The Role of Prolactin in Bone Metastasis and Breast Cancer Cell-Mediated Osteoclast Differentiation

Breast Tumor Metastasis and Its Microenvironment: It Takes Both Seed and Soil to Grow a Tumor and Target It for Treatment

Metastasis remains a major challenge in treating breast cancer. Breast tumors metastasize to organ-specific locations such as the brain, lungs, and bone, but why some organs are favored over others remains unclear. Breast tumors also show heterogeneity, plasticity, and distinct microenvironments. This contributes to treatment failure and relapse. The interaction of breast cancer cells with their metastatic microenvironment has led to the concept that primary breast cancer cells act as seeds, whereas the metastatic tissue microenvironment (TME) is the soil. Improving our understanding of this interaction could lead to better treatment strategies for metastatic breast cancer. Targeted treatments for different subtypes of breast cancers have improved overall patient survival, even with metastasis. However, these targeted treatments are based upon the biology of the primary tumor and often these patients' relapse, after therapy, with metastatic tumors. The advent of immunotherapy allowed the immune system to target metastatic tumors. Unfortunately, immunotherapy has not been as effective in metastatic breast cancer relative to other cancers with metastases, such as melanoma. This review will describe the heterogeneic nature of breast cancer cells and their microenvironments. The distinct properties of metastatic breast cancer cells and their microenvironments that allow interactions, especially in bone and brain metastasis, will also be described. Finally, we will review immunotherapy approaches to treat metastatic breast tumors and discuss future therapeutic approaches to improve treatments for metastatic breast cancer.

Predictive and prognostic biomarkers of bone metastasis in breast cancer: current status and future directions

The most common site of metastasis in breast cancer is the bone, where the balance between osteoclast-mediated bone resorption and osteoblast-mediated bone formation is disrupted. This imbalance causes osteolytic bone metastasis in breast cancer, which leads to bone pain, pathological fractures, spinal cord compression, and other skeletal-related events (SREs). These complications reduce patients' quality of life significantly and have a profound impact on prognosis. In this review, we begin by providing a brief overview of the epidemiology of bone metastasis in breast cancer, including current diagnostic tools, treatment approaches, and existing challenges. Then, we will introduce the pathophysiology of breast cancer bone metastasis (BCBM) and the animal models involved in the study of BCBM. We then come to the focus of this paper: a discussion of several biomarkers that have the potential to provide predictive and prognostic value in the context of BCBM-some of which may be particularly compatible with more comprehensive liquid biopsies. Beyond that, we briefly explore the potential of new technologies such as single-cell sequencing and organoid models, which will improve our understanding of tumor heterogeneity and aid in the development of improved biomarkers. The emerging biomarkers discussed hold promise for future clinical application, aiding in the prevention of BCBM, improving the prognosis of patients, and guiding the implementation of personalized medicine.

A Novel Mechanism of hPRL-G129R, a Prolactin Antagonist, Inhibits Human Breast Cancer Cell Proliferation and Migration

Prolactin (PRL) and its receptor, PRLR, are closely related to the occurrence and development of breast cancer. hPRL-G129R, an hPRLR antagonist, has been found to induce apoptosis in breast cancer cells via mechanisms currently unknown. Recent studies have indicated that PRLR exhibits dual functions based on its membrane/nucleus localization. In that context, we speculated whether hPRL-G129R is a dual-function antagonist. We studied the internalization of the hPRLR-G129R/PRLR complex using indirect immunofluorescence and Western blot assays. We found that hPRL-G129R not only inhibited PRLR-mediated intracellular signaling at the plasma membrane, but also blocked nuclear localization of the receptor in T-47D and MCF-7 cells in a time-dependent manner. Clone formation and transwell migration assays showed that hPRL-G129R inhibited PRL-driven proliferation and migration of tumor cells in vitro. Further, we found that increasing concentrations of hPRL-G129R inhibited the nuclear localization of PRLR and the levels of signal transducer and activator of transcription (STAT) 5 in tumor-bearing mice and hPRL-G129R also exerted an antiproliferative effect in vivo. These results indicate that hPRL-G129R is indeed a dual-function antagonist. This study lays a foundation for exploring and developing highly effective agents against the proliferation and progression of breast malignancies.

Organ-Specificity of Breast Cancer Metastasis

Breast cancer (BC) remains one of the most common malignancies among women worldwide. Breast cancer shows metastatic heterogeneity with priority to different organs, which leads to differences in prognosis and response to therapy among patients. The main targets for metastasis in BC are the bone, lung, liver and brain. The molecular mechanism of BC organ-specificity is still under investigation. In recent years, the appearance of new genomic approaches has led to unprecedented changes in the understanding of breast cancer metastasis organ-specificity and has provided a new platform for the development of more effective therapeutic agents. This review summarises recent data on molecular organ-specific markers of metastasis as the basis of a possible therapeutic approach in order to improve the diagnosis and prognosis of patients with metastatically heterogeneous breast cancer.

Modulation of JAK-STAT Signaling by LNK: A Forgotten Oncogenic Pathway in Hormone Receptor-Positive Breast Cancer

Breast cancer remains the most frequently diagnosed cancer in women worldwide. Tumors that express hormone receptors account for 75% of all cases. Understanding alternative signaling cascades is important for finding new therapeutic targets for hormone receptor-positive breast cancer patients. JAK-STAT signaling is commonly activated in hormone receptor-positive breast tumors, inducing inflammation, proliferation, migration, and treatment resistance in cancer cells. In hormone receptor-positive breast cancer, the JAK-STAT cascade is stimulated by hormones and cytokines, such as prolactin and IL-6. In normal cells, JAK-STAT is inhibited by the action of the adaptor protein, LNK. However, the role of LNK in breast tumors is not fully understood. This review compiles published reports on the expression and activation of the JAK-STAT pathway by IL-6 and prolactin and potential inhibition of the cascade by LNK in hormone receptor-positive breast cancer. Additionally, it includes analyses of available datasets to determine the level of expression of LNK and various members of the JAK-STAT family for the purpose of establishing associations between expression and clinical outcomes. Together, experimental evidence and in silico studies provide a better understanding of the potential implications of the JAK-STAT-LNK loop in hormone receptor-positive breast cancer progression.

Circulating biomarkers for diagnosis and therapeutic monitoring in bone metastasis

Bone is a frequent site of metastasis for multiple types of solid tumors in organs such as prostate, breast, lung, etc., accounting for significant morbidities and mortalities of afflicted patients. One of the major problems of bone metastasis is lack of biomarkers for early diagnosis and for monitoring therapeutic responses. Medical imaging modalities such as computerized tomography, magnetic resonance imaging, and radioactive isotope-based bone scans are currently standard clinical practices, yet these imaging techniques are limited to detect early lesions or to accurately monitor the metastatic disease progression during standard and/or experimental therapies. Accordingly, development of novel blood biomarkers rationalizes extensive basic research and clinical development. This review article covers the up-to-date information on protein- and cell-based biomarkers of bone metastasis that are currently used in the clinical practices and also are under development.

