Leptin signaling in breast cancer: an overview

The adipocyte-derived peptide leptin acts through binding to specific membrane receptors, of which six isoforms (obRa-f) have been identified up to now. Binding of leptin to its receptor induces activation of different signaling pathways, including the JAK/STAT, MAPK, IRS1, and SOCS3 signaling pathways. Since the circulating levels of leptin are elevated in obese individuals, and excess body weight has been shown to increase breast cancer risk in postmenopausal women, several studies addressed the role of leptin in breast cancer. Expression of leptin and its receptors has been demonstrated to occur in breast cancer cell lines and in human primary breast carcinoma. Leptin is able to induce the growth of breast cancer cells through activation of the Jak/STAT3, ERK1/2, and/or PI3K pathways, and can mediate angiogenesis by inducing the expression of vascular endothelial growth factor (VEGF). In addition, leptin induces transactivation of ErbB-2, and interacts in triple negative breast cancer cells with insulin like growth factor-1 (IGF-1) to transactivate the epidermal growth factor receptor (EGFR), thus promoting invasion and migration. Leptin can also affect the growth of estrogen receptor (ER)-positive breast cancer cells, by stimulating aromatase expression and thereby increasing estrogen levels through the aromatization of androgens, and by inducing MAPK-dependent activation of ER. Taken together, these findings suggest that the leptin system might play an important role in breast cancer pathogenesis and progression, and that it might represent a novel target for therapeutic intervention in breast cancer.     

Central Exendin‐4 Infusion Reduces Body Weight without Altering Plasma Leptin in (fa/fa) Zucker Rats

Objective: To investigate whether chronic administration of the long‐acting glucagon‐like peptide‐1 receptor agonist exendin‐4 can elicit sustained reductions in food intake and body weight and whether its actions require an intact leptin system. Research Methods and Procedures: Male lean and obese Zucker (fa/fa) rats were infused intracerebroventricularly with exendin‐4 using osmotic minipumps for 8 days. Results: Exendin‐4 reduced body weight in both lean and obese Zucker rats, maximum suppression being reached on Day 5 in obese (8%) and Day 7 in lean (16%) rats. However, epididymal white adipose tissue weight was not reduced, and only in lean rats was there a reduction in plasma leptin concentration. Food intake was maximally suppressed (by 81%) on Day 3 in obese rats but was reduced by only 18% on Day 8. Similarly, in lean rats food intake was maximally reduced (by 93%) on Day 4 of treatment and by 45% on Day 8. Brown adipose tissue temperature was reduced from Days 2 to 4. Plasma corticosterone was elevated by 76% in lean but by only 28% in obese rats. Discussion: Chronic exendin‐4 treatment reduced body weight in both obese and lean Zucker rats by reducing food intake: metabolic rate was apparently suppressed. These effects did not require an intact leptin system. Neither does the absence of an intact leptin system sensitize animals to exendin‐4. Partial tolerance to the anorectic effect of exendin‐4 in lean rats may have been due to elevated plasma corticosterone and depressed plasma leptin levels, but other counter‐regulatory mechanisms seem to play a role in obese Zucker rats.     

