Breast cancer incidence in ex-smokers in relation to body mass index, weight gain and blood lipid levels

Serum cholesterol and the risk of developing hormonally driven cancers: A narrative review

Although cholesterol has been hypothesized to promote cancer development through several potential pathways, its role in the risk of developing hormonally driven cancer is controversial. This literature review summarizes evidence from the highest quality studies to examine the consistency and strength of the relationship between serum cholesterol parameters and incidence of hormonally driven cancer. Articles were identified using EMBASE. Longitudinal observational studies published between January 2000 and December 2020 were considered for inclusion. The endpoint of interest was incident prostate, ovary, breast, endometrium, and uterine cancers. In total, 2732 reports were identified and screened; 41 studies were included in the review. No associations were found for ovarian cancer. Most endometrial cancer studies were null. The majority (76.9%) of studies reported no association between cholesterol and prostate cancer. Data on breast cancer were conflicting, associations limited, and effect sizes modest. Our results do not provide evidence for a clear association between cholesterol and different types of incident, hormonally driven reproductive cancers. Future studies should investigate the impact of lipid-lowering therapy.

World Cancer Research Fund International: Continuous Update Project-systematic literature review and meta-analysis of observational cohort studies on physical activity, sedentary behavior, adiposity, and weight change and breast cancer risk

PURPOSE

The purpose of the present study was to systematically review the complex associations between energy balance-related factors and breast cancer risk, for which previous evidence has suggested different associations in the life course of women and by hormone receptor (HR) status of the tumor.

METHODS

Relevant publications on adulthood physical activity, sedentary behavior, body mass index (BMI), waist and hip circumferences, waist-to-hip ratio, and weight change and pre- and postmenopausal breast cancer risk were identified in PubMed up to 30 April 2017. Random-effects meta-analyses were conducted to summarize the relative risks across studies.

RESULTS

One hundred and twenty-six observational cohort studies comprising over 22,900 premenopausal and 103,000 postmenopausal breast cancer cases were meta-analyzed. Higher physical activity was inversely associated with both pre- and postmenopausal breast cancers, whereas increased sitting time was positively associated with postmenopausal breast cancer. Although higher early adult BMI (ages 18-30 years) was inversely associated with pre- and postmenopausal breast cancers, adult weight gain and greater body adiposity increased breast cancer risk in postmenopausal women, and the increased risk was evident for HR+ but not HR- breast cancers, and among never but not current users of postmenopausal hormones. The evidence was less consistent in premenopausal women. There were no associations with adult weight gain, inverse associations with adult BMI (study baseline) and hip circumference, and non-significant associations with waist circumference and waist-to-hip ratio that were reverted to positive associations on average in studies accounting for BMI. No significant associations were observed for HR-defined premenopausal breast cancers.

CONCLUSION

Better understanding on the impact of these factors on pre- and postmenopausal breast cancers and their subtypes along the life course is needed.

Serum Lipids and Breast Cancer Risk: A Meta-Analysis of Prospective Cohort Studies

Purpose

Epidemiologic studies exploring causal associations between serum lipids and breast cancer risk have reported contradictory results. We conducted a meta-analysis of prospective cohort studies to evaluate these associations.

Methods

Relevant studies were identified by searching PubMed and EMBASE through April 2015. We included prospective cohort studies that reported relative risk (RR) estimates with 95% confidence intervals (CIs) for the associations of specific lipid components (i.e., total cholesterol [TC], high-density lipoprotein cholesterol [HDL-C], low-density lipoprotein cholesterol [LDL-C], and triglycerides [TG]) with breast cancer risk. Either a fixed- or a random-effects model was used to calculate pooled RRs.

Results

Fifteen prospective cohort studies involving 1,189,635 participants and 23,369 breast cancer cases were included in the meta-analysis. The pooled RRs of breast cancer for the highest versus lowest categories were 0.96 (95% CI: 0.86–1.07) for TC, 0.92 (95% CI: 0.73–1.16) for HDL-C, 0.90 (95% CI: 0.77–1.06) for LDL-C, and 0.93 (95% CI: 0.86–1.00) for TG. Notably, for HDL-C, a significant reduction of breast cancer risk was observed among postmenopausal women (RR = 0.77, 95% CI: 0.64–0.93) but not among premenopausal women. Similar trends of the associations were observed in the dose-response analysis.

