Susan N. Perkins

Calories and carcinogenesis: lessons learned from 30 years of calorie restriction research

Calorie restriction (CR) is arguably the most potent, broadly acting dietary regimen for suppressing the carcinogenesis process, and many of the key studies in this field have been published in Carcinogenesis. Translation of the knowledge gained from CR research in animal models to cancer prevention strategies in humans is urgently needed given the worldwide obesity epidemic and the established link between obesity and increased risk of many cancers. This review synthesizes the evidence on key biological mechanisms underlying many of the beneficial effects of CR, with particular emphasis on the impact of CR on growth factor signaling pathways and inflammatory processes and on the emerging development of pharmacological mimetics of CR. These approaches will facilitate the translation of CR research into effective strategies for cancer prevention in humans.

Physical activity and cancer prevention : pathways and targets for intervention.

The prevalence of obesity, an established epidemiological risk factor for many cancers, has risen steadily for the past several decades in the US and many other countries. Particularly alarming are the increasing rates of obesity among children, portending continuing increases in the rates of obesity and obesity-related cancers for many years to come. Modulation of energy balance, via increased physical activity, has been shown in numerous comprehensive epidemiological reviews to reduce cancer risk. Unfortunately, the effects and mechanistic targets of physical activity interventions on the carcinogenesis process have not been thoroughly characterized.Studies to date suggest that exercise can exert its cancer-preventive effects at many stages during the process of carcinogenesis, including both tumour initiation and progression. As discussed in this review, exercise may be altering tumour initiation events by modifying carcinogen activation, specifically by enhancing the cytochrome P450 system and by enhancing selective enzymes in the carcinogen detoxification pathway, including, but not limited to, glutathione-S-transferases. Furthermore, exercise may reduce oxidative damage by increasing a variety of anti-oxidant enzymes, enhancing DNA repair systems and improving intracellular protein repair systems.In addition to altering processes related to tumour initiation, exercise may also exert a cancer-preventive effect by dampening the processes involved in the promotion and progression stages of carcinogenesis, including scavenging reactive oxygen species (ROS); altering cell proliferation, apoptosis and differentiation; decreasing inflammation; enhancing immune function; and suppressing angiogenesis. A paucity of data exists as to whether exercise may be working as an anti-promotion strategy via altering ROS in initiated or preneoplastic models; therefore, no conclusions can be made about this possible mechanism. The studies directly examining cell proliferation and apoptosis have shown that exercise can enhance both processes, which is difficult to interpret in the context of carcinogenesis. Studies examining the relationship between exercise and chronic inflammation suggest that exercise may reduce pro-inflammatory mediators and reduce the state of low-grade, chronic inflammation. Additionally, exercise has been shown to enhance components of the innate immune response (i.e. macrophage and natural killer cell function). Finally, only a limited number of studies have explored the relationship between exercise and angiogenesis; therefore, no conclusions can be made currently about the role of exercise in the angiogenesis process as it relates to tumour progression.In summary, exercise can alter biological processes that contribute to both antiinitiation and anti-progression events in the carcinogenesis process. However, more sophisticated, detailed studies are needed to examine each of the potential mechanisms contributing to an exercise-induced decrease in carcinogenesis in order to determine the minimum dose, duration and frequency of exercise needed to yield significant cancer-preventive effects, and whether exercise can be used prescriptively to reverse the obesity-induced physiological changes that increase cancer risk.

Reducing the weight of cancer: mechanistic targets for breaking the obesity–carcinogenesis link

The prevalence of obesity, an established epidemiologic risk factor for many cancers, has risen steadily for the past several decades in the US. The increasing rates of obesity among children are especially alarming and suggest continuing increases in the rates of obesity-related cancers for many years to come. Unfortunately, the mechanisms underlying the association between obesity and cancer are not well understood. In particular, the effects on the carcinogenesis process and mechanistic targets of interventions that modulate energy balance, such as reduced-calorie diets and physical activity, have not been well characterized. The purpose of this review is to provide a strong foundation for the translation of mechanism-based research in this area by describing key animal and human studies of energy balance modulations involving diet or physical activity and by focusing on the interrelated pathways affected by alterations in energy balance. Particular attention is placed on signaling through the insulin and insulin-like growth factor-1 receptors, including components of the Akt and mammalian target of rapamycin (mTOR) signaling pathways downstream of these growth factor receptors. These pathways have emerged as potential targets for disrupting the obesity-cancer link. The ultimate goal of this work is to provide the missing mechanistic information necessary to identify targets for the prevention and control of cancers related to or caused by excess body weight.