PRLR and CACNA2D1 Impact the Prognosis of Breast Cancer by Regulating Tumor Immunity

Phosphatase and tensin homolog (PTEN) is one of the highly susceptible genes to breast cancer (BC); however, the role of PTEN-related RNAs in BC remains poorly understood. Understanding the effect of PTEN-related RNAs and their mechanisms may be helpful to clinicians. We screened the differentially expressed RNAs (deRNAs) related to PTEN and established the competitive endogenous RNA (ceRNA) network by integrating several databases. After that, the RNA model, prolactin receptor (PRLR)/calcium voltage-gated channel auxiliary subunit alpha2delta 1 (CACNA2D1), was obtained by KM survival analysis and logistic regression analysis. Finally, mutation, methylation, functional enrichment, and immune correlation were analyzed to explore the roles of these RNAs. Our results showed that PRLR might be harmful to BC, while CACNA2D1 might be beneficial to BC. Furthermore, the abnormal expression of PRLR in BC might result from mutation and hypomethylation, while the aberrant expression of CACNA2D1 might be ascribed to methylation. Mechanistically, PRLR might affect the prognosis of BC by inhibiting the expression of immune checkpoints, while CACNA2D1 might improve the prognosis of BC by increasing the immune cells infiltrating into BC and up-regulating the expression of immune checkpoints. The abnormal expression of PRLR and CACNA2D1 in BC is closely related to the prognosis of BC, and they may serve as targets for the treatment of BC.

The Binomial "Inflammation-Epigenetics" in Breast Cancer Progression and Bone Metastasis: IL-1β Actions Are Influenced by TET Inhibitor in MCF-7 Cell Line

The existence of a tight relationship between inflammation and epigenetics that in primary breast tumor cells can lead to tumor progression and the formation of bone metastases was investigated. It was highlighted how the induction of tumor progression and bone metastasis by Interleukin-1 beta, in a non-metastatic breast cancer cell line, MCF-7, was dependent on the de-methylating actions of ten-eleven translocation proteins (TETs). In fact, the inhibition of their activity by the Bobcat339 molecule, an inhibitor of TET enzymes, determined on the one hand, the modulation of the epithelial-mesenchymal transition process, and on the other hand, the reduction in the expression of markers of bone metastasis, indicating that the epigenetic action of TETs is a prerequisite for IL-1β-dependent tumor progression and bone metastasis formation.

The association between expression of prolactin receptor and lymph node involvement in triple-negative breast cancer

Background: Triple-negative breast cancer (TNBC) is associated with a poor prognosis and requires more aggressive treatment. Aim: The study aimed to evaluate the prophetic role of the prolactin receptor (PRLR) in TNBC stratification. Materials & methods: In a retrospective study, 58 formalin-fixed paraffin-embedded tumor tissues from patients diagnosed with TNBC were examined for PRLR expression using immunohistochemistry. The potential associations between PRLR expression and tumor characteristics were assessed. Result: PRLR expression was negative in 36 (62%) patients and positive in 22 (38%) patients. The number of positive PRLR tumors was significantly higher in patients without lymph node involvement (p = 0.019). Conclusion: PRLR expression was negatively associated with lymph node invasion in TNBC.

The Potential Prognostic and Therapeutic Implications of Prolactin Receptor and Growth Hormone-releasing Hormone Receptor Expression in Uterine Leiomyosarcomas

The management of uterine leiomyosarcomas (uLMS) remains challenging. The rate of recurrence and metastasis is high, with 5-yr survival reaching only 40% to 50% in patients with tumor confined to the uterus (FIGO stage I or II). Prolactin receptor (PRLR) and growth hormone-releasing hormone receptor (GHRHR) have been implicated in the carcinogenesis of malignant tumors of the breast, endometrium, ovary, liver, and prostate. GHRHR antagonists inhibit in vitro growth of many human tumors and the expression of PRLR is associated with resistance to chemotherapy. The immunohistochemical expression of PRLR and GHRH in 24 primary and 2 recurrent uLMS was evaluated. Representative sections were stained with PRLR and GHRHR antibodies and immunoreactivity was calculated using H -score. The results were correlated with clinicopathologic data using Kaplan-Meier survival and multivariable Cox proportion hazard regression analyses. All tumors were positive for both markers with predominantly moderate to strong expression of PRLR (89%) and GHRHR (82%). Patients with tumors showing moderate to strong expression of PRLR were significantly less likely to achieve disease-free survival ( P =0.004) and significantly more likely to have a poor overall survival ( P =0.049). No significant difference in mean PRLR expression was found between tumors with higher mitotic counts (>20/10 hpf) and lower mitotic counts (20 or less/10 hpf). Furthermore, in 2 patients where the primary and recurrent tumors were tested, there was stronger expression of PRLR in the recurrence compared with the primary. This correlation was not found with GHRHR. Both PRLR and GHRHR may play a role in carcinogenesis in uLMS, as they do in other malignant neoplasms. To our knowledge, this study is the first evaluating the expression of these receptors in uLMS. Moderate or high expression of PRLR may serve as a prognostic marker associated with recurrences and increased mortality in uLMS patients.

Breast Cancer and Prolactin - New Mechanisms and Models

The pathogenesis of breast cancer is driven by multiple hormones and growth factors. One of these, prolactin (PRL), contributes to both mammary differentiation and oncogenesis, and yet the basis for these disparate effects has remained unclear. The focus of this review is to examine and place into context 2 recent studies that have provided insight into the roles of PRL receptors and PRL in tumorigenesis and tumor progression. One study provides novel evidence for opposing actions of PRL in the breast being mediated in part by differential PRL receptor (PRLr) isoform utilization. Briefly, homomeric complexes of the long isoform of the PRLr (PRLrL-PRLrL) promotes mammary differentiation, while heteromeric complexes of the intermediate and long PRLr (PRLrI-PRLrL) isoforms trigger mammary oncogenesis. Another study describes an immunodeficient, prolactin-humanized mouse model, NSG-Pro, that facilitates growth of PRL receptor-expressing patient-derived breast cancer xenografts. Evidence obtained with this model supports the interactions of physiological levels of PRL with estrogen and ERBB2 gene networks, the modulatory effects of PRL on drug responsiveness, and the pro-metastatic effects of PRL on breast cancer. This recent progress provides novel concepts, mechanisms and experimental models expected to renew interest in harnessing/exploiting PRLr signaling for therapeutic effects in breast cancer.

