Breast Cancer Promotes Cardiac Dysfunction Through Deregulation of Cardiomyocyte Ca2+-Handling Protein Expression That is Not Reversed by Exercise Training

Abstract

Background Patients treated for breast cancer have a high incidence of cardiovascular complications. In this study, we evaluated the impact of breast cancer on cardiac function and cardiomyocyte Ca²⁺-handling protein expression. We also investigated whether exercise training (ET) would prevent these potential alterations. Methods and Results Transgenic mice with spontaneous breast cancer (mouse mammary tumor virus-polyomavirus middle T antigen [MMTV-PyMT+], n=15) and littermate mice with no cancer (MMTV-PyMT-, n=14) were studied. For the ET analysis, MMTV-PyMT+ were divided into sedentary (n=10) and exercise-trained (n=12) groups. Cardiac function was evaluated by echocardiography with speckle-tracking imaging. Exercise tolerance test was conducted on a treadmill. Both studies were performed when the tumor became palpable and when it reached 1 cm³. After euthanasia, Ca²⁺-handling protein expression (Western blot) was evaluated. Exercise capacity was reduced in MMTV-PyMT+ compared with MMTV-PyMT- (P_interaction=0.031). Longitudinal strain (P_group <0.001) and strain rate (P_group=0.030) were impaired. Cardiomyocyte phospholamban was increased (P=0.011), whereas phospho-phospholamban and sodium/calcium exchanger were decreased (P=0.038 and P=0.017, respectively) in MMTV-PyMT+. No significant difference in sarcoplasmic or endoplasmic reticulum calcium 2 ATPase (SERCA2a) was found. SERCA2a/phospholamban ratio was reduced (P=0.007). ET was not associated with increased exercise capacity. ET decreased left ventricular end-systolic diameter (P_group=0.038) and end-diastolic volume (P_group=0.026). Other morphological and functional cardiac parameters were not improved by ET in MMTV-PyMT+. ET did not improve cardiomyocyte Ca²⁺-handling protein expression. Conclusions Breast cancer is associated with decreased exercise capacity and subclinical left ventricular dysfunction in MMTV-PyMT+, which is at least partly associated with dysregulation of cardiomyocyte Ca²⁺ handling. ET did not prevent or reverse these changes.

Keywords: Ca2+ handling; breast cancer; cardiac function.

Figure 1. Experimental protocol.

ETT indicates exercise tolerance test; PNM, palpable and not measurable tumor; Pre, pre phase tumor growth (BASELINE); and Post, post phase tumor growth (END).

Figure 2. Morphological and functional cardiac parameters in mouse mammary tumor virus–polyomavirus middle T antigen (MMTV‐PyMT)− and MMTV‐PyMT+ mice in the pre‐experimental and post‐experimental protocol.

Left‐ventricular (LV) end‐diastolic diameter (LVEDD) (A); LV end‐diastolic volume (LVEDV) (B); LV end‐systolic diameter (LVESD) (C); LV end‐systolic volume (LVESV) (D); left atrium (LA) (E); and LV mass (F). Parameters were corrected by weight. LVEDD: P _group<0.001, P _interaction=0.417; LVEDV: P _group=0.003, P _interaction=0.437; LA: P _interaction=0.004; post hoc test: ^* P=0.030, between group difference and ^†† P<0.001, within MMTV‐PyMT+ group difference; and LV mass: P _group<0.001, P _interaction=0.365. Generalized estimation equations with normal distribution and identity link function using AR(1) correlation matrix between evaluations followed by Bonferroni multiple comparison. Values are expressed as means and SD. Four mice in the MMTV‐PyMT− and 2 mice in the MMTV‐PyMT+ were excluded from this analysis because of poor imaging quality on echocardiography. Thus, 10 mice in the MMTV‐PyMT− and 13 mice in the MMTV‐PyMT+ were involved in this analysis. Of note, 2 LA measures were not obtained in the MMTV‐PyMT+ mice.

Figure 3. Cardiac function by speckle tracking in mouse mammary tumor virus–polyomavirus middle T antigen (MMTV‐PyMT)− and MMTV‐PyMT+—longitudinal long‐axis strain (A) and strain rate (B).

Longitudinal strain (P _group<0.001, P _interaction=0.126) and strain rate (P _group=0.030, P _interaction=0.168) were lower in MMTV‐PyMT+ mice when compared with MMTV‐PyMT− mice. Pre: pre phase tumor growth and post: post phase tumor growth. Generalized estimation equations with normal distribution and identity link function using AR(1) correlation matrix between evaluations. Values are expressed as means and SD. Three mice in the MMTV‐PyMT− and 3 mice in the MMTV‐PyMT+ groups were excluded from the speckle‐tracking analysis because of poor imaging quality. Thus, 11 mice in the MMTV‐PyMT− and 12 mice in the MMTV‐PyMT+ were involved in this analysis.

Figure 4. Cardiac myocyte Ca²⁺‐handling proteins in mouse mammary tumor virus–polyomavirus middle T antigen (MMTV‐PyMT)− and MMTV‐PyMT+ mice—phospholamban (A); SERCA2a (sarcoplasmic or endoplasmic reticulum calcium 2 ATPase) (B); phosphorilated phospholamban (p‐phospholamban) (C); sodium/calcium exchanger (NCX) (D); and SERCA2a/phospholambam ratio (E).

Western blot bands are shown for each protein. In the MMTV‐PyMT+ mice, phospholamban protein expression was increased (P=0.011) and SERCA2a protein expression was not different between the MMTV‐PyMT+ and MMTV‐PyMT− groups (P=0.165). SERCA2a/phospholamban ratio was reduced in the MMTV‐PyMT+ mice (P=0.007). p‐Phospholamban and NCX protein expression were decreased (P=0.038 and P=0.017, respectively) compared with MMTV‐PyMT− mice. Mann‐Whitney test. Values are expressed as medians and interquartile ranges (IQRs). Animals per group: 7 MMTV‐PyMT− and 7 MMTV‐PyMT+.