Human Leptin Stimulates Systemic Nitric Oxide Production in the Rat

Objective: Apart from having an effect on energy balance, leptin is also involved in cardiovascular regulation and in the pathogenesis of obesity‐associated hypertension. We investigated the effect of leptin on nitric oxide (NO) production. Research Methods and Procedures: Wistar rats were placed in metabolic cages, and urine was collected in 2‐hour periods. After the control period, leptin (1 mg/kg intraperitoneal) was administered, and urine collection was continued for up to 6 hours. Blood was obtained 0.5, 1, 2, 4, and 6 hours after hormone injection. Results: Leptin increased plasma concentrations of NO metabolites (nitrates + nitrites, NO_x) by 32.5%, 58.0%, and 29.7% at 1, 2, and 4 hours, respectively. Urinary NO_x excretion increased by 28.8% in the first and by 20.1% in the second 2‐hour period after injection. The plasma concentration of the NO second messenger, cyclic guanosine 3′,5′‐monophosphate (cGMP), increased by 83% and 50.6% at 2 and 4 hours after leptin administration, respectively. Urinary excretion of cyclic GMP increased by 36.1% in the first and by 43.1% in the second 2‐hour period. Leptin had no effect on the plasma concentration of atrial natriuretic peptide (ANP). The effect of leptin on plasma and urinary NO_x was abolished by the NO synthase inhibitor, N^G‐nitro‐l ‐arginine methyl ester (l ‐NAME) (30 mg/kg intraperitoneal) administered 15 minutes before leptin injection. l ‐NAME alone caused a 32.2% increase in systolic blood pressure, but this increase was not observed in rats receiving l ‐NAME and leptin. Discussion: The results indicate that leptin stimulates systemic NO production; leptin prevents blood pressure elevation induced by acute NO blockade, suggesting that leptin also triggers additional hypotensive mechanisms; and ANP is not involved in renal and vascular effects of leptin.     

Soluble Leptin Receptor and Soluble Receptor‐Bound Fraction of Leptin in the Metabolic Syndrome

Objective: In obesity, plasma leptin is high and soluble leptin receptor (sOb‐R) levels are low, resulting in a low fraction of bound leptin. The aim of this study was to investigate the influence of insulin resistance (IR) and the metabolic syndrome (MS) on sOb‐R concentration and the bound‐free ratio of leptin. Research Methods and Procedures: sOb‐R, leptin levels, and homeostasis model assessment (HOMA) index for IR were determined in 76 middle‐aged obese or overweight men. Results: Concentration of sOb‐R and soluble receptor‐bound fraction of leptin were lowest in the highest tertile of HOMA‐IR. sOb‐R and the bound‐free ratio of leptin correlated with HOMA‐IR, leptin concentration, and waist‐to‐hip ratio independently of age, BMI, and fat mass. Leptin and waist‐to‐hip ratio were the sole independent determinants of sOb‐R concentration, and BMI, HOMA‐IR, and visceral adipose tissue were independent determinants of the bound fractin of leptin. sOb‐R concentration and the bound fraction of leptin decreased with increasing numbers of components of the MS, resulting in lower sOb‐R concentration and a lower fraction of bound leptin in men with the MS. Discussion: IR and abdominal obesity are associated with low sOb‐R concentration and low bound‐free ratio of leptin independent of fat mass. Low sOb‐R concentration and low bound‐free ratio of leptin segregate with components of the MS. We suggest that low sOb‐R levels and a low fraction of specifically bound leptin are markers of leptin resistance, which is independently associated with IR and abdominal obesity and may constitute an additional component of the MS.     

Alterations in brain leptin signalling in spite of unchanged CSF leptin levels in Alzheimer's disease

Several studies support the relation between leptin and Alzheimer's disease (AD). We show that leptin levels in CSF are unchanged as subjects progress to AD. However, in AD hippocampus, leptin signalling was decreased and leptin localization was shifted, being more abundant in reactive astrocytes and less in neurons. Similar translocation of leptin was found in brains from Tg2576 and apoE4 mice. Moreover, an enhancement of leptin receptors was found in hippocampus of young Tg2576 mice and in primary astrocytes and neurons treated with Aβ_1‐42. In contrast, old Tg2576 mice showed decreased leptin receptors levels. Similar findings to those seen in Tg2576 mice were found in apoE4, but not in apoE3 mice. These results suggest that leptin levels are intact, but leptin signalling is impaired in AD. Thus, Aβ accumulation and apoE4 genotype result in a transient enhancement of leptin signalling that might lead to a leptin resistance state over time.     