Characteristics and survival in bone metastatic breast cancer patients with different hormone receptor status: A population-based cohort study

Background

Accumulating preclinical evidence has uncovered the indispensable role of steroid hormone and their receptors, namely, estrogen receptor (ER) and progesterone receptor (PR), in the development of bone metastases in breast cancer. Limited data are available regarding the survival difference between different hormone receptor (HR) subgroups, and its prognostic significance is uncertain now. Such data are important for risk stratification and needed to formulate specialized regimen for bone metastatic breast cancer.

Methods

From the year of diagnosis 2010 to 2018, 554,585 breast cancer patients, among which are 19,439 with bone metastasis and 10,447 with bone-only metastasis, were extracted from the Surveillance, Epidemiology, and End Results (SEER) database. Kaplan–Meier survival analysis was performed to compare the survival difference between the different HR status subgroups. Univariate and multivariate Cox proportional hazard regression was used to validate the prognostic role of HR status and identify other prognostic factors in bone metastatic breast cancer.

Results

ER-positive/PR-positive breast cancer patients with bone metastasis showed the best breast cancer-specific survival (BCSS) and overall survival (OS) than those with other HR statuses, while single PR-positive bone metastatic breast cancers manifest similar survival with ER-negative/PR-negative ones. Adjusted Cox regression analysis demonstrated that patients with older age, male, black race, ILC, higher tumor grade, T3–T4, HER2-negative status, absence of surgery or adjuvant treatment, and HR status other than ER-positive/PR-positive tended to have worse outcomes. Further subgroup analysis based on HER2 status showed that within HER2-positive breast cancers, ER-positive/PR-positive ones still manifest better survival than the other three HR status subgroups, which are similar in survival outcomes.

Conclusion

Although collectively viewed as HR-positive breast cancers, certain distinctions exist between bone metastatic breast cancers with different HR statuses in survival outcome. Our findings indicate that despite metastasizing to the same location, the different survival rate is determined by the HR status of breast cancer. The selection and intensity of the regimen should consider HR status, and HER2 status occasionally, when treating bone metastatic breast cancer.

Prolactin receptor signaling: A novel target for cancer treatment - Exploring anti-PRLR signaling strategies

Prolactin (PRL) is a peptide hormone mainly secreted from the anterior pituitary gland. PRL is reported to play a role in pregnancy, mammary gland development, immune modulation, reproduction, and differentiation of islet cells. PRL binds to its receptor PRLR, which belongs to a superfamily of the class I cytokine receptor that has no intrinsic kinase activity. In canonical signaling, PRL binding to PRLR induces downstream signaling including JAK-STAT, AKT and MAPK pathways. This leads to increased cell proliferation, stemness, migration, apoptosis inhibition, and resistance to chemotherapy. PRL-signaling is upregulated in numerous hormone-dependent cancers including breast, prostate, ovarian, and endometrial cancer. However, more recently, the pathway has been reported to play a tumor-promoting role in other cancer types such as colon, pancreas, and hepatocellular cancers. Hence, the signaling pathway is an attractive target for drug development with blockade of the receptor being a potential therapeutic approach. Different strategies have been developed to target this receptor including modification of PRL peptides (Del1-9-G129R-hPRL, G129R-Prl), growth hormone receptor/prolactin receptor bispecific antibody antagonist, neutralizing antibody LFA102, an antibody-drug conjugate (ABBV-176) of the humanized antibody h16f (PR-1594804) and pyrrolobenzodiazepine dimer, a bispecific antibody targeting both PRLR and CD3, an in vivo half-life extended fusion protein containing PRLR antagonist PrlRA and albumin binding domain. There have also been attempts to discover and develop small molecular inhibitors targeting PRLR. Recently, using structure-based virtual screening, we identified a few antipsychotic drugs including penfluridol as a molecule that inhibits PRL-signaling to inhibit PDAC tumor progression. In this review, we will summarize the recent advances in the biology of this receptor in cancer and give an account of PRLR antagonist development for the treatment of cancer.

Prolactin: The Third Hormone in Breast Cancer

Prolactin coordinates with the ovarian steroids to orchestrate mammary development and lactation, culminating in nourishment and an increasingly appreciated array of other benefits for neonates. Its central activities in mammary epithelial growth and differentiation suggest that it plays a role(s) in breast cancer, but it has been challenging to identify its contributions, essential for incorporation into prevention and treatment approaches. Large prospective epidemiologic studies have linked higher prolactin exposure to increased risk, particularly for ER+ breast cancer in postmenopausal women. However, it has been more difficult to determine its actions and clinical consequences in established tumors. Here we review experimental data implicating multiple mechanisms by which prolactin may increase the risk of breast cancer. We then consider the evidence for role(s) of prolactin and its downstream signaling cascades in disease progression and treatment responses, and discuss how new approaches are beginning to illuminate the biology behind the seemingly conflicting epidemiologic and experimental studies of prolactin actions across diverse breast cancers.

The Expression of Prolactin Receptors in Benign Breast Tumors Is Not Associated with Serum Prolactin Level

The role of prolactin (PRL) and its receptors in the initiation and development of benign breast tumors (BBT) has not been sufficiently studied. An imbalance in the system of hormone homeostasis is crucial in the development of BBT. In particular, an association between elevated prolactin levels and the development of BBT has been reported. Our study showed no significant differences between PRL receptor (PRL-R) expression in BBT tissue under normal and elevated serum PRL levels. There was also no significant correlation between age, PRL-R expression in BBT tissue, intact tissue, and PRL level in the serum. There was a strong significant correlation (p < 0.01; r = 0.92) between PRL-R expression in BBT samples and intact breast tissue, which did not depend on the serum PRL level. There was also no significant difference in the expression of the proliferative marker Ki-67 in BBT tissues from women with normal and elevated levels of serum PRL (p > 0.05). No signs of PRL and its receptors were detected in the BBT cystic fluid women with elevated serum PRL levels. In summary, our prospective study showed that the expression of PRL-R in the tissue of BBT and physiological breast tissue does not depend on the level of serum PRL.

