HMGN2 inducibly binds a novel transactivation domain in nuclear PRLr to coordinate Stat5a-mediated transcription

Prolactin-specific induction of the nuclear translocation of porcine prolactin receptor in porcine mammary epithelial cells

Prolactin (PRL) translocation to the nucleus is a known phenomenon in patients with breast cancer. There is no evidence of this phenomenon in domestic animals (like pigs) at this time. Furthermore, a comprehensive understanding of the molecular mechanisms driving PRLR nuclear translocation remains elusive. In this study, a cell model consisting of porcine mammary epithelial cells (PMECs) was developed. The induction of nuclear localization of porcine PRLR in PMECs was observed in response to porcine prolactin (pPRL). Afterwards, an analysis was conducted on the dynamics of pPRL-induced nuclear localization of pPRLR, which revealed that this process is time-dependent. After that, we utilized several pPRLR ligands to investigate how pPRLR localizes to the nucleus, and we showed that the nuclear translocation of pPRLR is PRL(s)-dependent. Additionally, we discovered that the nuclear translocation of the pPRL-PRLR complex is influenced by importin β1 (IMP β1), and EEA1 was involved in the nuclear translocation of pPRL-PRLR complex. In cell nuclei, the pPRL-PRLR complex has the potential to form a pPRL-PRLR-JAK2 multimer complex, suggesting that the nuclear-localized pPRL-PRLR complex may remain capable of transmitting signals, analogous to its function in the cell membrane.

The growth hormone receptor interacts with transcriptional regulator HMGN1 upon GH-induced nuclear translocation

Growth hormone (GH) actions are mediated through binding to its cell-surface receptor, the GH receptor (GHR), with consequent activation of downstream signalling. However, nuclear GHR localisation has also been observed and is associated with increased cancer cell proliferation. Here we investigated the functional implications of nuclear translocation of the GHR in the human endometrial cancer cell-line, RL95-2, and human mammary epithelial cell-line, MCF-10A. We found that following GH treatment, the GHR rapidly translocates to the nucleus, with maximal localisation at 5-10 min. Combined immunoprecipitation-mass spectrometry analysis of RL95-2 whole cell lysates identified 40 novel GHR binding partners, including the transcriptional regulator, HMGN1. Moreover, microarray analysis demonstrated that the gene targets of HMGN1 were differentially expressed following GH treatment, and co-immunoprecipitation showed that HMGN1 associates with the GHR in the nucleus. Therefore, our results suggest that GHR nuclear translocation might mediate GH actions via interaction with chromatin factors that then drive changes in specific downstream transcriptional programs.

HMGN2 represses gene transcription via interaction with transcription factors Lef-1 and Pitx2 during amelogenesis

The chromatin-associated high mobility group protein N2 (HMGN2) cofactor regulates transcription factor activity through both chromatin and protein interactions. Hmgn2 expression is known to be developmentally regulated, but the post-transcriptional mechanisms that regulate Hmgn2 expression and its precise roles in tooth development remain unclear. Here, we demonstrate that HMGN2 inhibits the activity of multiple transcription factors as a general mechanism to regulate early development. Bimolecular fluorescence complementation, pull-down, and coimmunoprecipitation assays show that HMGN2 interacts with the transcription factor Lef-1 through its HMG-box domain as well as with other early development transcription factors, Dlx2, FoxJ1, and Pitx2. Furthermore, EMSAs demonstrate that HMGN2 binding to Lef-1 inhibits its DNA-binding activity. We found that Pitx2 and Hmgn2 associate with H4K5ac and H3K4me2 chromatin marks in the proximal Dlx2 promoter, demonstrating Hmgn2 association with open chromatin. In addition, we demonstrate that microRNAs (miRs) mir-23a and miR-23b directly target Hmgn2, promoting transcriptional activation at several gene promoters, including the amelogenin promoter. In vivo, we found that decreased Hmgn2 expression correlates with increased miR-23 expression in craniofacial tissues as the murine embryo develops. Finally, we show that ablation of Hmgn2 in mice results in increased amelogenin expression because of increased Pitx2, Dlx2, Lef-1, and FoxJ1 transcriptional activity. Taken together, our results demonstrate both post-transcriptional regulation of Hmgn2 by miR-23a/b and post-translational regulation of gene expression by Hmgn2–transcription factor interactions. We conclude that HMGN2 regulates tooth development through its interaction with multiple transcription factors.