Weight Loss Increases Soluble Leptin Receptor Levels and the Soluble Receptor Bound Fraction of Leptin

Objective: Soluble leptin receptor (sOB‐R) represents the main binding site for leptin in human blood. The aim of this study was to investigate the relationship between leptin and soluble leptin receptor and the bound/free ratio after pronounced weight reduction. Research Methods and Procedures: A total of 18 morbidly obese women participated in this prospective study. Subjects were examined for fat mass, leptin, and sOB‐R concentrations before and 1 year after Swedish adjustable gastric banding. Results: Anthropomorphic measures displayed a significant reduction of body mass index [(42.9 ± 5.6 to 32.9 ± 6.0 kg/m² (mean ± SD)]. Fat mass decreased from 56.3 ± 9.0 to 33.9 ± 12.5 kg. Plasma leptin concentration decreased from 44.6 ± 18.0 to 20.0 ± 13.1 ng/mL (p < 0.001), whereas the sOB‐R levels increased from 11.1 ± 3.6 to 16.6 ± 6.0 U/mL after weight‐reducing surgery. Thus, the sOB‐R bound fraction of leptin increased from 7% to 33%. Discussion: This work demonstrates a relationship between weight loss, leptin, and sOB‐R concentrations in vivo. During weight loss, leptin levels decreased, whereas sOB‐R levels and the receptor bound fraction of leptin increased. Thus, sOB‐R may negatively regulate free leptin.     

Expressions of Leptin and Insulin‐Like Growth Factor‐I Are Highly Correlated and Region‐Specific in Adipose Tissue of Growing Rats

Objective: Anatomically distinct adipose tissue regions differ in their predominant modality of growth (i.e., cellular hypertrophy vs. hyperplasia). We examined site‐specific patterns of expression of two genes whose products, leptin and insulin‐like growth factor‐I (IGF‐I), could be involved in mediating differential growth and metabolism of white adipose tissue. We also related these patterns of expression to measures of adipose depot cellularity. Research Methods and Procedures: Male Wistar rats were fed ad libitum and studied from ages 7 weeks to ～12 months. Terminal measures of body weights; weights, composition, and cellularity of four white adipose depots; circulating leptin and IGF‐I; and adipose depot‐specific expression levels of leptin and IGF‐I were measured in subsets of rats at 7, 12, 22, 42, and 46 weeks of age. Results: Both leptin and IGF‐I mRNAs are quantitatively expressed in a depot‐specific manner, in the following order: retroperitoneal ? epididymal > mesenteric > subcutaneous inguinal. Furthermore, there is a marked correlation between the expressions of these hormones in the various regions of adipose tissue of rats during the first year of life. The mechanisms that underlie the parallel expressions of leptin and IGF‐I appear to be related to fat‐cell volume. Discussion: Because both leptin and IGF‐I have been implicated in the regulation of energy homeostasis and are both expressed in adipose tissue, the depot‐specific linkage between the two genes suggests interaction at the autocrine level. This interaction may have an important role in determining functional properties particular to individual adipose depots.     

GATA binding protein 3 is correlated with leptin regulation of PPARγ1 in hepatic stellate cells

Accumulating evidence reveals that hormone leptin, mainly produced by adipocyte, plays a unique role in promotion of liver fibrosis. Hepatic stellate cell (HSC) activation is a key step in liver fibrosis and peroxisome‐proliferator activated receptor γ (PPARγ) exerts a crucial role in inhibition of HSC activation. Our previous researches demonstrated that leptin reduced PPARγ1 (a major subtype of PPARγ in HSCs) expression through GATA binding protein 2 (GATA2) binding to a site around ?2323 in PPARγ1 promoter. The present researches aimed to examine the effect of GATA3 on leptin‐induced inhibition of PPARγ1 and elucidate the relationship between GATA3 and GATA2. Gene expressions were analysed by real‐time PCR, western blot, luciferase assay and immunostaining. C57BL/6J ob/ob mouse model of thioacetamide‐induced liver injury was used in vivo. Results demonstrate that leptin significantly induces GATA3 expression in HSCs by multiple signalling pathways including NADPH oxidase pathway. There exist crosstalks between NADPH oxidase pathway and the other pathways. GATA3 can bind to GATA2‐binding site in PPARγ1 promoter and interacts with GATA2, contributing to leptin inhibition of PPARγ1 expression in HSCs. These data demonstrated novel molecular events for leptin inhibition of PPARγ1 expression in HSCs and thus might have potential implications for clarifying the detailed mechanisms underlying liver fibrosis in diseases in which circulating leptin levels are elevated such as non‐alcoholic steatohepatitis in obese patients.     