Obesity Accelerates Mouse Mammary Tumor Growth in the Absence of Ovarian Hormones

Obesity increases incidence and mortality of breast cancer in postmenopausal women. Mechanisms underlying this association are poorly understood. Suitable animal models are needed to elucidate potential mechanisms for this association. To determine the effects of obesity on mammary tumor growth, nonovariectomized and ovariectomized C57BL/6 mice of various body weights (lean, overweight, and obese) were implanted subcutaneously with mammary tumor cells from syngeneic Wnt-1 transgenic mice. In mice, the lean phenotype was associated with reduced Wnt-1 tumor growth regardless of ovarian hormone status. Ovariectomy delayed Wnt-1 tumor growth consistent with the known hormone responsiveness of these tumors. However, obesity accelerated tumor growth in ovariectomized but not in nonovariectomized animals. Diet-induced obesity in a syngeneic mouse model of breast cancer enhanced tumor growth, specifically in the absence of ovarian hormones. These results support epidemiological evidence that obesity is associated with increased breast cancer incidence and mortality in postmenopausal but not premenopausal women. In contrast, maintaining a lean body weight phenotype was associated with reduced Wnt-1 tumor growth regardless of ovarian hormone status.

Leptin, Insulin-Like Growth Factor-1, and Insulin-Like Growth Factor-2 Are Mitogens in ApcMin/+ but not Apc+/+ Colonic Epithelial Cell Lines

The obese state is associated with elevated circulating levels of insulin, insulin-like growth factors (IGF), and leptin. Research is contradictory regarding the role of these elevated growth factors in colon cancer risk. We hypothesized that colonic epithelial cells that were Apc deficient (ApcMin/+) but not those expressing wild-type Apc (Apc+/+) would experience a hyperproliferative and antiapoptotic phenotype when exposed to these growth factors. This hypothesis was addressed using two nontumorigenic murine colonic epithelial cell lines with distinct Apc genotypes: Apc+/+ YAMC cells and ApcMin/+ IMCE cells. Cells were treated for 48 hours with various concentrations of leptin (0.001-50 ng/mL), IGF-1 (0.1-200 ng/mL), or IGF-2 (0.1-600 ng/mL). In YAMC cells, leptin caused a significant decrease in cell proliferation (P < 0.01) compared with controls due to induction of caspase activity and apoptosis. In contrast, in the IMCE cells, leptin induced a 75% increase in cell proliferation compared with controls (P < 0.0001). IGF-1 and IGF-2 also induced 50% greater proliferation in the IMCE cells (P < 0.001) compared with controls. Cotreatment of IMCE cells with leptin and either IGF-1 or IGF-2 induced greater proliferation than either growth factor alone (P < 0.0001). IMCE cell proliferation caused by leptin only treatment was associated with activation of p42/44 mitogen-activated protein kinase (MAPK), p38 MAPK, and nuclear factor-κB nuclear translocation but not with MAPK kinase or Janus-activated kinase/signal transducers and activators of transcription activation. These data provide the first evidence that leptin may interact with IGFs to promote survival and expansion of colonic epithelial cells that were Apc deficient (ApcMin/+) but not those expressing wild-type Apc (Apc+/+).

Accelerated Tumor Formation in a Fatless Mouse with Type 2 Diabetes and Inflammation

Epidemiologic studies show a positive association between obesity and cancer risk. In addition to increased body adiposity and secretion of fat-derived hormones, obesity is also linked to insulin resistance, type 2 diabetes, and chronic inflammation. We used the fatless A-ZIP/F-1 transgenic mouse to dissociate the relative role of each of these underlying factors in the development of cancer. These mice are unique in that they do not have white fat but do develop type 2 diabetes. In two cancer models, the classic two-stage skin carcinogenesis protocol and the C3(1)/T-Ag transgenic mouse mammary tumor model, A-ZIP/F-1 mice displayed higher tumor incidence, tumor multiplicity, and decreased tumor latency than wild-type mice. We examined circulating levels of adipokines, growth factors, and cytokines. As expected, adipokines (i.e., leptin, adiponectin, and resistin) were undetectable or found at very low levels in the blood of fatless mice. However, insulin, insulin-like growth factor-I, growth hormone, vascular endothelial growth factor, and proinflammatory Th2 cytokines, such as interleukin (IL)-1beta, IL-4, and IL-6, were elevated in A-ZIP/F-1 mice. Additionally, we examined multiple phosphorylated proteins (i.e., protein kinase B/Akt and ErbB2/HER-2 kinase) associated with cancer development. Results show that many of these phosphorylated proteins were activated specifically in the A-ZIP/F-1 skin but not in the wild-type skin. These findings suggest that adipokines are not required for the promotion of tumor development and thus contradict the epidemiologic data linking obesity to carcinogenesis. We postulate that insulin resistance and inflammation are responsible for the positive correlation with cancer observed in A-ZIP/F-1 mice.

