Erin D. Giles

Regular exercise attenuates the metabolic drive to regain weight after long-term weight loss

Weight loss is accompanied by several metabolic adaptations that work together to promote rapid, efficient regain. We employed a rodent model of regain to examine the effects of a regular bout of treadmill exercise on these adaptations. Obesity was induced in obesity-prone rats with 16 wk of high-fat feeding and limited physical activity. Obese rats were then weight reduced (approximately 14% of body wt) with a calorie-restricted, low-fat diet and maintained at that reduced weight for 8 wk by providing limited provisions of the diet with (EX) or without (SED) a daily bout of treadmill exercise (15 m/min, 30 min/day, 6 days/wk). Weight regain, energy balance, fuel utilization, adipocyte cellularity, and humoral signals of adiposity were monitored during eight subsequent weeks of ad libitum feeding while the rats maintained their respective regimens of physical activity. Regular exercise decreased the rate of regain early in relapse and lowered the defended body weight. During weight maintenance, regular exercise reduced the biological drive to eat so that it came closer to matching the suppressed level of energy expenditure. The diurnal extremes in fuel preference observed in weight-reduced rats were blunted, since exercise promoted the oxidation of fat during periods of feeding (dark cycle) and promoted the oxidation of carbohydrate (CHO) later in the day during periods of deprivation (light cycle) . At the end of relapse, exercise reestablished the homeostatic steady state between intake and expenditure to defend a lower body weight. Compared with SED rats, relapsed EX rats exhibited a reduced turnover of energy, a lower 24-h oxidation of CHO, fewer adipocytes in abdominal fat pads, and peripheral signals that overestimated their adiposity. These observations indicate that regimented exercise altered several metabolic adaptations to weight reduction in a manner that would coordinately attenuate the propensity to regain lost weight.

The role for adipose tissue in weight regain after weight loss

Weight regain after weight loss is a substantial challenge in obesity therapeutics. Dieting leads to significant adaptations in the homeostatic system that controls body weight, which promotes overeating and the relapse to obesity. In this review, we focus specifically on the adaptations in white adipose tissues that contribute to the biological drive to regain weight after weight loss. Weight loss leads to a reduction in size of adipocytes and this decline in size alters their metabolic and inflammatory characteristics in a manner that facilitates the clearance and storage of ingested energy. We present the hypothesis whereby the long‐term signals reflecting stored energy and short‐term signals reflecting nutrient availability are derived from the cellularity characteristics of adipose tissues. These signals are received and integrated in the hypothalamus and hindbrain and an energy gap between appetite and metabolic requirements emerges and promotes a positive energy imbalance and weight regain. In this paradigm, the cellularity and metabolic characteristics of adipose tissues after energy‐restricted weight loss could explain the persistence of a biological drive to regain weight during both weight maintenance and the dynamic period of weight regain.

Obesity and Overfeeding Affecting Both Tumor and Systemic Metabolism Activates the Progesterone Receptor to Contribute to Postmenopausal Breast Cancer

Obese postmenopausal women have increased risk of breast cancers with poorer clinical outcomes than their lean counterparts. However, the mechanisms underlying these associations are poorly understood. Rodent model studies have recently identified a period of vulnerability for mammary cancer promotion, which emerges during weight gain after the loss of ovarian function (surgical ovariectomy; OVX). Thus, a period of transient weight gain may provide a life cycle-specific opportunity to prevent or treat postmenopausal breast cancer. We hypothesized that a combination of impaired metabolic regulation in obese animals prior to OVX plus an OVX-induced positive energy imbalance might cooperate to drive tumor growth and progression. To determine if lean and obese rodents differ in their metabolic response to OVX-induced weight gain, and whether this difference affects later mammary tumor metabolism, we performed a nutrient tracer study during the menopausal window of vulnerability. Lean animals preferentially deposited excess nutrients to mammary and peripheral tissues rather than to the adjacent tumors. Conversely, obese animals deposited excess nutrients into the tumors themselves. Notably, tumors from obese animals also displayed increased expression of the progesterone receptor (PR). Elevated PR expression positively correlated with tumor expression of glycolytic and lipogenic enzymes, glucose uptake, and proliferation markers. Treatment with the antidiabetic drug metformin during ovariectomy-induced weight gain caused tumor regression and downregulation of PR expression in tumors. Clinically, expression array analysis of breast tumors from postmenopausal women revealed that PR expression correlated with a similar pattern of metabolic upregulation, supporting the notion that PR+ tumors have enhanced metabolic capacity after menopause. Our findings have potential explanative power in understanding why obese, postmenopausal women display an increased risk of breast cancer.