An integrated analysis to predict micro‐RNAs targeting both stemness and metastasis in breast cancer stem cells

Several evidences support the idea that a small population of tumour cells representing self‐renewal potential are involved in initiation, maintenance, metastasis, and outcomes of cancer therapy. Elucidation of microRNAs/genes regulatory networks activated in cancer stem cells (CSCs) is necessary for the identification of new targets for cancer therapy. The aim of the present study was to predict the miRNAs pattern, which can target both metastasis and self‐renewal pathways using integration of literature and data mining. For this purpose, mammospheres derived from MCF‐7, MDA‐MB231, and MDA‐MB468 were used as breast CSCs model. They had higher migration, invasion, and colony formation potential, with increasing in stemness‐ and EMT‐related genes expression. Our results determined that miR‐204, ‐200c, ‐34a, and ‐10b contemporarily could target both self‐renewal and EMT pathways. This core regulatory of miRNAs could increase the survival rate of breast invasive carcinoma via up‐regulation of OCT4, SOX2, KLF4, c‐MYC, NOTCH1, SNAI1, ZEB1, and CDH2 and down‐regulation of CDH1. The majority of those target genes were involved in the regulation of pluripotency, MAPK, WNT, Hedgehog, p53, and transforming growth factor β pathways. Hence, this study provides novel insights for targeting core regulatory of miRNAs in breast CSCs to target both self‐renewal and metastasis potential and eradication of breast cancer.     

Genetic Variation in the Leptin Receptor Gene,Leptin, and Weight Gain in Young Dutch Adults

Objective: To investigate the association between leptin levels, polymorphisms in the leptin receptor (LEPR) gene, and weight gain. Research Methods and Procedures: From two large prospective cohorts in The Netherlands (n = 17, 500), we compared the baseline leptin of 259 subjects who had gained an average of 12.6 kg (range 5.5 to 33 kg) with 277 subjects who kept stable weight (range ?2.6 to 3.1 kg) after a mean follow‐up of 6.8 years. Three polymorphisms in the LEPR gene (Lys109Arg, Gln223Arg, and Lys656Asn) were determined. Results: Weight gainers had significantly higher baseline leptin levels than those who kept stable weight (odds ratio = 1.27, 95% confidence interval 1.1 to 1.5, per SD increase in log_e‐transformed leptin). Weight gainers with the Arg109 or the Arg223 alleles had higher leptin levels compared with the noncarriers of these alleles. Only among men, the association between leptin and weight gain tended to be stronger among those with an Arg223 allele compared with those without this mutation. Discussion: Relatively high leptin levels predict weight gain, suggesting that leptin resistance plays a role in the development of obesity in the general population. Higher leptin levels for those with a Lys109Arg or Gln223Arg mutation (or a linked other marker) may imply that these subjects have a modified functional leptin receptor. However, the role of these mutations on weight gain is limited.     

IL-1 family in breast cancer: Potential interplay with leptin and other adipocytokines

Obesity is associated with an increased risk of breast cancer. interleukin-1 (IL-1), a pro-inflammatory cytokine secreted by adipose tissue, is involved in breast cancer development. There is also convincing evidence that other adipocytokines including leptin not only have a role in haematopoiesis, reproduction and immunity but are also growth factors in cancer. Therefore, IL-1 family and leptin family are adipocytokines which could represent a major link between obesity and breast cancer progression. This minireview provides insight into recent findings on the prognostic significance of IL-1 and leptin in mammary tumours, and discusses the potential interplay between IL-1 family members and adipocyte-derived hormones in breast cancer.     

