LACTATION AND MENSTRUAL FUNCTION AS RELATED TO CANCER OF THE BREAST

Significance of rat mammary tumors for human risk assessment

We have previously indicated that the ideal animal tumor model should mimic the human disease. This means that the investigator should be able to ascertain the influence of host factors on the initiation of tumorigenesis, mimic the susceptibility of tumor response based on age and reproductive history, and determine the response of the tumors induced to chemotherapy. The utilization of experimental models of mammary carcinogenesis in risk assessment requires that the influence of ovarian, pituitary, and placental hormones, among others, as well as overall reproductive events are taken into consideration, since they are important modifiers of the susceptibility of the organ to neoplastic development. Several species, such as rodents, dogs, cats, and monkeys, have been evaluated for these purposes; however, none of them fulfills all the criteria specified previously. Rodents, however, are the most widely used models; therefore, this work will concentrate on discussing the rat rodent model of mammary carcinogenesis.

Increased standardized incidence ratio of breast cancer in female electronics workers

BACKGROUND

In 1994, a hazardous waste site, polluted by the dumping of solvents from a former electronics factory, was discovered in Taoyuan, Taiwan. This subsequently emerged as a serious case of contamination through chlorinated hydrocarbons with suspected occupational cancer. The objective of this study was to determine if there was any increased risk of breast cancer among female workers in a 23-year follow-up period.

METHODS

A total of 63,982 female workers were retrospectively recruited from the database of the Bureau of Labor Insurance (BLI) covering the period 1973-1997; the data were then linked with data, up to 2001, from the National Cancer Registry at the Taiwanese Department of Health, from which standardized incidence ratios (SIRs) for different types of cancer were calculated as compared to the general population.

RESULTS

There were a total of 286 cases of breast cancer, and after adjustment for calendar year and age, the SIR was close to 1. When stratified by the year 1974 (the year in which the regulations on solvent use were promulgated), the SIR of the cohort of workers who were first employed prior to 1974 increased to 1.38 (95% confidence interval, 1.11-1.70). No such trend was discernible for workers employed after 1974. When 10 years of employment was considered, there was a further increase in the SIR for breast cancer, to 1.62. Those workers with breast cancer who were first employed prior to 1974 were employed at a younger age and for a longer period. Previous qualitative studies of interviews with the workers, corroborated by inspection records, showed a short-term high exposure to chlorinated alkanes and alkenes, particularly trichloroethylene before 1974. There were no similar findings on other types of cancer.

CONCLUSION

Female workers with exposure to trichloroethylene and/or mixture of solvents, first employed prior to 1974, may have an excess risk of breast cancer.

Differences in plasma and nipple aspirate carotenoid by lactation status

BACKGROUND

Dietary antioxidants, such as provitamin A carotenoid, have a protective effect against breast cancer. The transport of carotenoid from the blood into the breast microenvironment may be enhanced by lactation.

OBJECTIVE

To examine the association between plasma and nipple aspirate carotenoid levels by lactation and post-wean status.

METHODS

The sample consisted of 43 women, ages 18-45, who were at least 12 months postpartum. Women who had breastfed their last infant were at least 3 months post-wean. Women collected breast fluid every other day for 17 days and had a venipuncture for total nipple aspirate and plasma carotenoid, and completed a written health assessment.

RESULTS

The association between plasma and nipple aspirate carotenoid levels was significant for breastfeeding women (r =.39, p=.03), but not for non-breastfeeding women (r =.31, p =.27). However, while the association between plasma and nipple aspirate carotenoid levels was significant for women at or less than 9 months post-wean (r =.65, p = .01), the effect for women after 9 months post-wean (r = .21, p =.45) was not significant.

CONCLUSION

Lactation may be protective by enhancing the delivery of chemopreventive substances available in the blood to the cell level of the breast, even after breast involution has occurred post lactation.

