Enhanced breast cancer protection in Brca1/P53-deficient mice post-cabergoline treatment in pregnancy context
The well-established protective effect of pregnancy against breast cancer, documented across diverse mammalian models including mice and rats (16, 21-23), was further explored in this study using the Brca1/P53-deficient mouse model. These mice, predisposed to developing triple-negative basal breast cancer (45), were utilized to determine if pregnancy's protective effects could be potentiated in a genetically susceptible context.
In this comprehensive longitudinal study, female Brca1/P53-deficient mice underwent two full reproductive cycles, including pregnancy and lactation. For the purposes of this manuscript, these mice will be referred to interchangeably as either 'parous and breastfeeding mice' or 'multiparous mice' (Fig. 1a). This cohort was compared to nulliparous counterparts to assess the protective effect of pregnancy. The results indicated a significant delay in breast cancer onset in multiparous mice (median onset shifted from 175 to 224 days; P = 0.0002) and a notable reduction in tumor incidence (from 88.6% to 59.4%; P = 0.0061) (Fig. 1b, c and Extended Data Fig. 1a, b).
Given the known relationship between postlactational involution and breast cancer risk in several genetically modified mouse models, where delayed involution is associated with increased risk and accelerated involution with decreased risk (27), we explored the impact of cabergoline, a drug recognized for hastening postlactational involution in dairy cows (41, 42). Our focus was on its efficacy in altering breast cancer development post-pregnancy.
Administering a single dose of cabergoline at the conclusion of the second lactation period resulted in a marked extension of tumor latency in multiparous mice, compared to both untreated multiparous and nulliparous groups (extension to 259 days; P = 0.0119 and P < 0.0001, respectively) (Fig. 1b and Extended Data Fig. 1c, d). Moreover, cabergoline treatment notably decreased tumor incidence in multiparous mice, with a reduction to 27.8%, in contrast to 88.6% in nulliparous and 59.4% in untreated multiparous counterparts. These differences were statistically significant (P < 0.0001 and P < 0.0065, respectively) (Fig. 1c and Extended Data Fig. 1e, f).
Further exploration of cabergoline's potential protective effect in nulliparous Brca1/P53-deficient mice revealed a marginal and statistically non-significant decrease in tumor latency (P = 0.0771), with no observable change in tumor incidence (P = 0.3460) (Fig. 1d, f). A biweekly cabergoline regimen in a separate nulliparous cohort showed no significant alterations in tumor incidence or latency (Extended Data Fig. 1g, h), indicating the absence of a dose-dependent protective effect of cabergoline in the context of non-pregnancy.
These findings collectively underscore the significance of post-pregnancy cabergoline treatment in substantially reducing breast cancer risk in a genetically susceptible model, an effect not replicated in the absence of pregnancy.
Cabergoline's role in enhancing postlactational involution in Brca1/P53-deficient mice
Building on the premise that accelerated postlactational involution is inversely correlated with breast cancer risk (27), this study delved into the mechanisms by which cabergoline augments pregnancy-related breast cancer protection. A key focus was to determine if cabergoline indeed amplifies postlactational involution in a Brca1/P53-deficient mouse model.
In our detailed investigations, mammary glands of cabergoline-treated Brca1/P53-deficient mice were examined at intervals post-lactation. The findings revealed an accelerated involution process, characterized by an expanded adipocyte area at three and seven days post-treatment (P < 0.0001), suggesting an early onset of involution (Fig. 2a-c). A corresponding increase in apoptotic epithelial cells at the 3-day mark (P < 0.0001) further confirmed this observation (Fig. 2d-f). These trends were also evident in wild-type mice, reinforcing the role of cabergoline in involution acceleration (Fig. 2a-f).
We then explored the molecular underpinnings of this accelerated involution. The focus was on pSTAT3 and pSTAT5, molecules pivotal in initiating postlactational involution (38, 46). In cabergoline-treated mice, an upregulation of pSTAT3 was observed (Fig. 2g), aligning with the drug's involution-enhancing effects. Subsequent RNA-seq analysis of mammary glands 24 hours post-lactation identified 14 differentially expressed genes (DEGs) (P Adj ≤ 0.05) between untreated and cabergoline-treated groups, although initial enrichment analysis did not directly link these changes to involution (Supplementary Tables S1, S2).