Serine residues 726 and 780 have nonredundant roles regulating STAT5a activity in luminal breast cancer

In breast cancer, prolactin-induced activation of the transcription factor STAT5a results from the phosphorylation of STAT5a tyrosine residue 694. However, its role in mammary oncogenesis remains an unsettled debate as STAT5a exhibits functional dichotomy with both pro-differentiative and pro-proliferative target genes. Phosphorylation of STAT5a serine residues, S726 and S780, may regulate STAT5a in such a way to underlie this duality. Given hematopoiesis studies showing phospho-serine STAT5a as necessary for transformation, we hypothesized that serine phosphorylation regulates STAT5a activity to contribute to its role in mammary oncogenesis, specifically in luminal breast cancer. Here, phosphorylation of S726-, S780-, and Y694-STAT5a in response to prolactin in MCF7 luminal breast cancer cells was investigated with STAT5a knockdown and rescue with Y694F-, S726A-, or S780A-STAT5a, where the phospho-sites were mutated. RNA-sequencing and subsequent Ingenuity Pathway Analysis predicted that loss of each phospho-site differentially affected both prolactin-induced gene expression as well as functional pathways of breast cancer (e.g. cell survival, proliferation, and colony formation). In vitro studies of anchorage-independent growth and proliferation confirmed distinct phenotypes: whereas S780A-STAT5a decreased clonogenicity, S726A-STAT5a decreased proliferation in response to prolactin compared to wild type STAT5a. Collectively, these studies provide novel insights into STAT5a activation in breast cancer pathogenesis.

Prolactin Drives a Dynamic STAT5A/HDAC6/HMGN2 Cis-Regulatory Landscape Exploitable in ER+ Breast Cancer

The hormone prolactin has been implicated in breast cancer pathogenesis and regulates chromatin engagement by the transcription factor, STAT5A. STAT5A is known to inducibly bind promoters and cis-regulatory elements genome-wide, though the mechanisms by which it exerts specificity and regulation of target gene expression remain enigmatic. We previously identified HDAC6 and HMGN2 as cofactors that facilitate prolactin-induced, STAT5A-mediated gene expression. Here, multicondition STAT5A, HDAC6, and HMGN2 chromatin immunoprecipitation and sequencing with parallel condition RNA-seq are utilized to reveal the cis-regulatory landscape and cofactor dynamics underlying prolactin-stimulated gene expression in breast cancer. We find that prolactin-regulated genes are significantly enriched for cis-regulatory elements bound by HDAC6 and HMGN2, and that inducible STAT5A binding at enhancers, rather than promoters, conveys specificity for prolactin-regulated genes. The selective HDAC6 inhibitor, ACY-241, blocks prolactin-induced STAT5A chromatin engagement at cis-regulatory elements as well as a significant proportion of prolactin-stimulated gene expression. We identify functional pathways known to contribute to the development and/or progression of breast cancer that are activated by prolactin and inhibited by ACY-241. Additionally, we find that the DNA sequences underlying shared STAT5A and HDAC6 binding sites at enhancers are differentially enriched for estrogen response elements (ESR1 and ESR2 motifs) relative to enhancers bound by STAT5A alone. Gene set enrichment analysis identifies significant overlap of ERα-regulated genes with genes regulated by prolactin, particularly prolactin-regulated genes with promoters or enhancers co-occupied by both STAT5A and HDAC6. Lastly, the therapeutic efficacy of ACY-241 is demonstrated in in vitro and in vivo breast cancer models, where we identify synergistic ACY-241 drug combinations and observe differential sensitivity of ER+ models relative to ER- models.