Predicting Bone Metastasis Using Gene Expression-Based Machine Learning Models

Bone is the most common site of distant metastasis from malignant tumors, with the highest prevalence observed in breast and prostate cancers. Such bone metastases (BM) cause many painful skeletal-related events, such as severe bone pain, pathological fractures, spinal cord compression, and hypercalcemia, with adverse effects on life quality. Many bone-targeting agents developed based on the current understanding of BM onset's molecular mechanisms dull these adverse effects. However, only a few studies investigated potential predictors of high risk for developing BM, despite such knowledge being critical for early interventions to prevent or delay BM. This work proposes a computational network-based pipeline that incorporates a ML/DL component to predict BM development. Based on the proposed pipeline we constructed several machine learning models. The deep neural network (DNN) model exhibited the highest prediction accuracy (AUC of 92.11%) using the top 34 featured genes ranked by betweenness centrality scores. We further used an entirely separate, "external" TCGA dataset to evaluate the robustness of this DNN model and achieved sensitivity of 85%, specificity of 80%, positive predictive value of 78.10%, negative predictive value of 80%, and AUC of 85.78%. The result shows the models' way of learning allowed it to zoom in on the featured genes that provide the added benefit of the model displaying generic capabilities, that is, to predict BM for samples from different primary sites. Furthermore, existing experimental evidence provides confidence that about 50% of the 34 hub genes have BM-related functionality, which suggests that these common genetic markers provide vital insight about BM drivers. These findings may prompt the transformation of such a method into an artificial intelligence (AI) diagnostic tool and direct us towards mechanisms that underlie metastasis to bone events.

Regulatory roles of miRNAs 16, 133a, and 223 on osteoclastic bone destruction caused by breast cancer metastasis

Osteolytic bone metastasis leads to skeletal-related events, resulting in a decline in the patient activities and survival; therefore, it is important to understand the mechanism underlying bone metastasis. Recent studies have suggested that microRNAs (miRNAs or miRs) are involved in osteoclast differentiation and/or osteolytic bone metastasis; however, the roles of miRNAs have not been elucidated. In the present study, the roles of miRNAs in bone destruction caused by breast cancer metastasis were investigated in vitro and in vivo. miR-16, miR-133a and miR-223 were transfected into a human breast cancer cell line, MDA-MB-231. The expression of osteolytic factors in conditioned medium (miR-CM) collected from the culture of transfected cells was assessed. To evaluate the effects of miRNAs on osteoclast differentiation and activities, tartrate-resistant acid phosphatase (TRAP) staining and bone resorptive assays were performed in osteoclasts following miR-CM treatment. To create in vivo bone metastasis models for histological and morphometric evaluation, miRNA-transfected MDA-MB-231 cells were transplanted into the proximal tibia of nude mice. Expression of osteolytic factors, including receptor activator for nuclear factor-κB ligand (RANKL), interleukin (IL)-1β, IL-6, parathyroid hormone-related protein (PTHrP), and tumor necrosis factor (TNF), was increased in miR-16-CM, whereas it was decreased in both miR-133a-CM and miR-223-CM. TRAP staining and bone resorptive assays revealed that osteoclast function and activities were promoted by miR-16-CM treatment, whereas they were suppressed by miR-133a-CM and miR-223-CM. Consistent with in vitro findings, in vivo experiments revealed that the overexpression of miR-16 increased osteoclast activities and bone destruction in MDA-MB-231 cells, whereas the opposite results were observed in both miR-133a- and miR-223-transfected MDA-MB-231 cells. Our results indicated that miR-16 promoted osteoclast activities and bone destruction caused by breast cancer metastasis in the bone microenvironment, whereas miR-133a and miR-223 suppressed them. These miRNAs could be potential biomarkers and therapeutic targets for breast cancer bone metastasis.

Prolactin and endocrine therapy resistance in breast cancer: The next potential hope for breast cancer treatment

Breast cancer, a hormone‐dependent tumour, generally includes four molecular subtypes (luminal A, luminal B, HER2 enriched and triple‐negative) based on oestrogen receptor, progesterone receptor and human epidermal growth factor receptor‐2. Multiple hormones in the body regulate the development of breast cancer. Endocrine therapy is one of the primary treatments for hormone‐receptor‐positive breast cancer, but endocrine resistance is the primary clinical cause of treatment failure. Prolactin (PRL) is a protein hormone secreted by the pituitary gland, mainly promoting mammary gland growth, stimulating and maintaining lactation. Previous studies suggest that high PRL levels can increase the risk of invasive breast cancer in women. The expression levels of PRL and PRLR in breast cancer cells and breast cancer tissues are elevated in most ER⁺ and ER⁻ tumours. PRL activates downstream signalling pathways and affects endocrine therapy resistance by combining with prolactin receptor (PRLR). In this review, we illustrated and summarized the correlations between endocrine therapy resistance in breast cancer and PRL, as well as the pathophysiological mechanisms and clinical practices. The study on PRL and its receptor would help explore reversing endocrine therapy‐resistance for breast cancer.

NSG-Pro mouse model for uncovering resistance mechanisms and unique vulnerabilities in human luminal breast cancers

Most breast cancer deaths are caused by estrogen receptor-α–positive (ER+) disease. Preclinical progress is hampered by a shortage of therapy-naïve ER+ tumor models that recapitulate metastatic progression and clinically relevant therapy resistance. Human prolactin (hPRL) is a risk factor for primary and metastatic ER+ breast cancer. Because mouse prolactin fails to activate hPRL receptors, we developed a prolactin-humanized Nod-SCID-IL2Rγ (NSG) mouse (NSG-Pro) with physiological hPRL levels. Here, we show that NSG-Pro mice facilitate establishment of therapy-naïve, estrogen-dependent PDX tumors that progress to lethal metastatic disease. Preclinical trials provide first-in-mouse efficacy of pharmacological hPRL suppression on residual ER+ human breast cancer metastases and document divergent biology and drug responsiveness of tumors grown in NSG-Pro versus NSG mice. Oncogenomic analyses of PDX lines in NSG-Pro mice revealed clinically relevant therapy-resistance mechanisms and unexpected, potently actionable vulnerabilities such as DNA-repair aberrations. The NSG-Pro mouse unlocks previously inaccessible precision medicine approaches for ER+ breast cancers.

Genome-wide Association Study Identified Chromosome 8 Locus Associated with Medication-Related Osteonecrosis of the Jaw

Medication‐related osteonecrosis of the jaw (MRONJ) is a rare but serious drug‐related adverse event. To identify pharmacogenomic markers of MRONJ associated with bisphosphonate therapy, we conducted a genomewide association study (GWAS) meta‐analysis followed by functional analysis of 5,008 individuals of European ancestry treated with bisphosphonates, which includes the largest number of MRONJ cases to date (444 cases and 4,564 controls). Discovery GWAS was performed in randomly selected 70% of the patients with cancer and replication GWAS was performed in the remaining 30% of the patients with cancer treated with intravenous bisphosphonates followed by meta‐analysis of all 3,639 patients with cancer. GWAS was also performed in 1,369 patients with osteoporosis treated with oral bisphosphonates. The lead single‐nucleotide polymorphism (SNP), rs2736308 on chromosome 8, was associated with an increased risk of MRONJ with an odds ratio (OR) of 2.71 and 95% confidence interval (CI) of 1.90–3.86 (P = 3.57*10⁻⁸) in the meta‐analysis of patients with cancer. This SNP was validated in the MRONJ GWAS in patients with osteoporosis (OR: 2.82, 95% CI: 1.55–4.09, P = 6.84*10⁻⁴). The meta‐analysis combining patients with cancer and patients with osteoporosis yielded the same lead SNP rs2736308 on chromosome 8 as the top SNP (OR: 2.74, 95% CI: 2.09–3.39, P = 9.65*10⁻¹¹). This locus is associated with regulation of the BLK, CTSB, and FDFT1 genes, which had been associated with bone mineral density. FDFT1 encodes a membrane‐associated enzyme, which is implicated in the bisphosphonate pathway. This study provides insights into the potential mechanism of MRONJ.

