats had a higher proportion of tumors in which >25% of the cells expressed ERα, when compared to nonobese (lean + mid-weight) rats (P < 0.05). (c–d) Humoral cytokine and metabolite profiles are expressed as the % difference from the SHAM, comparing (c) nonobese (lean and mid-weight rats) and obese rats and (d) rats with high-feed efficiency (>27 mg/kcal) and those with low-feed efficiency (<22 mg/kcal). * Significant difference between SHAM and all OVX animals, P < 0.05. ^†Difference between (c) nonobese vs. obese or (d) high-vs. low-feed efficiency; P < 0.05. ERα, estrogen receptor α; GRO/KC, growth related oncogene chemokine; IFN-γ, interferon-γ; IL, interleukin; MCP-1, monocyte chemoattractant protein 1; NEFAs, nonesterified free fatty acids; OVX, ovariectomized; TG, triglyceride.