Introduction: Deregulation of the cell cycle machinery is often found in human cancers. Modulations in the cell cycle regulator function and expression result not only in proliferative advantages, but also lead to tumor progression and invasiveness of the cancer. In particular, cyclin D1 and p21 are often over-expressed in human cancers, correlating with high tumor grade, poor prognosis and increased metastasis. This prompted us to investigate the role of the cyclin D1/p21 signaling axis downstream of transforming growth factor beta (TGFβ) in breast cancer progression. Methods: Cyclins mRNA and protein expressions were assessed by quantitative real-time PCR and Western blot in triple negative breast cancer cell lines. Co-localization and interaction between cyclin D1 and p21 were performed by immunocytochemistry and co-immunoprecipitation, respectively. Cell migration was assessed by wound healing and quantitative time-lapse imaging assays. In addition, the effects of cyclin D1 on cellular structure and actin organization were examined by staining with F-actin marker phalloidin and mesenchymal intermediate filament vimentin. Finally, a mammary fat pad xenograft mouse model was used to assess mammary tumor growth and local invasion. Results: We found TGFβ to specifically up-regulate the expression of cyclin D1 in triple negative breast cancer cells. Induction of cyclin D1 is also required for TGFβ-mediated cell migration. Suppression of cyclin D1 expression not only resulted in a rounded and epithelial-like phenotype, but also prevented TGFβ-induced vimentin and F-actin co-localization at the cell edge as well as invadopodia formation. Furthermore, TGFβ promoted the nuclear co-localization and physical interaction between cyclin D1 and p21. The co-expression of cyclin D1 and p21 proteins are required for the initial steps of tumor development, as double knockdown of these two molecules prevented primary tumor formation in a Xenograft mouse model. Moreover, the in vivo studies indicated that locally advanced features of the invasive tumors, including skeletal muscle, mammary fat pad and lymphovascular invasion, as well as ulcerated skin, were attenuated in the absence of cyclin D1 and p21. Conclusions: Thus, our findings highlight the cyclin D1/p21 signaling axis as a critical regulator of TGFβ-mediated tumor growth initiation and local tumor cell invasion, both in vitro and in vivo.

Introduction: Tumor cell migration and invasion are critical initiation steps in the process of breast cancer metastasis, the primary cause of breast cancer morbidity and death. Here we investigated the role of p21Cip1 (p21), a member of the core cell cycle machinery, in transforming growth factor-beta (TGFβ)-mediated breast cancer cell migration and invasion. Methods: A mammary fat pad xenograft mouse model was used to assess the mammary tumor growth and local invasion. The triple negative human breast cancer cell lines MDA-MB231 and its sub-progenies SCP2 and SCP25, SUM159PT, SUM149PT, SUM229PE and SUM1315MO₂ were treated with 5 ng/ml TGFβ and the protein expression levels were measured by Western blot. Cell migration and invasion were examined using the scratch/wound healing and Transwell assay. TGFβ transcriptional activity was measured by a TGFβ/Smad reporter construct (CAGA12-luc) using luciferase assay. q-PCR was used for assessing TGFβ downstream target genes. The interactions among p21, p/CAF and Smad3 were performed by co-immunoprecipitation. In addition, Smad3 on DNA binding ability was measured by DNA immunoprecipitation using biotinylated Smad binding element DNA probes. Finally, the association among active TGFβ/Smad signaling, p21 and p/CAF with lymph node metastasis was examined by immunohistochemistry in tissue microarray containing 50 invasive ductal breast tumors, 25 of which are lymph node positive. Results: We found p21 expression to correlate with poor overall and distant metastasis free survival in breast cancer patients. Furthermore, using xenograft animal models and in vitro studies, we found p21 to be essential for tumor cell invasion. The invasive effects of p21 were found to correlate with Smad3, and p/CAF interaction downstream of TGFβ. p21 and p/CAF regulates TGFβ-mediated transcription of pro-metastatic genes by controlling Smad3 acetylation, DNA binding and transcriptional activity. In addition, we found that active TGFβ/Smad signaling correlates with high p21 and p/CAF expression levels and lymph node involvement using tissue microarrays from breast cancer patients. Conclusions: Together these results highlight an important role for p21 and p/CAF in promoting breast cancer cell migration and invasion at the transcriptional level and may open new avenues for breast cancer therapy.

Prolactin (PRL) hormone plays an important role in the development of the mammary gland and terminal differentiation of the mammary epithelial cells. While initial studies suggested that PRL may contribute to the development of breast cancer through PRL/prolactin receptor (PRLR) autocrine function, mounting evidence indicate a different role for PRL, highlighting this hormone as a regulator of epithelial plasticity and as a potential tumor suppressor. To gain further insights into the role of PRL in human breast carcinogenesis, immunohistochemistry analyses of PRLR protein expression levels using tissue microarray of 102 cases were done in comparison with various clinical/pathologic parameters and molecular subtypes. In addition, gene expression level of PRLR was also evaluated in relation to intrinsic molecular subtypes, tumor grade, and patient outcome using GOBO database for 1881 breast cancer patients. Interestingly, PRLR expression was found to be significantly downregulated in invasive breast cancer (21.4%) in comparison with normal/benign (80%) and in situ carcinoma (60%) (P=0.003498). Moreover, PRLR expression was associated with lymph node negativity and low-grade well-differentiated tumors. PRLR expression was strongest in luminal A subtype, and was virtually undetectable in the worse prognosis triple-negative breast cancer subtype (P=0.00001). Furthermore, PRLR expression was independent of ER, PR, HER-2, and P53 status. Finally, PRLR expression was significantly (P<0.01) associated with prolonged distant metastasis–free survival in breast cancer patients. In conclusion, our results highlight PRLR as an independent predictor of favorable prognosis in human breast cancer.