Prolactin enhances T regulatory cell promotion of breast cancer through the long form prolactin receptor

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Systemic knockdown of the long form prolactin receptor in vivo increases survival in an aggressive, immunocompetent model of stage IV, triple negative breast cancer.

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Knockdown of the long form prolactin receptor reduces Treg recruitment to tumors by reducing tumor parenchymal production of CCL-17.

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Those Tregs still recruited to primary tumors have a substantial reduction in their ability to promote epithelial to mesenchymal transition of tumor parenchyma.

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For the Tregs in the primary tumor, there is transcript downregulation of components of the T cell receptor complex and CTLA-4.

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Tregs outside of the tumor have normal ability to suppress T effector cell proliferation after 1–5 months of treatment.

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Knockdown of the long form of the prolactin receptor therefore seems to have an intra-tumor immunotherapeutic effect without effect on peripheral Treg function.

Previous work has shown systemic knockdown of the long form prolactin receptor (LFPRLR) in vivo markedly reduced metastasis in mouse models of breast cancer, but whether this translated to prolonged survival was unknown. Here we show that LFPRLR knockdown in the highly metastatic, immunocompetent 4T1 model prolonged survival and reduced recruitment of T regulatory cells (Tregs) to the tumor through effects on the production of CCL17. For the Tregs still recruited to the primary tumor, LFPRLR knockdown both directly and indirectly reduced their ability to promote tumor parenchymal epithelial to mesenchymal transition. Importantly, effects of prolactin on expression of mesenchymal genes by the tumor parenchyma were very different in the absence and presence of Tregs. While systemic knockdown of the LFPRLR downregulated transcripts important for immune synapse function in the remaining tumor Tregs, splenic Tregs seemed unaffected by LFPRLR knockdown, as demonstrated by their continued ability to suppress anti-CD3/CD28-stimulated effector cell proliferation at 1–5 months. These results demonstrate that knockdown of the LFPRLR achieves intra-tumor immunotherapeutic effects and suggest this occurs with reduced likelihood of peripheral inflammatory/autoimmune sequelae.

Progesterone receptor isoform-dependent cross-talk between prolactin and fatty acid synthase in breast cancer

Progesterone receptor (PR) isoforms can drive unique phenotypes in luminal breast cancer (BC). Here, we hypothesized that PR-B and PR-A isoforms differentially modify the cross-talk between prolactin and fatty acid synthase (FASN) in BC. We profiled the responsiveness of the FASN gene promoter to prolactin in T47D_co BC cells constitutively expressing PR-A and PR-B, in the PR-null variant T47D-Y cell line, and in PR-null T47D-Y cells engineered to stably re-express PR-A (T47D-YA) or PR-B (T47D-YB). The capacity of prolactin to up-regulate FASN gene promoter activity in T47D_co cells was lost in T47D-Y and TD47-YA cells. Constitutively up-regulated FASN gene expression in T47-YB cells and its further stimulation by prolactin were both suppressed by the prolactin receptor antagonist hPRL-G129R. The ability of the FASN inhibitor C75 to decrease prolactin secretion was more conspicuous in T47-YB cells. In T47D-Y cells, which secreted notably less prolactin and downregulated prolactin receptor expression relative to T47D_co cells, FASN blockade resulted in an augmented secretion of prolactin and up-regulation of prolactin receptor expression. Our data reveal unforeseen PR-B isoform-specific regulatory actions in the cross-talk between prolactin and FASN signaling in BC. These findings might provide new PR-B/FASN-centered predictive and therapeutic modalities in luminal intrinsic BC subtypes.

Prolactin: A hormone with diverse functions from mammary gland development to cancer metastasis

Prolactin has a rich mechanistic set of actions and signaling in order to elicit developmental effects in mammals. Historically, prolactin has been appreciated as an endocrine peptide hormone that is responsible for final, functional mammary gland development and lactation. Multiple signaling pathways impacted upon by the microenvironment contribute to cell function and differentiation. Endocrine, autocrine and paracrine signaling are now apparent in not only mammary development, but also in cancer, and involve multiple cell types including those of the immune system. Multiple ligands agonists are capable of binding to the prolactin receptor, potentially expanding receptor function. Prolactin has an important role not only in tumorigenesis of the breast, but also in a number of hormonally responsive cancers such as prostate, ovarian and endometrial cancer, as well as pancreatic and lung cancer. Although pituitary and extra-pituitary sources of prolactin such as the epithelium are important, stromal sourced prolactin is now also being recognized as an important factor in tumor progression, all of which potentially signal to multiple cell types in the tumor microenvironment. While prolactin has important roles in milk production including calcium and bone homeostasis, in the disease state it can also affect bone homeostasis. Prolactin also impacts metastatic cancer of the breast to modulate the bone microenvironment and promote bone damage. Prolactin has a fascinating contribution in both physiologic and pathologic settings of mammals.

Personal Medicine and Bone Metastases: Biomarkers, Micro-RNAs and Bone Metastases

Bone metastasis is a major cause of morbidity within solid tumours of the breast, prostate, lung and kidney. Metastasis to the skeleton is associated with a wide range of complications including bone fractures, spinal cord compression, hypercalcaemia and increased bone pain. Improved treatments for bone metastasis, such as the use of anti-bone resorptive bisphosphonate agents, within post-menopausal women have improved disease-free survival; however, these treatments are not without side effects. There is thus a need for biomarkers, which will predict the risk of developing the spread to bone within these cancers. The application of molecular profiling techniques, together with animal model systems and engineered cell-lines has enabled the identification of a series of potential bone-metastasis biomarker molecules predictive of bone metastasis risk. Some of these biomarker candidates have been validated within patient-derived samples providing a step towards clinical utility. Recent developments in multiplex biomarker quantification now enable the simultaneous measurement of up to 96 micro-RNA/protein molecules in a spatially defined manner with single-cell resolution, thus enabling the characterisation of the key molecules active at the sites of pre-metastatic niche formation as well as tumour-stroma signalling. These technologies have considerable potential to inform biomarker discovery. Additionally, a potential future extension of these discoveries could also be the identification of novel drug targets within cancer spread to bone. This chapter summarises recent findings in biomarker discovery within the key bone metastatic cancers (breast, prostate, lung and renal cell carcinoma). Tissue-based and circulating blood-based biomarkers are discussed from the fields of genomics, epigenetic regulation (micro-RNAs) and protein/cell-signalling together with a discussion of the potential future development of these markers towards clinical development.