Conclusions

Our findings suggest that serum levels of TG but not TC and LDL-C may be inversely associated with breast cancer risk. Serum HDL-C may also protect against breast carcinogenesis among postmenopausal women.

Serum triglycerides and cancer risk in the metabolic syndrome and cancer (Me-Can) collaborative study

OBJECTIVE

To assess the association between serum triglyceride levels and cancer risk.

METHODS

The metabolic syndrome and cancer project (Me-Can) includes cohorts from Norway, Austria, and Sweden; the current study included data on 257,585 men and 256,512 women. The mean age at study entry was 43.8 years for men and 44.2 years for women. The mean follow-up time was 13.4 years (SD = 8.5) for men and 11.9 years (SD = 7.2) for women. Excluding the first year of follow-up, 23,060 men and 15,686 women were diagnosed with cancer. Cox regression models were used to calculate relative risk (RR) of cancer for triglyceride levels in quintiles and as a continuous variable. RRs were corrected for random error by use of regression dilution ratio.

RESULTS

Relative risk for top quintile versus bottom quintile of triglycerides of overall cancer was 1.16 (95% confidence interval 1.06-1.26) in men and 1.15 (1.05-1.27) in women. For specific cancers, significant increases for top quintile versus bottom quintile of triglycerides among men were found for cancers of the colon, respiratory tract, the kidney, melanoma and thyroid and among women, for respiratory, cervical, and non-melanoma skin cancers.

CONCLUSION

Data from our study provided evidence for a possible role of serum triglycerides in cancer development.

Low triglyceride and nonuse of statins is associated with cancer in type 2 diabetes mellitus: the Hong Kong Diabetes Registry

BACKGROUND

Patients with type 2 diabetes mellitus (T2DM) have increased cancer risks. The authors reported nonlinear associations of cancer with triglyceride and other lipids in T2DM. Crosstalk between lipid metabolism and the renin-angiotensin system may increase cancer risk via activation of insulin-like growth factor-1 pathway in T2DM. In this analysis, the authors explored associations of cancer risk with high/low triglyceride in T2DM and possible modifying effects of statins on this risk association, if any.

METHODS

A consecutive cohort of 5166 Chinese patients with T2DM, free of cancer at enrollment and not using statins at or before enrollment, was analyzed using Cox models. Biological interactions were estimated using relative excess risk because of interaction, attributable proportion because of interaction, and synergy index. Relative excess risk because of interaction > 0, attributable proportion because of interaction > 0, or synergy index > 1 indicates biological interaction.

RESULTS

During 5.25 years of follow-up (median), 4.7% (n = 243) patients developed cancer. Triglyceride < 1.70 mmol/L was associated with increased cancer risk in the entire cohort and in statin nonusers, but not in statin users. Patients with triglyceride < 1.70 mmol/L plus nonuse of statins during follow-up had 2.74-fold increased cancer risk compared with their counterparts with either triglyceride ≥ 1.70 mmol/L or use of statins or both. There was significant interaction between triglyceride < 1.70 mmol/L and nonuse of statins (relative excess risk because of interaction, 0.99; 95% confidence interval [CI], 0.07-1.90 and attributable proportion because of interaction, 0.36; 95% CI, 0.02-0.70).

CONCLUSIONS

In Chinese T2DM patients, triglyceride < 1.70 mmol/L might be associated with increased cancer risk, which was attenuated in the presence of use of statins.

Serum calcium and tumour aggressiveness in breast cancer: a prospective study of 7847 women