Leptin action in normal and pathological pregnancies

Leptin is now considered an important signalling molecule of the reproductive system, as it regulates the production of gonadotrophins, the blastocyst formation and implantation, the normal placentation, as well as the foeto‐placental communication. Leptin is a peptide hormone secreted mainly by adipose tissue, and the placenta is the second leptin‐producing tissue in humans. Placental leptin is an important cytokine which regulates placental functions in an autocrine or paracrine manner. Leptin seems to play a crucial role during the first stages of pregnancy as it modulates critical processes such as proliferation, protein synthesis, invasion and apoptosis in placental cells. Furthermore, deregulation of leptin levels has been correlated with the pathogenesis of various disorders associated with reproduction and gestation, including polycystic ovary syndrome, recurrent miscarriage, gestational diabetes mellitus, pre‐eclampsia and intrauterine growth restriction. Due to the relevant incidence of the mentioned diseases and the importance of leptin, we decided to review the latest information available about leptin action in normal and pathological pregnancies to support the idea of leptin as an important factor and/or predictor of diverse disorders associated with reproduction and pregnancy.     

Improvement of Metabolism among Obese Breast Cancer Survivors in Differing Weight Loss Regimens

Objective: To compare the efficacy of different weight loss regimens on body weight loss and metabolic improvement in breast cancer survivors. Research Methods and Procedures: Forty‐eight obese breast cancer survivors were randomly divided into four groups and were followed for 1 year: 1) the Control group (subjects did not receive specific nutrition counseling); 2) the Weight Watchers group (subjects were given free coupons to attend weekly Weight Watchers meetings); 3) the Individualized group (a registered dietitian provided one‐on‐one nutritional counseling); and 4) the Comprehensive group (subjects received individualized dietary counseling and free coupons for the weekly Weight Watchers meetings). At baseline and 3‐, 6‐, and 12‐month data collection visits, a fasting blood sample was obtained for assays. A three‐day dietary record was kept during the week before these visits and dietary intake was analyzed. Results: Subjects in the three intervention groups lost weight (Control: 1.1 ± 1.7 kg; Weight Watchers: ?2.7 ± 2.1 kg; Individualized: ?8.0 ± 1.9 kg; Comprehensive: ?9.5 ± 2.7 kg) and percentage body fat, but only the Individualized and Comprehensive groups had significant losses. Subjects in the Comprehensive group showed the most improvement in cholesterol levels and had reductions in blood leptin levels. Discussion: Because insulin resistance and high blood leptin levels are associated with breast cancer, losing weight to improve these parameters may reduce the risk of recurrence. Only subjects in the Comprehensive group showed significant reductions in body weight and fat, energy intake, and leptin levels. For breast cancer survivors, different weight loss strategies should be considered to assist them in losing weight.     

Restoration of leptin responsiveness in diet‐induced obese mice using an optimized leptin analog in combination with exendin‐4 or FGF21