Current and Emerging Biomarkers Predicting Bone Metastasis Development

Bone is one of the preferential sites of distant metastases from malignant tumors, with the highest prevalence observed in breast and prostate cancers. Patients with bone metastases (BMs) may experience skeletal-related events, such as severe bone pain, pathological fractures, spinal cord compression, and hypercalcemia, with negative effects on the quality of life. In the last decades, a deeper understanding of the molecular mechanisms underlying the BM onset has been gained, leading to the development of bone-targeting agents. So far, most of the research has been focused on the pathophysiology and treatment of BM, with only relatively few studies investigating potential predictors of risk for BM development. The ability to select such "high-risk" patients could allow early identification of those most likely to benefit from interventions to prevent or delay BM. This review summarizes several evidences for the potential use of specific biomarkers able to predict early the BM development.

Prolactin receptor expression as a novel prognostic biomarker for triple negative breast cancer patients

Prolactin receptor (PRLR) is a novel emerging prognostic biomarker in different cancers, especially in breast cancer. However, there is limited information about the association of PRLR expression and triple-negative breast cancers (TNBC) prognosis. In this study, 80 TNBC patients were evaluated for PRLR expression by immunohistochemistry. The correlation of PRLR expression with clinicopathological features, patient recurrence, and survival was investigated. PRLR expression was considered positive if >10% of tumor cells were stained. The Fisher's exact test was used to analyze PRLR expression relation with the clinicopathological parameters. Survival distribution was estimated by the Kaplan-Meier method. Positive immunoreactivity for PRLR was observed in 50 out of 80 (62%) specimens. Although expression of PRLR was associated with TNBC patients' stage, no-correlation was observed between its expression and tumor size, grade, lymph node status, and Ki-67 expression. In addition, patients with positive expression of PRLR exhibited lower recurrence (P = 0.0027) and higher overall survival (P = 0.0285) in comparison with negative expression group. In multivariate analyses, positive expression of PRLR was an independent prognostic marker for lower recurrence (P < 0.001) and higher overall survival (P < 0.001). Therefore, PRLR plays a crucial role in TNBC and has to be considered as an independent prognostic biomarker for TNBC patients.

[Analysis of prolactin receptor expression in breast cancer subtypes]

Breast cancer (BC) is the most common cancer among women. It is known that the prolactin receptor (PRLR) may play a role in breast carcinogenesis, but the available data are often contradictory. To get a more complete picture of the relationship between the receptor and mammary gland carcinogenesis, we examined the association between changes in PRLR expression level and tumor subtype (and its main characteristics). To do this, using real-time PCR, we evaluated the level of PRLR mRNA in BC tissue samples and untransformed adjoining tissue samples (89 pairs). Since the androgen receptor (AR) has begun to be seen as a prognostic marker in breast cancer, we also evaluated the association between mRNA levels of AR and PRLR. We found a significant increase in PRLR expression in luminal subtypes; the highest level of PRLR mRNA was detected in luminal A subtype. In HER2-positive ER-, PR-negative BC, the PRLR mRNA level decreases in tumor tissues compared with untransformed tissues. High PRLR expression is also associated with smaller tumor size in luminal B HER2-negative subtype. In ER-, PR-negative tumors, PRLR expression is associated with AR expression: PRLR mRNA level is increased when AR mRNA level is reduced by more than 8 times in triple-negative tumors; in contrast, in HER2-positive subtype it decreases more significantly when AR expression is reduced by more than 3 times. A tendency towards an increase in PRLR expression with an increase in the AR mRNA level was also discovered in luminal subtypes. The level of PRLR expression depends on the age of patients. In luminal A, PRLR expression is higher in patients under 65 years. In contrast, in luminal B HER2-negative and triple-negative BC, reduced PRLR expression was observed in patients under the age of 40 years and under the age of 50 years, respectively. In this group of patients under the age of 40 years with luminal B HER2-negative BC, ER expression was also reduced (0-4 score according to the IHC assay). Thus, PRLR probably plays a different role in the development and progression of BC: in luminal A and luminal B HER2-positive subtypes PRLR may act as an oncogen, and in luminal B HER2-negative and ER-, PR-negative subtypes can play a tumor suppressor role.

Pharmacogenomics of osteonecrosis of the jaw

Osteonecrosis of the jaw (ONJ) is a rare but serious drug induced adverse event, mainly associated with the use of antiresorptive medications, such as intravenous (IV) bisphosphonates (BPs) in cancer patients. In this review, we evaluated all the pharmacogenomic association studies for ONJ published up to December 2018. To date, two SNPs (CYP2C8 rs1934951 and RBMS3 rs17024608) were identified to be associated with ONJ by two genome-wide association studies (GWAS). However, all six subsequent candidate gene studies failed to replicate these results. In addition, six discovery candidate gene studies tried to identify the genetic markers in several genes associated with bone remodeling, bone mineral density, or osteoporosis. After evaluating the results of these 6 studies, none of the SNPs was significantly associated with ONJ. Recently, two whole-exome sequencing (WES) analysis (including one from our group) were performed to identify variants associated with ONJ. So far, only our study successfully replicated discovery result indicating SIRT1 SNP rs7896005 to be associated with ONJ. However, this SNP also did not reach genome-wide significance. The major limitations of these studies include lack of replication phases and limited sample sizes. Even though some studies had larger sample sizes, they recruited healthy individuals as controls, not subjects treated with BPs. We conclude that a GWAS with a larger sample size followed by replication phase will be needed to fully investigate the pharmacogenomic markers of ONJ.