Breast cancer represents a major health challenge. The majority of breast cancer deaths are due to cancer progression/recurrence for which no efficient therapies exist. Aggressive breast cancers are characterized by loss of cellular differentiation. Defining molecular mechanisms/targets contributing to cancer aggressiveness is needed to guide the design of new screening and targeted treatments. Here, we describe a novel tumor promoting function for the Cleavage and Polyadenylation Factor-6 (CPSF6). Importantly, aggressive breast cancer cells of luminal B, HER2-overexpressing and triple negative subtypes show dependency on CPSF6 for viability and tumorigenic capacity. Mechanistically, we found CPSF6 to interact with components of the A-to-I RNA editing machinery, paraspeckles and ADAR1 enzyme, and to be required for their physical integrity. Clinically, we found CPSF6 and all core paraspeckles proteins to be overexpressed in human breast cancer cases and their expression to correlate with poor patient outcomes. Finally, we found prolactin, a key mammary differentiation factor, to suppress CPSF6/RNA editing activity. Together, this study revealed CPSF6 as a molecular target with clinical relevance for prognosis and therapy in breast cancer.

Although the cyclin-dependent kinases CDK4 and CDK6 play fundamental roles in cancer, the specific pathways and downstream targets by which they exert their tumorigenic effects remain elusive. In this study, we uncover distinct and novel functions for these kinases in regulating tumor formation and metastatic colonization in various solid tumors, including those of the breast, prostate, and pancreas. Combining in vivo CRISPR-based CDK4 and CDK6 gene editing with pharmacologic inhibition approaches in orthotopic transplantation and patient-derived xenograft preclinical models, we defined clear functions for CDK4 and CDK6 in facilitating tumor growth and progression in metastatic cancers. Transcriptomic profiling of CDK4/6 CRISPR knockouts in breast cancer revealed these two kinases to regulate cancer progression through distinct mechanisms. CDK4 regulated prometastatic inflammatory cytokine signaling, whereas CDK6 mainly controlled DNA replication and repair processes. Inhibition of CDK6 but not CDK4 resulted in defective DNA repair and increased DNA damage. Multiple CDK6 DNA replication/repair genes were not only associated with cancer subtype, grades, and poor clinical outcomes, but also facilitated primary tumor growth and metastasis in vivo. CRISPR-based genomic deletion of CDK6 efficiently blocked tumor formation and progression in preestablished cell- and patient-derived xenograft preclinical models of breast cancer, providing a potential novel targeted therapy for these deadly tumors. Significance: In-depth transcriptomic analysis identifies cyclin-dependent kinases CDK4 and CDK6 as regulators of metastasis through distinct signaling pathways and reveals the DNA replication/repair pathway as central in promoting these effects.

Epithelial mesenchymal plasticity (EMP) is deemed vital in breast cancer progression, metastasis, stemness and resistance to therapy. Therefore, characterizing molecular mechanisms contributing to EMP are in need enabling the development of more advanced therapeutics against breast cancer. While kinesin superfamily proteins (KIFs) are well known for their role in intracellular cargo movement, our knowledge of their function in breast tumorigenesis is still limited. Various breast cancer cell lines representing different molecular subtypes were used to determine the role of kinesine-1 subunits KIF5B/KLC1 in regulation of EMP. In breast cancer, we show that kinesin family member 5B (KIF5B) and its partner protein kinesin light chain 1 (KLC1), subunits of kinesin-1, to play differential roles in regulating EMP and tumorigenesis. Indeed, we found KIF5B to be expressed in triple negative (TN)-basal-like/claudin low breast cancer subtype and to be an inducer of epithelial-mesenchymal transition (EMT), stemness, invasiveness, tumor formation and metastatic colonization. Whereas, we found KLC1 to be expressed in epithelial/luminal breast cancer subtypes and to be a suppressor of EMT, invasion, metastasis and stem cell markers expression as well as to be an inducer of epithelial/luminal phenotype. Interestingly, in TN-basal-like/claudin low cells we found a novel nuclear accumulation of KIF5B and its interaction with the EMT transcriptional regulator Snail1 independent of KLC1. In addition, TGF-β mediated pro-invasive activity was found to be dependent on KIF5B expression. In contrast, the epithelial differentiation factor and EMT suppressor prolactin (PRL) was found to repress KIF5B gene expression and KIF5B-Snail1 nuclear accumulation, but enhanced KLC1 gene expression and KIF5B-KLC1 interaction. Together, these results highlight a new paradigm for kinesin-1 function in breast tumorigenesis by regulating EMP programing and aggressiveness. FUND: This work was supported by the Canadian Institutes of Health Research (operating grants #233437 and 233438) granted to Suhad Ali.