The identification of leptin as a mediator of body weight regulation provided much initial excitement for the treatment of obesity. Unfortunately, leptin monotherapy is insufficient in reversing obesity in rodents or humans. Recent findings suggest that amylin is able to restore leptin sensitivity and when used in combination with leptin enhances body weight loss in obese rodents and humans. However, as the uniqueness of this combination therapy remains unclear, we assessed whether co‐administration of leptin with other weight loss‐inducing hormones equally restores leptin responsiveness in diet‐induced obese (DIO) mice. Accordingly, we report here the design and characterization of a series of site‐specifically enhanced leptin analogs of high potency and sustained action that, when administered in combination with exendin‐4 or fibroblast growth factor 21 (FGF21), restores leptin responsiveness in DIO mice after an initial body weight loss of 30%. Using either combination, body weight loss was enhanced compared with either exendin‐4 or FGF21 monotherapy, and leptin alone was sufficient to maintain the reduced body weight. In contrast, leptin monotherapy proved ineffective when identical weight loss was induced by caloric restriction alone over a comparable time. Accordingly, we find that a hypothalamic counter‐regulatory response to weight loss, assessed using changes in hypothalamic agouti related peptide (AgRP) levels, is triggered by caloric restriction, but blunted by treatment with exendin‐4. We conclude that leptin re‐sensitization requires pharmacotherapy but does not appear to be restricted to a unique signaling pathway. Our findings provide preclinical evidence that high activity, long‐acting leptin analogs are additively efficacious when used in combination with other weight‐lowering agents. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd.     

A physiological role of breast milk leptin in body weight control in developing infants

Objective: Leptin, a hormone that regulates food intake and energy metabolism, is present in breast milk. The aim of this study was to determine whether milk leptin concentration is correlated with maternal circulating leptin and BMI and with body weight gain of infants. Research Methods and Procedures: A group of 28 non‐obese women (BMI between 16.3 and 27.3 kg/m²) who breast‐fed their infants for at least 6 months and their infants were studied. Venous blood and milk samples were obtained from mothers at 1, 3, 6, and 9 months of lactation, and leptin concentration was determined. Infant body weight and height were followed until 2 years of age. Results: During the whole lactation period, milk leptin concentration correlated positively with maternal plasma leptin concentration and with maternal BMI. In addition, milk leptin concentration at 1 month of lactation was negatively correlated with infant BMI at 18 and 24 months of age. A better negative correlation was also found between log milk leptin concentration at 1 and at 3 months of lactation and infant BMI from 12 to 24 months of age. Discussion: We concluded that, in a group of non‐obese mothers, infant body weight during the first 2 years may be influenced by milk leptin concentration during the first stages of lactation. Thus, moderate milk‐borne maternal leptin appears to provide moderate protection to infants from an excess of weight gain. These results seem to point out that milk leptin is an important factor that could explain, at least partially, the major risk of obesity of formula‐fed infants with respect to breast‐fed infants.     

β‐sheet breaker peptide inhibitor of Alzheimer's amyloidogenesis with increased blood–brain barrier permeability and resistance to proteolytic degradation in plasma

Short synthetic peptides homologous to the central region of Aβ but bearing proline residues as β‐sheet blockers have been shown in vitro to bind to Aβ with high affinity, partially inhibit Aβ fibrillogenesis, and redissolve preformed fibrils. While short peptides have been used extensively as therapeutic drugs in medicine, two important problems associated with their use in central nervous system diseases have to be addressed: (a) rapid proteolytic degradation in plasma, and (b) poor blood–brain barrier (BBB) permeability. Recently, we have demonstrated that the covalent modification of proteins with the naturally occurring polyamines significantly increases their permeability at the BBB. We have extended this technology to iAβ11, an 11‐residue β‐sheet breaker peptide that inhibits Aβ fibrillogenesis, by covalently modifying this peptide with the polyamine, putrescine (PUT), and evaluating its plasma pharmacokinetics and BBB permeability. After a single intravenous bolus injection in rats, both ¹²⁵I‐YiAβ11 and ¹²⁵I‐PUT‐YiAβ11 showed rapid degradation in plasma as determined by trichloroacetic acid (TCA) precipitation and paper chromatography. By switching to the all d ‐enantiomers of YiAβ11 and PUT‐YiAβ11, significant protection from degradation by proteases in rat plasma was obtained with only 1.9% and 5.7% degradation at 15 min after intravenous bolus injection, respectively. The permeability coefficient × surface area product at the BBB was five‐ sevenfold higher in the cortex and hippocampus for the ¹²⁵I‐PUT‐d ‐YiAβ11 compared to the ¹²⁵I‐d ‐YiAβ11, with no significant difference in the residual plasma volume. In vitro assays showed that PUT‐d ‐YiAβ11 retains its ability to partially inhibit Aβ fibrillogenesis and dissolve preformed amyloid fibrils. Because of its five‐ to sevenfold increase in permeability at the BBB and its resistance to proteolysis in the plasma, this polyamine‐modified β‐sheet breaker peptide may prove to be an effective inhibitor of amyloidogenesis in vivo and, hence, an important therapy for Alzheimer's disease. © 1999 John Wiley & Sons, Inc. J Neurobiol 39: 371–382, 1999     