From latency to overt bone metastasis in breast cancer: potential for treatment and prevention

Bone metastasis is present in a high percentage of breast cancer (BCa) patients with distant disease, especially in those with the estrogen receptor‐positive (ER⁺) subtype. Most cells that escape primary tumors are unable to establish metastatic lesions, which suggests that target organ microenvironments are hostile for tumor cells. This implies that BCa cells must achieve a process of speciation to adapt to the new conditions imposed in the new organ. Bone has unique characteristics that can be exploited by cancer cells: it undergoes constant remodeling and comprises diverse environments (including osteogenic, perivascular, and hematopoietic stem cell niches). This allows colonizing cells to take advantage of numerous adhesion molecules, matrix proteins, and soluble factors that facilitate homing, survival, and, eventually, metastatic outgrowth. However, in most cases, metastatic lesions enter into a latency state that can last months, years, or even decades, before forming a clinically detectable macrometastasis. This dormant state challenges the effectiveness of adjuvant chemotherapy. Detecting which tumors are more prone to metastasize to bone and developing new specific therapies that target bone metastasis represent urgent clinical needs. Here, we review the biological mechanisms of BCa bone metastasis and provide the latest options of treatments and predictive markers that are currently in clinical use or are being tested in clinical assays. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Prolactin modulates TNBC aggressive phenotype limiting tumorigenesis

Triple-negative breast cancer (TNBC) accounts for ~20% of all breast cancer cases. The management of TNBC represents a challenge due to its aggressive phenotype, heterogeneity and lack of targeted therapy. Loss of cell differentiation and enrichment with breast cancer stem-like cells (BCSC) are features of TNBC contributing to its aggressive nature. Here, we found that treatment of TNBC cells with PRL significantly depletes the highly tumorigenic BCSC subpopulations CD44+/CD24- and ALDH+ and differentiates them to the least tumorigenic CD44-/CD24- and ALDH- phenotype with limited tumorsphere formation and self-renewal capacities. Importantly, we found PRL to induce a heterochromatin phenotype marked by histone H3 lysine 9 trimethylation (H3K9me3) and accompanied by ultra-structural cellular architecture associated with differentiation and senescence rendering the cells refractory to growth signals. Crucially, we found PRL to mediate these effects in vivo in a pre-clinical animal xenograft of TNBC controlling tumor growth. These results reveal that the lactogenic hormone PRL may exert its anti-tumorigenic effects on TNBC through cellular reprogramming indicative of differentiation resulting in the depletion of BCSCs and restricting tumorigenesis.

Kadsurenone is a useful and promising treatment strategy for breast cancer bone metastases by blocking the PAF/PTAFR signaling pathway

Breast cancer (BC) is characterized by high incidences of bone metastases. Current treatment strategies for BC bone metastases primarily focused on breaking the ‘vicious osteolytic cycle’. Platelet-activating factor (PAF) is a potent phospholipid mediator, which has previously reported biological activities in BC progression and osteoclast differentiation by activating its receptor PAF receptor (PTAFR). However, the role of PAF in the mediation of BC bone metastases remains elusive. In the present study, it was revealed that the upregulation of PTAFR was associated with an increased incidence of bone metastases. It was also revealed that PAF significantly enhanced the processes of BC cell migration and BC mediated osteoclastogenesis. These results suggest that PAF serves a promotion role in BC bone metastases. It was further demonstrated that the natural PAF antagonist Kadsurenone may effectively attenuate each process by partially blocking the PAF/PTAFR signaling pathway. Therefore, targeting PAF/PTAFR by Kadsurenone may be a promising treatment strategy for BC bone metastases.

Involvement of miR-106b in tumorigenic actions of both prolactin and estradiol

Prolactin promotes a variety of cancers by an array of different mechanisms. Here, we have investigated prolactin's inhibitory effect on expression of the cell cycle-regulating protein, p21. Using a miRNA array, we identified a number of miRNAs upregulated by prolactin treatment, but one in particular that was strongly induced by prolactin and predicted to bind to the 3′UTR of p21 mRNA, miR-106b. By creating a p21 mRNA 3′UTR-luciferase mRNA construct, we demonstrated degradation of the construct in response to prolactin in human breast, prostate and ovarian cancer cell lines. Increased expression of miR-106b replicated, and anti-miR-106b counteracted, the effects of prolactin on degradation of the 3′UTR construct, p21 mRNA levels, and cell proliferation in breast (T47D) and prostate (PC3) cancer cells. Increased expression of miR-106b also stimulated migration of the very epithelioid T47D cell line. By contrast, anti-miR-106b dramatically decreased expression of the mesenchymal markers, SNAIL-2, TWIST-2, VIMENTIN, and FIBRONECTIN. Using signaling pathway inhibitors and the 3′UTR construct, induction of miR-106b by prolactin was determined to be mediated through the MAPK/ERK and PI3K/Akt pathways and not through Jak2/Stat5 in both T47D and PC3 cells. Prolactin activation of MAPK/ERK and PI3K/Akt also activates ERα in the absence of an ERα ligand. 17β-estradiol promoted degradation of the construct in both cell lines and pre-incubation in the estrogen antagonist, Fulvestrant, blocked the ability of both prolactin and 17β-estradiol to induce the construct-degrading activity. Together, these data support a convergence of the prolactin and 17β-estradiol miR-106b-elevating signaling pathways at ERα.

Prolactin signaling through focal adhesion complexes is amplified by stiff extracellular matrices in breast cancer cells

Estrogen receptor α positive (ERα+) breast cancer accounts for most breast cancer deaths. Both prolactin (PRL) and extracellular matrix (ECM) stiffness/density have been implicated in metastatic progression of this disease. We previously demonstrated that these factors cooperate to fuel processes involved in cancer progression. Culture of ERα+ breast cancer cells in dense/stiff 3D collagen-I matrices shifts the repertoire of PRL signals, and increases crosstalk between PRL and estrogen to promote proliferation and invasion. However, previous work did not distinguish ECM stiffness and collagen density. In order to dissect the ECM features that control PRL signals, we cultured T47D and MCF-7 cells on polyacrylamide hydrogels of varying elastic moduli (stiffness) with varying collagen-I concentrations (ligand density). Increasing stiffness from physiological to pathological significantly augmented PRL-induced phosphorylation of ERK1/2 and the SFK target, FAK-Y925, with only modest effects on pSTAT5. In contrast, higher collagen-I ligand density lowered PRL-induced pSTAT5 with no effect on pERK1/2 or pFAK-Y925. Disrupting focal adhesion signaling decreased PRL signals and PRL/estrogen-induced proliferation more efficiently in stiff, compared to compliant, extracellular environments. These data indicate that matrix stiffness shifts the balance of PRL signals from physiological (JAK2/STAT5) to pathological (FAK/SFK/ERK1/2) by increasing PRL signals through focal adhesions. Together, our studies suggest that PRL signaling to FAK and SFKs may be useful targets in clinical aggressive ERα+ breast carcinomas.