Effects of Free Fatty Acids with Different Chain Lengths and Degrees of Saturability on the Milk Fat Synthesis in Primary Cultured Bovine Mammary Epithelial Cells

How short-chain fatty acids (FAs) affect cell membrane morphology and milk fat biosynthesis in mammary epithelial cells (MECs) is yet unclear. This study investigated the primary bovine MEC response to different FAs. We observed that the cell surface ultrastructures were influenced by chain length and degree of saturability of FAs. The CD36, FATP1, and FABP3 gene expression was affected independent of the type of FA. FASN, LPIN1, PPARα, and PPARγ transcripts were more sensitive to the short-chain FAs (acetic and β-hydroxybutyric acids). Furthermore, short-chain FAs inclined to regulate FA degradation-, elongation-, and metabolism-associated pathways, while long-chain FAs (stearic and trans-10,cis-12 conjugated linolenic acids) modulated extracellular matrix-receptor interaction-, transcriptional misregulation-, microRNA-, and ribosome biogenesis-related pathways. However, triacylglycerol accumulation in the cytoplasm was not changed by all of the FAs. Overall, FAs with different chain lengths and degrees of saturability could differentially alter primary bovine MEC cell morphology and influence protein profiles involved in milk fat synthesis pathways.

Targeting STATs in neuroinflammation: The road less traveled!

JAK/STAT transduction pathway is a highly conserved pathway implicated in regulating cellular proliferation, differentiation, survival and apoptosis. Dysregulation of this pathway is involved in the onset of autoimmune, haematological, oncological, metabolic and neurological diseases. Over the last few years, the research of anti-neuroinflammatory agents has gained considerable attention. The ability to diminish the STAT-induced transcription of inflammatory genes is documented for both natural compounds (such as polyphenols) and chemical drugs. Among polyphenols, quercetin and curcumin directly inhibit STAT, while Berberis vulgaris L. and Sophora alopecuroides L extracts act indirectly. Also, the Food and Drug Administration has approved several JAK/STAT inhibitors (direct or indirect) for treating inflammatory diseases, indicating STAT can be considered as a therapeutic target for neuroinflammatory pathologies. Considering the encouraging data obtained so far, clinical trials are warranted to demonstrate the effectiveness and potential use in the clinical practice of STAT inhibitors to treat inflammation-associated neurodegenerative pathologies.

Increased expression of high-mobility group nucleosomal-binding domain 2 protein in various tumor cell lines

High mobility group nucleosomal-binding domain 2 (HMGN2) is an abundant non-histone nuclear protein of vertebrates and invertebrates. The aim of the present study was to characterize the endogenous expression of HMGN2 in various types of tumor cell. Western blotting was performed to analyze HMGN2 expression in the following tumor cell lines: H1975, HSC-4, MDA-MB-468, MDA-MB-231, SCC-25 and THP-1. Periodontal ligament cells (PDLCs) were included as a noncancerous control. HMGN2 was detected in human oral squamous cell carcinoma tissues by immunohistochemical analysis. The results demonstrated that the expression of HMGN2 was increased in the majority of tumor cell lines, particularly MDA-MB-468 and THP-1 cells, compared with PDLCs. The expression of HMGN2 in oral squamous cell carcinoma tissue was significantly increased compared with the expression in normal tissue. Furthermore, the expression of HMGN2 in metastatic oral squamous cell carcinoma tissues was increased compared with that in its non-metastatic counterpart. These results indicated that HMGN2 may serve an important function in the growth and metastasis of tumor cells.

Prolactin/androgen-inducible carboxypeptidase-D increases with nitrotyrosine and Ki67 for breast cancer progression in vivo, and upregulates progression markers VEGF-C and Runx2 in vitro

PURPOSE

Carboxypeptidase-D (CPD) cleaves C-terminal arginine (Arg) to produce nitric oxide (NO). Upregulation of CPD and NO by 17β-estradiol, prolactin (PRL), and androgen increases survival of human breast cancer (BCa) cells in vitro. To demonstrate similar events in vivo, CPD, nitrotyrosine (NT, hallmark of NO action), androgen receptor (AR), prolactin receptor (PRLR), and phospho-Stat5a (for activated PRLR) levels were evaluated in benign and malignant human breast tissues, and correlated with cell proliferation (Ki67) and BCa progression (Cullin-3) biomarkers.