Epigenetic silencing of the WNT antagonist Dickkopf 3 disrupts normal Wnt/β‐catenin signalling and apoptosis regulation in breast cancer cells

Dickkopf‐related protein 3 (DKK3) is an antagonist of Wnt ligand activity. Reduced DKK3 expression has been reported in various types of cancers, but its functions and related molecular mechanisms in breast tumorigenesis remain unclear. We examined the expression and promoter methylation of DKK3 in 10 breast cancer cell lines, 96 primary breast tumours, 43 paired surgical margin tissues and 16 normal breast tissues. DKK3 was frequently silenced in breast cell lines (5/10) by promoter methylation, compared with human normal mammary epithelial cells and tissues. DKK3 methylation was detected in 78% of breast tumour samples, whereas only rarely methylated in normal breast and surgical margin tissues, suggesting tumour‐specific methylation of DKK3 in breast cancer. Ectopic expression of DKK3 suppressed cell colony formation through inducing G0/G1 cell cycle arrest and apoptosis of breast tumour cells. DKK3 also induced changes of cell morphology, and inhibited breast tumour cell migration through reversing epithelial‐mesenchymal transition (EMT) and down‐regulating stem cell markers. DKK3 inhibited canonical Wnt/β‐catenin signalling through mediating β‐catenin translocation from nucleus to cytoplasm and membrane, along with reduced active‐β‐catenin, further activating non‐canonical JNK signalling. Thus, our findings demonstrate that DKK3 could function as a tumour suppressor through inducing apoptosis and regulating Wnt signalling during breast tumorigenesis.     

Antiestrogenic and anticancer activities of peptides derived from the active site of alpha‐fetoprotein

Cyclo[EKTOVNOGN] (AFPep), a cyclic 9‐amino acid peptide derived from the active site of alpha‐fetoprotein, has been shown to prevent carcinogen‐induced mammary cancer in rats and inhibit the growth of ER⁺ human breast cancer xenografts in mice. Recently, studies using replica exchange molecular dynamics predicted that the TOVN region of AFPep might form a dynamically stable putative Type I beta‐turn, and thus be biologically active without additional amino acids. The studies presented in this paper were performed to determine whether TOVN and other small analogs of AFPep would inhibit estrogen‐stimulated cancer growth and exhibit a broad effective‐dose range. These peptides contained nine or fewer amino acids, and were designed to bracket or include the putative pharmacophoric region (TOVN) of AFPep. Biological activities of these peptides were evaluated using an immature mouse uterine growth inhibition assay, a T47D breast cancer cell proliferation assay, and an MCF‐7 breast cancer xenograft assay. TOVN had very weak antiestrogenic activity in comparison to AFPep's activity, whereas TOVNO had antiestrogenic and anticancer activities similar to AFPep. OVNO, which does not form a putative Type I beta‐turn, had virtually no antiestrogenic and anticancer activities. A putative proteolytic cleavage product of AFPep, TOVNOGNEK, significantly inhibited E₂‐stimulated growth in vivo and in vitro over a wider dose range than AFPep or TOVNO. We conclude that TOVNO has anticancer potential, that TOVNOGNEK is as effective as AFPep in suppressing growth of human breast cancer cells, and that it does so over a broader effective‐dose range. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.