Postnatal neurogenesis in the vasopressin and oxytocin-containing nucleus of the pig hypothalamus

The vasopressin and oxytocin-containing nucleus (VON) of the pig hypothalamus demonstrates dramatic postnatal growth in nucleus size, both volume and neuron number, during puberty, and continues to increase in size in the adult sexually mature female pig throughout its reproductive prime. This study was designed to show that postnatal neurogenesis is responsible for the VON growth that occurs between adolescence and maturity. Recently divided neurosecretory cells of the hypothalamus were identified in adolescent and mature non-lactating female pigs using a sequential immunohistochemistry double-labeling technique with monoclonal mouse antibodies to detect vasopressin and proliferating cell nuclear antigen (PCNA), a protein associated with the S phase of the cell cycle. A computer-assisted image-analysis system was used to assess nucleus volume and neuron counts. The VON of the mature dry sows was significantly larger in volume and number of vasopressin neurons than the VON of the adolescent pigs. Double-labeled cells were noted in the VON of both adolescent and mature dry sows, but the number and proportion of double-labeled cells was significantly higher in adolescent pigs. Our results indicate the presence of neurons containing PCNA in the VON of the pig hypothalamus. This suggests that mitosis of neurogenic precursors plays a role in the growth of the nucleus.

Effect of the estrous cycle and surgical ovariectomy on energy balance, fuel utilization, and physical activity in lean and obese female rats

This study presents an in-depth analysis of the effects of obesity on energy balance (EB) and fuel utilization in adult female rats, over the estrous cycle and immediately after surgical ovariectomy (OVX), to model pre- and postmenopausal states, respectively. Female Wistar rats were fed a high-fat (46%) diet for 16 wk to produce mature lean and obese animals. Stage of estrous was identified by daily vaginal lavage, while energy intake (EI), total energy expenditure (TEE), and fuel utilization were monitored in a multichamber indirect calorimeter and activity was monitored by infrared beam breaks. Metabolic monitoring studies were repeated during the 3-wk period of rapid OVX-induced weight gain. Component analysis of TEE was performed to determine the nonresting and resting portions of energy expenditure. Obesity was associated with a greater fluctuation in EB across the estrous cycle. Cycling obese rats were less active, expended more energy per movement, and oxidized more carbohydrate than lean rats. The changes in EB over the cycle in lean and obese rats were driven by changes in EI. Finally, OVX induced a large positive energy imbalance in obese and lean rats. This resulted primarily from an increase in EI in both groups, with little change in TEE following OVX. These observations reveal a dominant effect of obesity on EB, fuel utilization, and activity levels in cycling rats, which has implications for studies focused on obesity and EB in female rodents.

Exercise reduces appetite and traffics excess nutrients away from energetically efficient pathways of lipid deposition during the early stages of weight regain.