Lactation reduces breast cancer risk in Shandong Province, China

Results from studies of western populations investigating lactation and breast cancer risk have been inconsistent. To examine this issue, the authors conducted a hospital-based case-control study in Shandong Province, China, in 1997-1999. A total of 404 cases and an equal number of controls were included. Detailed information regarding lactation, menstruation, and reproduction was collected through in-person interview. The authors found a significant inverse association between duration of lactation and breast cancer risk. For women who breastfed for more than 24 months per child, the odds ratio was 0.46 (95% confidence interval (CI): 0.27, 0.78) when compared with those who breastfed for 1-6 months per child. A significantly reduced risk of breast cancer was also found for those whose lifetime duration of lactation totaled 73-108 months (odds ratio = 0.47, 95% CI: 0.23, 0.95) and for those who breastfed for > or =109 months (odds ratio = 0.24, 95% CI: 0.11, 0.53). The test for trend was statistically significant for both mean duration of lactation per child (p = 0.02) and lifetime duration of lactation (p = 0.00). Further stratification by menopausal status resulted in the same conclusion. These data suggest that prolonged lactation reduces breast cancer risk.

Use of artificial neural networks in modeling associations of discriminant factors: towards an intelligent selective breast cancer screening

In order to improve the costs/benefits ratio of breast cancer (BC) screenings, the author evaluated the performance of a back-propagation artificial neural network (ANN) to predict an outcome (cancer/not cancer) to be used as classificator. Networks were trained on data from familial history of cancer, and sociodemographic, gynecoobstetric and dietary variables. The ANN achieved up to 94.04% of positive predictive value and 97.60% of negative predictive value. Results could operate as guidelines for preselecting women who would be considered as a BC high-risk subpopulation. The procedure--not only based on age factor, but on a multifactorial basis--appears to be a promising method of achieving a more efficient detection of preclinical, asymptomatic BC cases.

Influence of age and parity on the development of the human breast

Breast cancer is heavily influenced by the reproductive history of the individual. Pregnancy has a protective effect which is attributed to differences in the degree of differentiation of the breast. The purpose of this work was to determine whether the quantity and the type of parenchymal structures present in the human breast were related to the age and parity history of a woman. Fifty-one human breast samples were obtained from bilateral or unilateral reduction mammoplasties performed in 40 parous women ranging in age from 18 to 57 years, and 11 nulliparous women ranging in age from 14 to 54 years. An average of 100 grams of tissue/specimen were processed for whole mount. A total of 650 slides were examined and 31,222 structures were classified and counted under the light microscope. The following mammary structures were identified: terminal structures (TS), and lobules (LOB) type 1, 2, and 3. Results were plotted for the total patient population and separately for nulliparous and parous women against age. The total patient population contained similar proportions of LOb1, 2, and 3 between ages 14-18, with a reduction of percentage of Lob1 and increase in Lob3 between ages 23 to the middle forties, when Lob3 decreased and Lob1 increased to 70%. Lob2 and TS did not exhibit significant changes throughout the period of life analyzed. When analyzed separately it was found that the breasts of nulliparous women were predominantly composed of Lob1, fewer Lob2, with Lob3 almost completely absent, whereas parous women had a high frequency of Lob3, which were the predominant structures until the fourth decade of life.(ABSTRACT TRUNCATED AT 250 WORDS)

Reproductive factors in breast cancer epidemiology

There is abundant epidemiologic evidence showing that early menarche, late menopause, low parity and late age at first birth are related to increased risk of breast cancer. However, in younger age groups, uniparous women seem to be at higher risk than nulliparous, and the effect of later pregnancies is less clear in this group. Intervals between pregnancies may modify the general protective effect. Some studies have indicated an adverse effect of late age at pregnancies after the first. Further studies are necessary to determine if the general protective effect of pregnancies after the first is preceded by a transient increase in breast cancer risk. No clear association has been established with number of abortions. Results from two large prospective studies suggest that breast feeding is not strongly related to risk of breast cancer among Western populations.