To elucidate the underlying mechanisms of our observed increase in cell death during post-lactation involution, we refined our analytical approach. Initially, we identified differentially expressed genes (DEGs) using a threshold of P ≤ 0.05. Subsequent enrichment analysis of these DEGs enabled us to focus on specific biological processes relevant to our study. This approach, in line with established methodologies (47-51), enabled the identification of 263 overexpressed and 347 underexpressed transcripts in treated mice, facilitating distinct clustering of mammary gland samples from treated and untreated mice (Extended Data Fig. S2a and Supplementary Table S3).
Enrichment analysis pinpointed underexpressed genes in cabergoline-treated mice associated with homeostasis and ion transport, alongside overexpressed genes linked to immune processes (Fig. 2h, i, Extended Data Fig. 2b, and Supplementary Table S4). Ion channel homeostasis, particularly involving calcium ions, plays a crucial role in apoptosis (52-58). The observed changes in specific ion transporters in cabergoline-treated mice are consistent with the initiation of involution, potentially through elevated intracellular calcium levels (53-58). These patterns were consistent across various segments of the analysis (Supplementary Table S5).
The data collectively indicate that cabergoline enhances post-lactational involution, marked by increased apoptosis and accelerated adipocyte expansion (Fig. 2a-f). The drug appears to facilitate involution by regulating ion channel homeostasis, particularly impacting calcium ion transport, thus enhancing cell death and promoting involution.
Long-term impact of cabergoline on mammary gland ductal structure and proliferation dynamics in Brca1/P53-deficient mice
Given the established correlation between mammographic breast density and breast cancer risk (59, 60), and the inverse relationship between glandular fraction and mammary involution (61), we focused on the long-term effects of cabergoline on mammary gland composition as an indirect measure of breast cancer susceptibility (62, 63).
This study employed a cross-sectional and longitudinal approach to assess the glandular tissue evolution in Brca1/P53-deficient mice post-cabergoline treatment (Fig. 3).
In the cross-sectional analysis, we quantified ductal tissue in Brca1/P53-deficient mice post-involution. The comparison involved multiparous mice, both untreated and treated with cabergoline, at 30- and 60-days post-involution, alongside nulliparous, age-matched controls. Nulliparous mice exhibited an increased glandular component over time (P < 0.0001), while multiparous mice showed a decrease (P = 0.0038). Notably, cabergoline-treated mice consistently demonstrated the least glandular component, with minimal variation between the two time points (Fig. 3a).
In the longitudinal analysis, which included mice previously assessed for breast cancer susceptibility (Fig. 1), we observed a progressive increase in ductal area in nulliparous mice (P < 0.0001; r = 0.7842) (Fig. 3b). This pattern was not evident in multiparous mice (Fig. 3c). Intriguingly, cabergoline-treated mice maintained a smaller initial ductal area that increased minimally with age (Fig. 3d), but consistently less than in untreated mice. The minimal ductal area in the cabergoline-treated group was confirmed at 60 days post-involution (Fig. 3e, f), aligning with our hypothesis of cabergoline's role in reducing breast cancer susceptibility. Comparing cabergoline's effect on multiparous wild-type mice, we observed a consistent reduction in ductal area (Extended Data Fig. 3a-c).
To further explore the impact of cabergoline on mammary gland dynamics, we assessed apoptosis and epithelial proliferation in Brca1/P53-deficient mice. The cabergoline-treated group showed a significantly higher apoptotic rate than untreated multiparous mice (P = 0.0268) (Extended Data Fig. 3d, e), albeit minimal across all groups. Ki-67 expression, a marker for basal proliferation and a recognized breast cancer risk factor (64-66), was employed to quantify glandular tissue proliferation. Thus, we used this marker to assess glandular tissue proliferation as an indicator of breast cancer susceptibility. We found a significant increase in epithelial cell proliferation from 30 to 60 days in nulliparous mice (P = 0.0006), a trend absent in multiparous mice. Cabergoline-treated mice exhibited markedly lower proliferation rates at both time points (Fig. 3g).
Our longitudinal study further demonstrated that in nulliparous mice, the proliferation rate of cells in the mammary gland did not exhibit significant temporal variations (P = 0.74144; r = -0.0311) (Fig. 3h). However, this rate significantly decreased post-pregnancy (P < 0.0001; r = -0.4165) (Fig. 3i). The cabergoline-treated group consistently exhibited the lowest proportion of proliferating cells throughout their lifespan (P = 0.1691; r = -0.1352) (Fig. 3j), with the most pronounced decrease in epithelial cell proliferation confirmed at 60 days post-involution (Fig. 3k, l).