METHODS

Paraffin-embedded breast tissues were analyzed by immunohistochemistry (IHC). BCa progression markers in human MCF-7 and T47D BCa cell lines treated with NO donor SIN-1 or PRL, ±CPD inhibitors were analyzed by RT-qPCR and immunoblotting.

RESULTS

IHC showed progressive increases in CPD, NT, Ki67, and Cullin-3 from low levels in benign tissues to high levels in ductal carcinoma in situ, low-grade, high-grade, and triple-negative BCa. CPD and NT staining were closely associated, implicating CPD in NO production. Phospho-Stat5a increased significantly from benign to high-grade BCa and was mostly nuclear. AR and PRLR were abundant in benign breast and BCa, including triple-negative tumors. SIN-1 and PRL increased VEGF-C and Runx2, but not Cullin-3, in BCa cell lines. PRL induction of VEGF-C and Runx2 was inhibited partly by CPD inhibitors, implicating NO, produced by PRL-regulated CPD, in BCa progression.

CONCLUSIONS

The CPD-Arg-NO pathway contributes to BCa progression in vitro and in vivo. PRL/androgen activation of the pathway support combined AR and PRLR blockade as an additional therapy for BCa.

Histone H1 and Chromosomal Protein HMGN2 Regulate Prolactin-induced STAT5 Transcription Factor Recruitment and Function in Breast Cancer Cells

The hormone prolactin (PRL) contributes to breast cancer pathogenesis through various signaling pathways, one of the most notable being the JAK2/signal transducer and activator of transcription 5 (STAT5) pathway. PRL-induced activation of the transcription factor STAT5 results in the up-regulation of numerous genes implicated in breast cancer pathogenesis. However, the molecular mechanisms that enable STAT5 to access the promoters of these genes are not well understood. Here, we show that PRL signaling induces chromatin decompaction at promoter DNA, corresponding with STAT5 binding. The chromatin-modifying protein high mobility group nucleosomal binding domain 2 (HMGN2) specifically promotes STAT5 accessibility at promoter DNA by facilitating the dissociation of the linker histone H1 in response to PRL. Knockdown of H1 rescues the decrease in PRL-induced transcription following HMGN2 knockdown, and it does so by allowing increased STAT5 recruitment. Moreover, H1 and STAT5 are shown to function antagonistically in regulating PRL-induced transcription as well as breast cancer cell biology. While reduced STAT5 activation results in decreased PRL-induced transcription and cell proliferation, knockdown of H1 rescues both of these effects. Taken together, we elucidate a novel mechanism whereby the linker histone H1 prevents STAT5 binding at promoter DNA, and the PRL-induced dissociation of H1 mediated by HMGN2 is necessary to allow full STAT5 recruitment and promote the biological effects of PRL signaling.

The modulation of MiR-155 and MiR-23a manipulates Klebsiella pneumoniae Adhesion on Human pulmonary Epithelial cells via Integrin α5β1 Signaling

Micro-RNAs (miRNAs) critically regulate several host defense mechanisms, but their roles in the bacteria-epithelium interplay remain unclear. Our results displayed that the expression of miR-155 and miR-23a were down-regulated in K. pneumoniae-infected pulmonary epithelial cells. The elevated bacterial adhesion on A549 cells followed the enhancement of the cellular levels of these two miRNAs. Meanwhile, a mechanistic study demonstrated that miR-155 promoted integrin α5β1 function and resulted in the increased actin polymerization. Moreover, a non-histone nuclear protein, high mobility group nucleosomal-binding domain 2 (HMGN2) served as the potential target of miR-155 and miR-23a to regulate the integrin α5β1 expression and K. pneumoniae adhesion. Furthermore, the expression of a known integrin transcription suppressor-Nuclear Factor-I (NFI) was also repressed by miR-155, which paralleled with its chromatin location in the promoter regions of integrin α5 and β1. These results uncover novel links between miRNAs and integrin function to regulate bacterial adhesion, indicating a potential mechanism of host cell autonomous immune response to K. pneumoniae infection.