Epidemiological and biochemical support for a theory on the cause and prevention of breast cancer

Damage to the breast epithelium by chemical carcinogens as products of oxygen free radical release can lead to fibroblast proliferation, hyperplasia of epithelium, cellular atypia and breast cancer. Chemical carcinogens may accumulate in breast fluid in the non-lactating breast consequent to superoxide free radical production which occurs via the adenosine triphosphate (ATP) hypoxanthine pathway. This pathway is initiated by hypoxia of local tissue. Under hypoxic conditions ATP is broken down to form hypoxanthine. Hypoxanthine itself is broken down to produce xanthine and then uric acid. This results in the production of superoxide free radicals, the products of which are carcinogenic. The development of localized hypoxia, which is central to this hypothesis, is caused by acinal gland distention from fluid secreted by raised prolactin levels in the absence of oxytocin. Stimulation of the nipple in a non-lactating breast may raise plasma oxytocin and lower plasma prolactin levels. Contraction of the myoepithelial cells of the breast under the influence of oxytocin would relieve distention of the acinal glands and thus reduce hypoxia and the generation of lipid peroxidoses as products of free radical damage. The epidemiology of breast fibrosis and cancer support the notion that lack of nipple stimulation over time may be a significant variable. A review of this literature linked with current biochemical work on fibrosis and carcinogenesis suggest that draining the breasts of the products of superoxide free-radical release by the encouragement of regular nipple erections may prevent such breast disease.

Number of children and death from hormone-dependent cancers

The association between number of children of current marriage reported by 431,604 women aged 45-74 years at the Norwegian Census in 1970 and mortality, at follow-up through 1985, from hormone-dependent cancers, i.e., cancers of breast, corpus uteri and ovary combined, has been investigated. The reduction in age-adjusted mortality was 9.6% (95% confidence interval; 8.3-10.9%) for each child with no deviation from linearity. Women with 8-11 children had a relative risk of 0.34 (0.25-0.47) compared to nullipara. Adjustment for age at first birth slightly changed the effect of number of children on mortality to 9.3% per child. The reduction in mortality per child was for cancer of the breast 7.2%, corpus uteri 12.2% and ovary 12.7%. Our findings indicate that having few children is a major risk factor for death from the 3 hormone-dependent cancers combined.

Menstrual factors and breast cancer risk

Relationships between menstrual factors and breast cancer risk were investigated in a prospective study of 63,090 Norwegian women. A total of 1565 cases of breast cancer occurred during follow-up from 1961 through 1980. The risk of breast cancer decreased with increasing age at menarche (P = 0.06) and increased with increasing age at menopause (P = 0.005) in analyses adjusted for age, urban/rural place of residence, parity, and age at first and last birth. The results correspond to an average increase in breast cancer risk of 4.0% for each year of decrease in age at menarche, and an increase in risk of 3.6% for each year of increase in age at menopause. The protective effect of early menopause was strongest for breast cancer diagnosed in patients 80 years of age or older. No clear relationship was seen between menstrual irregularities and breast cancer risk.

Influence of pregnancy and lactation on serum and breast fluid estrogen levels: implications for breast cancer risk

We investigated estrogen (estrone and estradiol) levels in serum and in nipple aspirates of breast fluid in relation to reproductive and menopausal characteristics in 104 normal women. In general, breast fluid and serum estrogen levels were not correlated and breast fluid estrogen levels were approximately 5 to 45 times higher than serum levels. Serum estrogen levels were lower in post-menopausal than in pre-menopausal women. In contrast, breast fluid estrogen levels were approximately the same in pre- and post-menopausal women. Breast fluid estrogen mean levels were lower in pre-menopausal parous women than in nulligravidous or nulliparous women whereas serum estrogen levels did not differ in these 3 groups. Breast fluid estrogen levels were positively correlated with months since last birth or since last breast-feeding. Estrogen levels were low in nipple aspirates of breast milk but gradually increased in breast fluid of non-lactating women over a period of several years after cessation of lactation. Serum estrogen levels did not increase with months since last breast-feeding. We were unable to evaluate the post-partum effect of pregnancy without lactation due to the small numbers of these subjects. The high concentrations of estrogen in breast fluid and the absence of a relationship to serum estrogen levels may explain why prior serum studies have failed to link variations in serum estrogens with breast cancer risk. The prolonged low levels of breast fluid estrogens following full-term birth and lactation may, in part, provide a mechanism by which parity reduces breast cancer risk.