Extreme obesity reduces bone mineral density: complementary evidence from mice and women.

Objective: To evaluate the effects of body adiposity on bone mineral density in the presence and absence of ovarian hormones in female mice and postmenopausal women.

Diet‐induced adiposity alters the serum profile of inflammation in C57BL/6N mice as measured by antibody array

Morbid obesity is considered a systemic inflammatory state. The objective of this project was to characterize the adipokine, cytokine and chemokine protein profile in serum from control, lean and obese mice. We hypothesized that chemokines and cytokines are altered by caloric restriction and diet‐induced obesity as a function of changes in body composition. Six‐week‐old female C57BL/6N mice (n = 12 per group) were randomized to one of three diets: control (fed ad libitum); lean (30% calorie‐restricted regimen relative to control) and diet‐induced obese (DIO; high calorie diet, fed ad libitum). Body weight, body composition and food intake were monitored throughout the study. After 10 weeks on the diets, blood samples were collected, and adipokine/cytokine/chemokine serum profiles were measured by antibody array. Lean mice, relative to the control group, displayed increased concentrations of insulin‐like growth factor (IGF) binding protein‐3, ‐5 and ‐6 and adiponectin and decreased IGF‐1. These mice also showed increased concentrations of interleukin (IL)‐10, IL‐12 p40/p70, eotaxin, monocyte chemoattractant protein‐5 and SDF‐1. In contrast, DIO mice displayed increased leptin, IL‐6 and LPS‐induced chemokine and decreased concentrations of all chemokines/cytokines measured relative to control mice. As such, these data indicate that DIO may lead to an inflammatory state characterized as a shift towards a T helper lymphocyte type 1–skewed responsiveness. The demonstration of differential adipokine, cytokine and chemokine protein profile in control, lean and DIO mice may have implications for immune responsiveness and risk of disease.

Reduced Susceptibility to Two-Stage Skin Carcinogenesis in Mice with Low Circulating Insulin-Like Growth Factor I Levels

Calorie restriction has been shown to inhibit epithelial carcinogenesis and this method of dietary restriction reduces many circulating proteins, including insulin-like growth factor I (IGF-I). Previously, we identified a relationship between elevated tissue IGF-I levels and enhanced susceptibility to chemically induced skin tumorigenesis. In this study, liver IGF-I-deficient (LID) mice, which have a 75% reduction in serum IGF-I, were subjected to the standard two-stage skin carcinogenesis protocol using 7,12-dimethylbenz(a)anthracene as the initiator and 12-O-tetradecanoylphorbol-13-acetate (TPA) as the promoter. We observed a significant reduction in epidermal thickness and labeling index in LID mice treated with either vehicle or TPA. A significant decrease in both tumor incidence and tumor multiplicity was observed in LID mice undergoing two-stage skin carcinogenesis relative to wild-type littermates. Western blot analyses of epidermal extracts revealed reduced activation of both the epidermal growth factor and IGF-I receptors in response to TPA treatment in LID mice. In addition, reduced activation of both Akt and the mammalian target of rapamycin (mTOR) was observed in LID mice following TPA treatment relative to wild-type controls. Signaling downstream of mTOR was also reduced. These data suggest a possible mechanism whereby reduced circulating IGF-I leads to attenuated activation of the Akt and mTOR signaling pathways, and thus, diminished epidermal response to tumor promotion, and ultimately, two-stage skin carcinogenesis. The current data also suggest that reduced circulating IGF-I levels which occur as a result of calorie restriction may lead to the inhibition of skin tumorigenesis, at least in part, by a similar mechanism.

Energy balance and carcinogenesis: underlying pathways and targets for intervention.

The prevalence of obesity, an established epidemiologic risk factor for many cancers, has risen steadily for the past several decades in the U.S. Particularly alarming are the increasing rates of obesity among children, portending continuing increases in the rates of obesity and obesity-related cancers for many years to come. Unfortunately, the mechanisms underlying the association between obesity and cancer are not well understood. In particular, the effects and mechanistic targets of interventions that modulate energy balance, such as reduced calorie diets and physical activity, on the carcinogenesis process have not been well characterized. The purpose of this review is to provide a strong foundation for future mechanistic-based research in this area by describing key animal and human studies of energy balance modulations involving diet, exercise, or pharmaceutical agents and by focusing on the interrelated pathways affected by alterations in energy balance. Particular attention in this review is placed on the components of the insulin/IGF-1/Akt pathway, which has emerged as a predominant target for disrupting the obesity-cancer link. Also discussed is the promise of global approaches, including genomics, proteomics, and metabolomics, for the elucidation of energy balance-responsive pathways. The ultimate goal of this work is to provide the missing mechanistic information necessary to identify targets for the prevention and control of cancers related to or caused by excess body weight.