ATM Is Required for the Prolactin-Induced HSP90-Mediated Increase in Cellular Viability and Clonogenic Growth After DNA Damage

Prolactin (PRL) acts as a survival factor for breast cancer cells, but the PRL signaling pathway and the mechanism are unknown. Previously, we identified the master chaperone, heat shock protein 90 (HSP90) α, as a prolactin-Janus kinase 2 (JAK2)-signal transducer and activator of transcription 5 (STAT5) target gene involved in survival, and here we investigated the role of HSP90 in the mechanism of PRL-induced viability in response to DNA damage. The ataxia-telangiectasia mutated kinase (ATM) protein plays a critical role in the cellular response to double-strand DNA damage. We observed that PRL increased viability of breast cancer cells treated with doxorubicin or etoposide. The increase in cellular resistance is specific to the PRL receptor, because the PRL receptor antagonist, Δ1-9-G129R-hPRL, prevented the increase in viability. Two different HSP90 inhibitors, 17-allylamino-17-demethoxygeldanamycin and BIIB021, reduced the PRL-mediated increase in cell viability of doxorubicin-treated cells and led to a decrease in JAK2, ATM, and phosphorylated ATM protein levels. Inhibitors of JAK2 (G6) and ATM (KU55933) abolished the PRL-mediated increase in cell viability of DNA-damaged cells, supporting the involvement of each, as well as the crosstalk of ATM with the PRL pathway in the context of DNA damage. Drug synergism was detected between the ATM inhibitor (KU55933) and doxorubicin and between the HSP90 inhibitor (BIIB021) and doxorubicin. Short interfering RNA directed against ATM prevented the PRL-mediated increase in cell survival in two-dimensional cell culture, three-dimensional collagen gel cultures, and clonogenic cell survival, after doxorubicin treatment. Our results indicate that ATM contributes to the PRL-JAK2-STAT5-HSP90 pathway in mediating cellular resistance to DNA-damaging agents.

Bone metastasis risk factors in breast cancer

Bone is the single most frequent site for bone metastasis in breast cancer patients. Patients with bone-only metastasis have a fairly good prognosis when compared with patients with visceral disease. Nevertheless, cancer-induced bone disease carries an important risk of developing skeletal related events that impact quality of life (QoL). It is therefore particularly important to stratify patients according to their risk of developing bone metastasis. In this context, several risk factors have been studied, including demographic, clinicopathological, genetic, and metabolic factors. Most of them show conflicting or non-definitive associations and are not validated for clinical use. Nonetheless, tumour intrinsic subtype is widely accepted as a major risk factor for bone metastasis development and luminal breast cancer carries an increased risk for bone disease. Other factors such as gene signatures, expression of specific cytokines (such as bone sialoprotein and bone morphogenetic protein 7) or components of the extracellular matrix (like bone crosslinked C-telopeptide) might also influence the development of bone metastasis. Knowledge of risk factors related with bone disease is of paramount importance as it might be a prediction tool for triggering the use of targeted agents and allow for better patient selection for future clinical trials.

Prolactin receptor in breast cancer: marker for metastatic risk

Prolactin and prolactin receptor signaling and function are complex in nature and intricate in function. Basic, pre-clinical and translational research has opened up our eyes to the understanding that prolactin and prolactin receptor signaling function differently within different cellular contexts and microenvironmental conditions. Its multiple roles in normal physiology are subverted in cancer initiation and progression, and gradually we are teasing out the intricacies of function and therapeutic value. Recently, we observed that prolactin has a role in accelerating the time to bone metastasis in breast cancer patients and identified the mechanism by which prolactin stimulated breast cancer cell-mediated lytic osteoclast formation. The possibility that the prolactin receptor is a marker for metastasis, and specifically bone metastasis, is one that may have to be put into the context of the different variants of prolactin, different prolactin receptor isoforms and intricate signaling pathways that are regulated by the microenvironment. The more complete the picture, the better one can test biomarker identity and design clinical trials to test therapeutic intervention. This review will cover the recent advances and highlight the complexity of prolactin receptor biology.

What Is Breast in the Bone?

The normal developmental program that prolactin generates in the mammary gland is usurped in the cancerous process and can be used out of its normal cellular context at a site of secondary metastasis. Prolactin is a pleiotropic peptide hormone and cytokine that is secreted from the pituitary gland, as well as from normal and cancerous breast cells. Experimental and epidemiologic data suggest that prolactin is associated with mammary gland development, and also the increased risk of breast tumors and metastatic disease in postmenopausal women. Breast cancer spreads to the bone in approximately 70% of cases with advanced breast cancer. Despite treatment, new bone metastases will still occur in 30%–50% of patients. Only 20% of patients with bone metastases survive five years after the diagnosis of bone metastasis. The breast cancer cells in the bone microenvironment release soluble factors that engage osteoclasts and/or osteoblasts and result in bone breakdown. The breakdown of the bone matrix, in turn, enhances the proliferation of the cancer cells, creating a vicious cycle. Recently, it was shown that prolactin accelerated the breast cancer cell-mediated osteoclast differentiation and bone breakdown by the regulation of breast cancer-secreted proteins. Interestingly, prolactin has the potential to affect multiple proteins that are involved in both breast development and likely bone metastasis, as well. Prolactin has normal bone homeostatic roles and, combined with the natural “recycling” of proteins in different tissues that can be used for breast development and function, or in bone function, increases the impact of prolactin signaling in breast cancer bone metastases. Thus, this review will focus on the role of prolactin in breast development, bone homeostasis and in breast cancer to bone metastases, covering the molecular aspects of the vicious cycle.

Prolactin Pro-Differentiation Pathway in Triple Negative Breast Cancer: Impact on Prognosis and Potential Therapy

Triple negative breast cancer (TNBC) is a heterogeneous disease associated with poor clinical outcome and lack of targeted therapy. Here we show that prolactin (PRL) and its signaling pathway serve as a sub-classifier and predictor of pro-differentiation therapy in TNBC. Using immunohistochemistry and various gene expression in silica analyses we observed that prolactin receptor (PRLR) protein and mRNA levels are down regulated in TNBC cases. In addition, examining correlation of PRLR gene expression with metagenes of TNBC subtypes (580 cases), we found that PRLR gene expression sub-classifies TNBC patients into a new subgroup (TNBC-PRLR) characterized by epithelial-luminal differentiation. Importantly, gene expression of PRL signaling pathway components individually (PRL, PRLR, Jak2 and Stat5a), or as a gene signature is able to predict TNBC patients with significantly better survival outcomes. As PRL hormone is a druggable target we determined the biological role of PRL in TNBC biology. Significantly, restoration/activation of PRL pathway in TNBC cells representative of mesenchymal or TNBC-PRLR subgroups led to induction of epithelial phenotype and suppression of tumorigenesis. Altogether, these results offer potential new modalities for TNBC stratification and development of personalized therapy based on PRL pathway activation.