HDAC6 Deacetylates HMGN2 to Regulate Stat5a Activity and Breast Cancer Growth

Stat5a is a transcription factor utilized by several cytokine/hormone receptor signaling pathways that promotes transcription of genes associated with proliferation, differentiation, and survival of cancer cells. However, there are currently no clinically approved therapies that directly target Stat5a, despite ample evidence that it contributes to breast cancer pathogenesis. Here, deacetylation of the Stat5a coactivator and chromatin-remodeling protein HMGN2 on lysine residue K2 by HDAC6 promotes Stat5a-mediated transcription and breast cancer growth. HDAC6 inhibition both in vitro and in vivo enhances HMGN2 acetylation with a concomitant reduction in Stat5a-mediated signaling, resulting in an inhibition of breast cancer growth. Furthermore, HMGN2 is highly acetylated at K2 in normal human breast tissue, but is deacetylated in primary breast tumors and lymph node metastases, suggesting that targeting HMGN2 deacetylation is a viable treatment for breast cancer. Together, these results reveal a novel mechanism by which HDAC6 activity promotes the transcription of Stat5a target genes and demonstrate utility of HDAC6 inhibition for breast cancer therapy.

IMPLICATIONS

HMGN2 deacetylation enhances Stat5a transcriptional activity, thereby regulating prolactin-induced gene transcription and breast cancer growth. Mol Cancer Res; 14(10); 994-1008. ©2016 AACR.

The prolactin receptor transactivation domain is associated with steroid hormone receptor expression and malignant progression of breast cancer

The polypeptide hormone prolactin (PRL) stimulates breast epithelial cell growth, differentiation, and motility through its cognate receptor, PRLr. PRLr is expressed in most breast cancers; however, its exact role remains elusive. Our laboratory previously described a novel mode of PRLr signaling in which Stat5a-mediated transcription is regulated through ligand-induced phosphorylation of the PRLr transactivation domain (TAD). Herein, we used a PRLr transactivation-deficient mutant (PRLrYDmut) to identify novel TAD-specific target genes. Microarray analysis identified 120 PRL-induced genes up-regulated by wild type but not PRLrYDmut. Compared with control, PRLr expression significantly induced expression of approximately 4700 PRL-induced genes, whereas PRLrYDmut ablated induction of all but 19 of these genes. Ingenuity pathway analysis found that the PRLr TAD most profoundly affected networks involving cancer and proliferation. In support of this, PRLrYDmut expression reduced anchorage-dependent and anchorage-independent growth. In addition, pathway analysis identified a link between the PRLr TAD and the estrogen and progesterone receptors (ERα/PR). Although neither ERα nor PR was identified as a PRL target gene, a TAD mutation significantly impaired ERα/PR expression and estrogen responsiveness. TMA analysis revealed a marked increase in nuclear, but not cytoplasmic, PRLr TAD phosphorylation as a function of neoplastic progression. We propose that PRLr TAD phosphorylation contributes to breast cancer pathogenesis, in part through regulation of ERα and PR, and has potential utility as a biomarker in this disease.