Experimental, epidemiological and clinical studies suggest that calcium and/or its regulating hormones affect breast cancer risk. There has been no prospective cohort study investigating serum calcium levels and breast cancer aggressiveness, as determined by tumour histology and stage. Dichotomized prediagnostic serum calcium levels were investigated in relation to breast cancer aggressiveness as determined by grade (mitotic frequency, tubule formation, nuclear atypia) and stage (tumour size and axillary lymph node status). Cox's proportional hazards analysis and heterogeneity analysis were used to investigate the associations between low/high calcium and grade/stage in a prospective cohort study of 7847 women, out of whom 462 women were diagnosed with incident breast cancer during a mean follow-up of 17.2 years. All analyses were stratified for body mass index and menopausal status. Prediagnostic serum calcium levels in premenopausal women were positively associated with increased tumour aggressiveness as determined by a higher risk of nodal metastasis; relative risk (RR) for calcium above median as compared with calcium below median was 1.88 with a 95% confidence interval (CI) of 1.04-3.38. In overweight women, prediagnostic serum calcium levels were also associated with tumour aggressiveness, as determined by both a higher risk of nodal metastasis [RR (95% CI) 1.69 (0.95-3.02)] and severe nuclear atypia [RR (95% CI) 2.06 (1.10-3.86)]. Results also indicate that, in overweight women, calcium is positively associated with worse grade as determined by tubule formation and mitotic frequency. In conclusion, prediagnostic serum calcium levels are positively associated with increased tumour aggressiveness in premenopausal and/or overweight women.

A longitudinal study of the metabolic syndrome and risk of postmenopausal breast cancer

The metabolic syndrome, characterized by abdominal obesity, high blood glucose levels, impaired glucose tolerance, dyslipidemia, and hypertension, is associated with increased risk of type 2 diabetes and coronary heart disease. Several studies have examined the association of the individual components of the metabolic syndrome with breast cancer; to date, however, no study has assessed the metabolic syndrome per se in relation to breast cancer risk. Furthermore, previous studies have relied only on baseline assessment of components of the syndrome. Therefore, we assessed the association of the metabolic syndrome with the risk of postmenopausal breast cancer among women in the 6% sample of subjects in the Women's Health Initiative clinical trial and the 1% sample of women in the observational study who had repeated measurements of the components of the syndrome during follow-up. We used Cox proportional hazards models to estimate hazard ratios and 95% confidence intervals for the association of breast cancer risk with the presence of the metabolic syndrome, as well as its components, at baseline and in time-dependent analyses. After exclusion of women with diabetes, among 4,888 women with baseline measurements, 165 incident cases of breast cancer were ascertained over a median of 8 years of follow-up. The presence of the metabolic syndrome at baseline was not associated with altered risk. Of the individual components measured at baseline, diastolic blood pressure showed a borderline positive association with breast cancer. In time-dependent covariate analyses, however, certain scenarios indicated a positive association between the metabolic syndrome and breast cancer, due primarily to positive associations with serum glucose, serum triglycerides, and diastolic blood pressure.

Breast Cancer Risk Among Women Who Start Smoking as Teenagers

Objective: To examine the effect of smoking on breast cancer risk in a large population-based cohort of women, many of whom started smoking as teenagers.

Methods: We followed 102,098 women, ages 30 to 50 years, completing a mailed questionnaire at recruitment to the Norwegian-Swedish Cohort Study in 1991/1992, through December 2000. We used Cox proportional hazard regression models to estimate relative risk (RR) of breast cancer associated with different measures of smoking initiation, duration, and intensity adjusting for confounding variables. We conducted analyses on the entire study population, among women who had smoked for at least 20 years, among nondrinkers, and separately for each country.

Results: Altogether, 1,240 women were diagnosed with incident, invasive breast cancer. Compared with never smokers, women who smoked for at least 20 years and who smoked 10 cigarettes or more daily had a RR of 1.34 (95% CI, 1.06-1.70). Likewise, those who initiated smoking prior to their first birth (1.27, 1.00-1.62), before menarche (1.39, 1.03-1.87), or before age 15 (1.48, 1.03-2.13) had an increased risk. In contrast, women who had smoked for at least 20 years, but started after their first birth, did not experience an increased breast cancer risk. The increased RR associated with smoking was observed among nondrinkers of alcohol, women with and without a family history of breast cancer, premenopausal and postmenopausal women, and in both countries.

Conclusion: Our results support the notion that women who start smoking as teenagers and continue to smoke for at least 20 years may increase their breast cancer risk.