The impact of regular exercise on energy balance, fuel utilization, and nutrient availability, during weight regain was studied in obese rats, which had lost 17% of their weight by a calorie-restricted, low-fat diet. Weight reduced rats were maintained for 6 wk with and without regular treadmill exercise (1 h/day, 6 days/wk, 15 m/min). In vivo tracers and indirect calorimetry were then used in combination to examine nutrient metabolism during weight maintenance (in energy balance) and during the first day of relapse when allowed to eat ad libitum (relapse). An additional group of relapsing, sedentary rats were provided just enough calories to create the same positive energy imbalance as the relapsing, exercised rats. Exercise attenuated the energy imbalance by 50%, reducing appetite and increasing energy requirements. Expenditure increased beyond the energetic cost of the exercise bout, as exercised rats expended more energy to store the same nutrient excess in sedentary rats with the matched energy imbalance. Compared with sedentary rats with the same energy imbalance, exercised rats exhibited the trafficking of dietary fat toward oxidation and away from storage in adipose tissue, as well as a higher net retention of fuel via de novo lipogenesis in adipose tissue. These metabolic changes in relapse were preceded by an increase in the skeletal muscle expression of genes involved in lipid uptake, mobilization, and oxidation. Our observations reveal a favorable shift in fuel utilization with regular exercise that increases the energetic cost of storing excess nutrients during relapse and alterations in circulating nutrients that may affect appetite. The attenuation of the biological drive to regain weight, involving both central and peripheral aspects of energy homeostasis, may explain, in part, the utility of regular exercise in preventing weight regain after weight loss.

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

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 ∼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 ERα, 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 obesitys impact on breast cancer risk after menopause.

Developmental windows of breast cancer risk provide opportunities for targeted chemoprevention

The magnitude of the breast cancer problem implores researchers to aggressively investigate prevention strategies. However, several barriers currently reduce the feasibility of breast cancer prevention. These barriers include the inability to accurately predict future breast cancer diagnosis at the individual level, the need for improved understanding of when to implement interventions, uncertainty with respect to optimal duration of treatment, and negative side effects associated with currently approved chemoprevention therapies. None-the-less, the unique biology of the mammary gland, with its postnatal development and conditional terminal differentiation, may permit the resolution of many of these barriers. Specifically, lifecycle-specific windows of breast cancer risk have been identified that may be amenable to risk-reducing strategies. Here, we argue for prevention research focused on two of these lifecycle windows of risk: postpartum mammary gland involution and peri-menopause. We provide evidence that these windows are highly amenable to targeted, limited duration treatments. Such approaches could result in the prevention of postpartum and postmenopausal breast cancers, correspondingly.

Modeling Diet-Induced Obesity with Obesity-Prone Rats: Implications for Studies in Females

Obesity is a worldwide epidemic, and the comorbidities associated with obesity are numerous. Over the last two decades, we and others have employed an outbred rat model to study the development and persistence of obesity, as well as the metabolic complications that accompany excess weight. In this review, we summarize the strengths and limitations of this model and how it has been applied to further our understanding of human physiology in the context of weight loss and weight regain. We also discuss how the approach has been adapted over time for studies in females and female-specific physiological conditions, such as menopause and breast cancer. As excess weight and the accompanying metabolic complications have become common place in our society, we expect that this model will continue to provide a valuable translational tool to establish physiologically relevant connections to the basic science studies of obesity and body weight regulation.

Role of insulin-like growth factor binding proteins (IGFBPs) in breast cancer proliferation and metastasis.

Cancers of the breast, prostate, and lung commonly metastasize to the bone resulting in osteolysis, pathologic fracture, pain and significant clinical morbidity. To date, the reason for such selectivity in the site of metastasis remains largely unknown. The bone is a rich source of many chemokines and growth factors, including: insulin-like growth factor (IGF) I and II, transforming growth factor-β (TGF-β), interleukins, and tumour necrosis factor-α (TNF-α) [1]. We propose that exposure of breast cancer cells to the bone microenvironment results in alterations in gene expression that favour the growth and proliferation of tumour cells in the bone. To investigate this hypothesis, MDA-MB-231 breast carcinoma cells were exposed to bone-derived conditioned media (BDCM) generated by culturing fetal rat calvaria for 24 h under serum free conditions. Using cDNA microarray technology, we have identified the insulin-like growth factor family of binding proteins (IGFBPs) as genes whose expression profiles are consistently and significantly altered with exposure to this simulated bone environment in vitro, when compared to untreated controls. Our data suggests that the upregulation of IGFBP-3 seen with exposure to the bone microenvironment is directly linked to an increase in TGF-β mediated cell proliferation. Furthermore, this process appears to be functioning through an IGF-independent mechanism.