Nuclear functions and subcellular trafficking mechanisms of the epidermal growth factor receptor family

Accumulating evidence suggests that various diseases, including many types of cancer, result from alteration of subcellular protein localization and compartmentalization. Therefore, it is worthwhile to expand our knowledge in subcellular trafficking of proteins, such as epidermal growth factor receptor (EGFR) and ErbB-2 of the receptor tyrosine kinases, which are highly expressed and activated in human malignancies and frequently correlated with poor prognosis. The well-characterized trafficking of cell surface EGFR is routed, via endocytosis and endosomal sorting, to either the lysosomes for degradation or back to the plasma membrane for recycling. A novel nuclear mode of EGFR signaling pathway has been gradually deciphered in which EGFR is shuttled from the cell surface to the nucleus after endocytosis, and there, it acts as a transcriptional regulator, transmits signals, and is involved in multiple biological functions, including cell proliferation, tumor progression, DNA repair and replication, and chemo- and radio-resistance. Internalized EGFR can also be transported from the cell surface to several intracellular compartments, such as the Golgi apparatus, the endoplasmic reticulum, and the mitochondria, in addition to the nucleus. In this review, we will summarize the functions of nuclear EGFR family and the potential pathways by which EGFR is trafficked from the cell surface to a variety of cellular organelles. A better understanding of the molecular mechanism of EGFR trafficking will shed light on both the receptor biology and potential therapeutic targets of anti-EGFR therapies for clinical application.

The nucleosome binding protein HMGN1 interacts with PCNA and facilitates its binding to chromatin

Proliferating cell nuclear antigen (PCNA) is a ubiquitous protein that interacts with multiple partners and regulates nuclear activities, including chromatin assembly, histone modifications, replication, and DNA damage repair. The role of specific partners in regulating PCNA activities is not fully understood. Here we identify the nucleosome binding protein HMGN1 as a new PCNA-interacting protein that enhances the binding of PCNA to chromatin but not to purified DNA. Two tetrapeptides in the conservative domain of HMGN1 contain amino acids necessary for the binding of HMGN1 to PCNA. Deletion of both tetrapeptides abolishes the HMGN1-PCNA interaction. PCNA preferentially binds to the linker DNA adjacent to an HMGN-containing nucleosome. In living cells, loss of HMGN1 decreases the rate of PCNA recruitment to damaged DNA sites. Our study identifies a new factor that facilitates the interaction of PCNA with chromatin and provides insights into mechanisms whereby nucleosome binding architectural proteins affect the cellular phenotype.

Landscape of EGFR signaling network in human cancers: biology and therapeutic response in relation to receptor subcellular locations

The epidermal growth factor receptor (EGFR) pathway is one of the most dysregulated molecular pathways in human cancers. Despite its well-established importance in tumor growth, progression and drug-resistant phenotype over the past several decades, targeted therapy designed to circumvent EGFR has yielded only modest clinical success in cancer patients, except those with non-small cell lung cancer (NSCLC) carrying EGFR activation mutations. However, almost all of these NSCLC patients eventually developed resistance to small molecule EGFR kinase inhibitors. These disappointing outcomes are, in part, due to the high complexity and the interactive nature of the EGFR signaling network. More recent compelling evidence further indicates that EGFR functionality can be dependent on its subcellular location. In this regard, EGFR undergoes translocation into different organelles where it elicits distinctly different functions than its best known activity as a plasma membrane-bound receptor tyrosine kinase. EGFR can be shuttled into the cell nucleus and mitochondrion upon ligand binding, radiation, EGFR-targeted therapy and other stimuli. Nuclear EGFR behaves as transcriptional regulator, tyrosine kinase, and mediator of other physiological processes. The role of mitochondrial EGFR remains poorly understood but it appears to regulate apoptosis and autophagy. While studies using patient tumors have shown nuclear EGFR to be an indicator for poor clinical outcomes in cancer patients, the impact of mitochondrial EGFR on tumor behavior and patient prognosis remains to be defined. Most recently, several lines of evidence suggest that mislocated EGFR may regulate tumor response to therapy and that plasma membrane-bound EGFR elicits survival signals independent of its kinase activity. In light of these recent progresses and discoveries, we will outline in this minireview an emerging line of research that uncovers and functionally characterizes several novel modes of EGFR signaling that take center stage in the cell nucleus, mitochondrion and other subcellular compartments. We will also discuss the clinical implications of these findings in the rationale design for therapeutic strategy that overcomes tumor drug resistance.