Etiology of Hormone Receptor–Defined Breast Cancer: A Systematic Review of the Literature

Breast cancers classified by estrogen receptor (ER) and/or progesterone receptor (PR) expression have different clinical, pathologic, and molecular features. We examined existing evidence from the epidemiologic literature as to whether breast cancers stratified by hormone receptor status are also etiologically distinct diseases. Despite limited statistical power and nonstandardized receptor assays, in aggregate, the critically evaluated studies (n = 31) suggest that the etiology of hormone receptor–defined breast cancers may be heterogeneous. Reproduction-related exposures tended to be associated with increased risk of ER-positive but not ER-negative tumors. Nulliparity and delayed childbearing were more consistently associated with increased cancer risk for ER-positive than ER-negative tumors, and early menarche was more consistently associated with ER-positive/PR-positive than ER-negative/PR-negative tumors. Postmenopausal obesity was also more consistently associated with increased risk of hormone receptor–positive than hormone receptor–negative tumors, possibly reflecting increased estrogen synthesis in adipose stores and greater bioavailability. Published data are insufficient to suggest that exogenous estrogen use (oral contraceptives or hormone replacement therapy) increase risk of hormone-sensitive tumors. Risks associated with breast-feeding, alcohol consumption, cigarette smoking, family history of breast cancer, or premenopausal obesity did not differ by receptor status. Large population-based studies of determinants of hormone receptor–defined breast cancers defined using state-of-the-art quantitative immunostaining methods are needed to clarify the role of ER/PR expression in breast cancer etiology.

Cytochrome P450 1A2 (CYP1A2) activity and risk factors for breast cancer: a cross-sectional study

INTRODUCTION

Breast cancer risk may be determined by various genetic, metabolic, and lifestyle factors that alter sex hormone metabolism. Cytochrome P450 1A2 (CYP1A2) is responsible for the metabolism of estrogens and many exogenous compounds, including caffeine.

METHODS

In a cross-sectional study of 146 premenopausal and 149 postmenopausal women, we examined the relationships between CYP1A2 activity and known or suspected risk factors for breast cancer. Blood levels of sex hormones, lipids, and growth factors were measured. In vivo CYP1A2 activity was assessed by measuring caffeine metabolites in urine. Stepwise and maximum R regression analyses were used to identify covariates related to CYP1A2 activity after adjustment for ethnicity.

RESULTS

In both menopausal groups CYP1A2 activity was positively related to smoking and levels of sex hormone binding globulin. In premenopausal women, CYP1A2 activity was also positively related to insulin levels, caffeine intake, age, and plasma triglyceride levels, and negatively related with total cholesterol levels and body mass index. In postmenopausal women CYP1A2 activity was positively associated with insulin-like growth factor-1, and negatively associated with plasma triglyceride, high-density lipoprotein cholesterol, and age at menarche.

CONCLUSION

These results suggest that CYP1A2 activity is correlated with hormones, blood lipids, and lifestyle factors associated with breast cancer risk, although some of the observed associations were contrary to hypothesized directions and suggest that increased CYP1A2 function may be associated with increased risk for breast cancer.

A comparative review of the risks and benefits of hormone replacement therapy regimens

The Women's Health Initiative (a large, randomized, placebo-controlled trial) investigated the effect of conjugated equine estrogens combined with medroxyprogesterone acetate on specific potential long-term benefits and risks. A review of the clinical studies that have investigated different types and regimens of estrogens combined with progestins was conducted to assess how applicable the results of the Women's Health Initiative are to hormone replacement therapy regimens in general. The studies that were reviewed were limited to randomized clinical trials and observational studies that have been published over the last 15 years (1987-2002) and to meta-analyses and reviews that may have included the literature before 1987. The increased risks for venous thromboembolism, stroke, coronary heart disease, and breast cancer that were identified in the Women's Health Initiative trial have also been reported with postmenopausal hormone therapies that contain a variety of estrogen and progestin products. The beneficial effects that were noted in the Women's Health Initiative, with respect to reductions in fractures and colorectal cancer, have not been evaluated in large, randomized controlled trials that use different estrogen/progestin combinations; however, observational trials that used a variety of estrogen or hormone replacement therapy products and randomized clinical studies that evaluated bone mineral density (an excellent predictor of fracture risk) with different estrogen/hormone replacement therapy regimens would suggest that results would be similar to those found in the Women's Health Initiative. Although the relief of menopausal symptoms, the primary reason women seek treatment, was not included in the overall benefit/risk analysis of the Women's Health Initiative, numerous trials suggest that all therapies are effective. Overall, these data indicate that the benefit/risk analysis that was reported in the Women's Health Initiative can be generalized to all postmenopausal hormone replacement therapy products.