Comparisons in wild-type mice at 30 and 60 days post-involution, both treated and untreated with cabergoline, corroborated these findings (Extended Data Fig. 3f-h). Notably, the proliferation rate in nulliparous wild-type mice was lower than in their Brca1/P53-deficient counterparts (Extended Data Fig. 3i), emphasizing the potential role of basal breast tissue proliferation in breast cancer susceptibility (64-68).
Lastly, we linked the diminished proliferation in cabergoline-treated Brca1/P53-deficient mice to reduced pAKT and pERK levels in breast organoids, as determined by Western blot analysis (Fig. 3m, n).
These findings suggest that extended pregnancy, followed by cabergoline treatment, leads to a reduction in glandular components and suppresses ductal proliferation, thereby aligning with the reduced breast cancer susceptibility observed in Brca1/P53-deficient mice (Fig. 1b, c and Extended Data Fig. 1a-f).
Deciphering the molecular mechanisms underlying decreased breast cancer risk in multiparous Brca1/P53-deficient mice through RNA-seq analysis
In an effort to unravel the molecular basis for the observed reduction in breast cancer risk in multiparous Brca1/P53-deficient mice, we carried out a comprehensive RNA-seq study. We analyzed non-tumoral breast tissues from nulliparous and multiparous mice 60 days post-lactational involution. This comparative approach identified distinct sets of differentially expressed genes (DEGs) (P Adj ≤ 0.05) between these groups, providing insights into the molecular divergence contributing to the altered cancer susceptibility (Extended Data Fig. 4a and Supplementary Table S6).
Utilizing Gene Ontology (GO) enrichment, Reactome, and WikiPathways analyses, we categorized these DEGs into functionally relevant biological processes and pathways. The enrichment analysis highlighted a spectrum of biological processes and pathways associated with both overexpressed and underexpressed genes in the multiparous group compared to their nulliparous counterparts (Supplementary Table S7). Key biological processes are depicted in Extended Data Fig. 4b, c.
Correlating our findings with the progressive increase in ductal area over time in nulliparous mice (Fig. 3b, c), and the attenuated proliferation observed in multiparous mice (Fig. 3h, i, k), we focused on clusters related to tubular growth, breast development, and cellular proliferation (Fig 4a and Supplementary Tables S8, S9). Pregnancy consistently resulted in lower gene expression within these clusters in multiparous mice. In contrast, nulliparous mice showed a higher and broader range of transcriptomic variability in non-tumoral mammary glands (Fig. 4a, b), aligning with the observed differences in ductal area and proliferation rates.
The underexpression of genes associated with cell proliferation, mammary gland development, and branching in multiparous mice's breasts may be a key factor in their reduced cancer susceptibility. Critical genes in this context include Pgr, Sox9, Fgfr2, Erbb4, and the Hgf/Met signaling axis, among others (Extended Data Fig. 4d). The altered expression of Pgr suggests shifts in hormonal signaling pivotal to mammary tissue development and proliferation (69, 70). Sox9, essential in stem cell differentiation (71), alongside Fgfr2 and Erbb4, crucial for cell survival and signaling (72-77), indicate a protective shift against cancer cell proliferation and transformation. The role of the Hgf/Met pathway in cell proliferation and mammary morphogenesis further underscores its potential contribution to mitigating cancer initiation and progression (78, 79).
In summary, these molecular patterns suggest a link between diminished activity in specific pathways and the reduced glandular development and proliferation observed in multiparous mice. This molecular landscape potentially underlies the lowered breast cancer susceptibility in this specific mouse model, providing valuable insights into the interplay between genetic factors, pregnancy, and cancer risk.
RNA-Seq analysis of cabergoline-induced gene expression changes in mammary glands of multiparous mice
Later, we expanded our investigation to elucidate the impact of cabergoline treatment on gene expression in the mammary glands of multiparous mice. Through comparative RNA-Seq analysis, distinct differentially expressed genes (DEGs) (P Adj ≤ 0.05) were identified between cabergoline-treated and untreated groups, as detailed in Supplementary Table S10. Heatmap analyses effectively demarcated the gene expression profiles of these cohorts (Extended Data Fig. 5a).