Molecular and cellular basis of the mammary gland susceptibility to carcinogenesis

Mammary carcinomas induced by the administration of 7,12-dimethylbenz(a)anthracene (DMBA) to young virgin rats arise from undifferentiated terminal ductal structures called terminal end buds (TEBs). TEBs that normally differentiate into alveolar buds (ABs) and lobules under the influence of DMBA develop intraductal proliferations which progress to carcinoma. The high susceptibility of the young virgin rat TEBs to neoplastic transformation is due to its large proliferative compartment, with cells cycling every 10 hr, and to a higher (3)H-DMBA uptake. Progressive differentiation of TEBs into ABs and lobules or their regression to terminal ducts (TDs) is seen with aging. Complete differentiation of the gland is attained only through pregnancy and lactation. The greater differentiation of the gland is manifested as permanent structural changes, consisting in the disappearance of TEBs and in a diminution of the number of TDs due to their differentiation into ABs and lobules. This greater differentiation results in a diminished or total refractoriness of the gland to the carcinogen because ABs and lobules have a lower proliferative compartment and a longer cell cycle than TEBs and TDs. Cells of parous rats have both in vivo and in vitro a lower DMBA-DNA binding capacity, a lower DNA synthesis and a greater ability to repair DMBA damaged DNA than cells of young virgin rats. The more efficient DNA repair capacity of the parous rat mammary gland is demonstrated by the induction of unscheduled DNA synthesis and a removal of DMBA-DNA adducts.

Differentiation of the mammary gland and susceptibility to carcinogenesis

It has been demonstrated that in humans certain factors such as early menarche, late pregnancy, and nulliparity are associated with a higher risk of developing breast cancer, while early pregnancy acts as a protective factor. Induction of mammary cancer in rats by administration of the chemical carcinogen 7,12-dimethylbenz(a)anthracene reveals that the same factors influencing human breast cancer risk also affect the susceptibility of the rat mammary gland to the chemical carcinogen. Nulliparous rats and rats undergoing pregnancy interruption are more susceptible to developing carcinomas. This fact has been attributed to the incomplete differentiation of the gland at the time of carcinogen administration. Parous rats are resistant to the carcinogenic effect of DMBA, which is explained by the complete development of the gland attained during pregnancy and lactation. This development is manifested by the differentiation of terminal end buds into secretory units, which have a smaller proliferative compartment; the epithelial cells of these secretory units have a longer cell cycle, less avidity for binding DMBA, and possess a more efficient DNA excision repair capacity.

The epidemiology of breast cancer in 785 United States Caucasian women

A retrospective case-control hospital study of 785 Caucasian breast cancer patients and 2,231 age-stratified controls was conducted in New York City from 1969-1975. Patients were grouped by pre- peri- and postmenopausal status at diagnosis for the analysis to make a distinctive separation for variables showing a pre- and postmenopausal differential. Demographic characteristics were similar for cases and controls. Previously recorded hormone-related risk variables for this disease were largely confirmed for pre- and perimenopausal women, i.e., late age at first birth (greater than 25), premenstrual symptoms of breast swelling and premenopausal chills and flushes. Mother's history of breast cancer was also found to be a risk variable. Nulliparity was a risk factor only perimenopausally. No risk was foun for absolute height, weight or for obesity (Quetelet Index), prior breast diseases or previous usage of exogenous hormones of any type and no "protective" effect was found for multiparous women and for nursing. Perimenopausally diagnosed patients (menopause to 10 years after) were similar to premenopausally diagnosed women on most risk factors. Risk variables determined by this and other case-control studies cannot account for the magnitude of differences in the international incidence of breast cancer.