Utilizing Gene Ontology (GO), Reactome, and Wikipathways analyses, we categorized these DEGs into significant biological pathways impacted by cabergoline treatment in multiparous mice. These pathways, crucially altered by cabergoline, might elucidate the molecular underpinnings of its role in augmenting breast cancer protection post-pregnancy (Supplementary Table S11). Noteworthy among these pathways are those associated with attenuated inflammatory responses and modulated TNFα and insulin/IGF1 sensitivity (Supplementary Table S12), key elements in breast cancer susceptibility (80, 81).
Following the observation of a reduced glandular component and lower proliferation rates in Brca1/P53-deficient mice's mammary glands post-cabergoline treatment (Fig. 3), we honed in on pertinent DEGs related to proliferation and development (Fig. 5a, b; Supplementary Tables S12, S13). Among others, cabergoline was found to upregulate genes such as Krt14, Serpinb5, and Trim29 (Extended Data Fig. 5b), potentially augmenting cellular differentiation, DNA stability, and immune responses. Of particular interest, Trim29 may offer protective roles against DNA damage and inflammation-associated cancer, impacting estrogen receptor (ER) signaling (82, 83), while Serpinb5 (Maspin) is implicated in tumor suppression in mammary cells (84). Decreased expression of genes like Scd1, Fasn, Sox2, and Lep suggests a reduction in fat production and pluripotency (85, 86); additionally, elevated Leptin levels have been linked to mammary gland hyperplasia (87) (Extended Data Fig. 5b). These genetic alterations might underpin the observed reduced proliferation and diminished breast cancer risk in cabergoline-treated mice.
To further dissect cabergoline's specific effects within the context of parity, we compared gene expression profiles between untreated and cabergoline-treated multiparous mice with those of nulliparous mice. DEGs (P Adj ≤ 0.05) unique to cabergoline-treated multiparous mice were examined (Fig. 5c; Extended Data Fig. 5c; Supplementary Table S14), revealing pathways that might underlie cabergoline's enhanced protective effect against breast cancer, particularly those involving underexpressed DEGs related to inflammation and glandular development (Supplementary Table S15).
Our focus extended to genes implicated in proliferation and development, informed by literature and our enrichment study (Fig. 5d, e; Supplementary Table S16). In cabergoline-treated multiparous mice, key genes such as ErbB3, Klf5, Igfbp1, Fgf17, Msx2, and Sox10 (88-92), among others, were notably underexpressed compared to nulliparous counterparts (Extended Data Fig. 5d). This underexpression suggests a plausible mechanism for the observed reduction in breast cancer risk, potentially due to constrained mammary gland growth and regulated cell proliferation, particularly in these pivotal genes governing mammary gland development and cell signaling (88-92).
In sum, these results provide a detailed molecular landscape that might elucidate how cabergoline administration in multiparous mice might confer a reduced risk of breast cancer.
Influence of cabergoline on reducing Brca1/P53-deficient cell populations in mammary glands
Informed by the concept of "field cancerization," which links an increased number of pre-malignant cells in a tissue to heightened cancer risk (93, 94), our study examined the long-term impact of cabergoline on the prevalence of Brca1/P53-deficient cells in the mammary gland. This analysis is pivotal given that in Brca1/P53-deficient mice, these cells, generated through Cre-recombinase action on K14+ cells, are the potential tumor progenitors (45). We sought to ascertain whether pregnancy and cabergoline treatment would alter the ratio of these recombinant cells, using this as a barometer for breast cancer risk.
Quantifying the fraction of recombinant cells within the mammary gland of Brca1/P53-deficient mice, where 10-40% of cells are recombinant and predisposed to tumorigenesis (45), we utilized qPCR, an established method for cellular chimerism detection (95, 96). Our research reveals that the proportion of recombinant cells was significantly lower in cabergoline-treated multiparous mice compared to the other groups of mice in the longitudinal cohort (P < 0.001) (Fig. 6a).
An upward trend in recombinant cells deficient in Brca1/ P53 was observed over time in nulliparous mice (P = 0.0365; r = 0.6333.) (Fig. 6b). However, this increase was not evident in either untreated or cabergoline-treated multiparous mice (P = 0.2589 and P = 0.7092, respectively) (Fig. 6c, d). At 60 days post-weaning, the cross-sectional cohort revealed no significant differences in the proportion of recombinant cells (P = 0.3416) (Fig. 6e), suggesting that the impact of cabergoline on recombinant cell populations becomes more pronounced over time. Notably, after adjusting for the cabergoline-induced reduction in ductal area, a substantial decrease in P53 recombinant alleles was evident in treated multiparous mice (P = 0.0344) (Fig. 6f).
Further validation was pursued using a second mouse model with Red Fluorescent Protein (RFP)+ Brca1/P53-deficient cells (Fig. 6g, h and Supplementary Figure S1)(97). Flow cytometry quantification of these cells post-lactational involution showed no significant differences at 60 days (P = 0.3855), but a marked reduction in ductal area was observed in cabergoline-treated mice (P < 0.0001) (Fig. 6i, j), accompanied by a decrease in recombinant RFP+ cells after adjusting for ductal area (P = 0.0006) (Fig. 6k).
Flow cytometry was also employed to dissect changes in various RFP+ recombinant epithelial cell subpopulations (Fig. 6l). Pregnancy and cabergoline treatment led to a reduced proportion of luminal cells (characterized by low CD49f and high EpCAM), with this effect pronounced in untreated multiparous mice compared to nulliparous counterparts (Fig. 6m, left). No significant difference was observed between cabergoline-treated and untreated multiparous groups across all cell subpopulations (Fig. 6m). Yet, accounting for the cabergoline-induced reduction in ductal area, all recombinant cell subpopulations exhibited a decline in the treated group (Fig. 6n).
Given the hypothesis that BRCA1-deficient breast cancer originates from luminal precursors (98, 99), the observed decrease in luminal cells implies that both pregnancy and cabergoline may diminish the pool of potential tumor-initiating cells. This effect was also reflected in RFP-negative cells, indicating a broader impact of the interventions (Extended Data Fig. 6a, b).
In summary, cabergoline treatment in parous mice significantly lowers the count of Brca1/P53-deficient cells, chiefly by reducing the glandular component. This consequential decrease narrows the 'field cancerization' zone, potentially enhancing post-pregnancy protective effects against breast cancer development.
Assessment of cabergoline's post-pregnancy protective effect against breast cancer in women
Our study embarked on a retrospective dynamic cohort analysis (2005-2021) to explore the potential protective impact of cabergoline administered post-pregnancy against breast cancer. Utilizing data from the Information System for the Development of Research in Primary Care (SIDIAP) database (100, 101) (Extended Data Fig. 7), we scrutinized the breast cancer incidence among women who had their first child at age 30 or older. The cohort comprised 14,810 women, with 780 receiving cabergoline within one month of childbirth and 13,898 serving as the control group. The epidemiological characteristics of these groups are detailed in Supplementary Table S17.
In the control group, 132 breast cancer cases were documented, contrasting with only 2 cases in the cabergoline-treated group. The annual cumulative incidence rates per 1,000 population were 1.17 for the control group and 0.3 for the treated group, with average follow-up durations of 8.03 and 8.56 years, respectively (Fig. 7a). Bivariate Cox regression yielded a Hazard Ratio (HR) of 0.247 (95% CI: 0.061 - 0.998, P = 0.050) for the cabergoline-treated group (Fig. 7b), with additional bivariate analysis details in Supplementary Table S18. Further, a multivariate Cox regression, adjusting for significant epidemiological breast cancer risk factors, substantiated the diminished risk in the cabergoline-treated group (HR = 0.239; 95% CI: 0.059 - 0.968) (Fig. 7c).
Expanding the scope, we examined breast cancer incidence over a decade following the first pregnancy, irrespective of maternal age. This included 27,467 women, with 1,604 in the cabergoline-treated group and 25,863 controls. The control group reported 125 breast cancer cases, compared to just 2 in the treated group. Detailed epidemiological profiles are available in Supplementary Table S19. The annual cumulative incidence rates per 1,000 population were 0.66 for the control group and 0.17 for the treated group (Fig. 7d). Bivariate Cox regression analysis indicated an HR of 0.239 (95% CI: 0.059 - 0.968, P = 0.045) for the cabergoline group (Fig. 7e), with further information in Supplementary Table S20. The multivariate Cox regression analysis reaffirmed a consistently lower breast cancer risk in the cabergoline-treated group (adjusted P = 0.056) (Fig. 7f).
In summary, these results collectively suggest that administering cabergoline postpartum may significantly reduce the risk of breast cancer following pregnancy, highlighting its potential as a protective intervention in this specific context.