Reproductive history and risk of breast cancer: a case-control study in an unselected Swedish population

Variables in reproductive histories were studied in 179 consecutively detected, unselected breast cancer patients and age-matched controls selected from a computerized population register. The comparison between patients and controls showed no significant difference in age at meanarche, age at first birth, age at menopause or number of children. A subdivision into pre- and post-menopausal women yielded no further information. These results are at variance with most earlier reports, possible because the controls here were selected from the whole female population instead of hospitalized patients. Our data do not support the view that it is possible to define groups at high risk for breast cancer on the basis of productive histories.

Rat mammary deoxyribonucleic acid synthesis during the estrous cycle, pregnancy, and lactation in relation to mammary tumorigenesis: its implication for human breast cancer

Rat mammary DNA synthesis is greatly reduced during late pregnancy and lactation; this corresponds to the decreased incidence of induced mammary tumors by chemical carcinogens during these periods. The protective effects of pregnancy and lactation against mammary tumorigenesis of rats and human subjects may be explained by the prolonged phases of mitotic rest encountered during pregnancy and lactation.

Etiology of breast cancer. II. Epidemiologic aspects

The epidemiology of breast cancer is reviewed with particular emphasis on its etiology. A number of studies suggest that differences in breast cancer incidence are associated with differences in marital status, number of pregnancies, age at menarche, age at menopause, height and weight, socioeconomic status, geographic location and residence. However, in no case is the evidence adequate to establish a "cause and effect" relationship. The genetic component of these associations may be of primary importance, while other conditions such as marital status are probably indirect reflections of the operation of more fundamental factors.There is a general consensus that endocrine factors play an important part in mammary cancer occurrence. At present, the association between breast cancer and the presence of the virus-like (type B) particles in human milk is not established.

Social trauma as related to cancer of the breast

A number of writers, primarily in the field of psychophysiology, have suggested that breast cancer may be related to a variety of untoward psychological states and that these may be related in turn to having experienced misfortune in the social milieu. Other research has indicated that endocrine function may figure in the etiology of this disease. For these reasons, we wished to examine the relationship between the experiencing of social trauma which could induce endocrine effect and the development of cancer of the breast. We hypothesized that breast cancer cases, more often than controls, would have encountered traumatic incidents in their social milieu in the 5-year period prior to the diagnosis of their disease.Three hundred and fifty-two breast cancer cases and 670 controls with other types of cancer and non-neoplastic diseases of organs other than the breast and genitalia from Roswell Park Memorial Institute were interviewed. Comparisons were made concerning the extent to which the subjects and their immediate and extended families incurred such life events as death, divorce, illness, economic want, residential mobility, and feelings of being upset. No difference was found between the breast cancer cases and the controls either in the experiencing of single events or cumulative numbers of events by themselves or by members of their families. There may be events of a different type, not studied here, which are related to the development of cancer of the breast.

Lactation and cancer of the breast. A summary of an international study

An international collaborative study has been carried out to test the hypothesis that prolonged lactation protects women against cancer of the breast. While pregnancy itself seemed to confer some protection against breast cancer in all areas studied, no consistent differences in duration of lactation were found between breast cancer patients and unaffected women, once the fact that breast cancer patients have fewer pregnancies had been allowed for. Even in areas where some women had lactated for a total of 5 years or more, such women occurred proportionately no less frequently among breast cancer patients than among unaffected women. In the light of this and other recent evidence, it is unlikely that lactation has any protective effect against breast cancer in women, and other explanations must be sought for the remarkable international differences in the frequency of this disease.

Age at first birth and breast cancer risk

An international collaborative study of breast cancer and reproductive experience has been carried out in 7 areas of the world. In all areas studied, a striking relation between age at first birth and breast cancer risk was observed. It is estimated that women having their first child when aged under 18 years have only about one-third the breast cancer risk of those whose first birth is delayed until the age of 35 years or more. Births after the first, even if they occur at an early age, have no, or very little, protective effect. The reduced risk of breast cancer in women having their first child at an early age explains the previously observed inverse relationship between total parity and breast cancer risk, since women having their first birth early tend to become ultimately of high parity. The association with age at first birth requires different kinds of etiological hypotheses from those that have been invoked in the past to explain the association between breast cancer risk and reproductive experience.