Stress-induced nuclear condensation of NELF drives transcriptional downregulation.

In response to stress, human cells coordinately downregulate transcription and translation of housekeeping genes. To downregulate transcription, the negative elongation factor (NELF) is recruited to gene promoters impairing RNA polymerase II elongation. Here we report that NELF rapidly forms nuclear condensates upon stress in human cells. Condensate formation requires NELF dephosphorylation and SUMOylation induced by stress. The intrinsically disordered region (IDR) in NELFA is necessary for nuclear NELF condensation and can be functionally replaced by the IDR of FUS or EWSR1 protein. We find that biomolecular condensation facilitates enhanced recruitment of NELF to promoters upon stress to drive transcriptional downregulation. Importantly, NELF condensation is required for cellular viability under stressful conditions. We propose that stress-induced NELF condensates reported here are nuclear counterparts of cytosolic stress granules. These two stress-inducible condensates may drive the coordinated downregulation of transcription and translation, likely forming a critical node of the stress survival strategy.

ZFP451-mediated SUMOylation of SATB2 drives embryonic stem cell differentiation.

The establishment of cell fates involves alterations of transcription factor repertoires and repurposing of transcription factors by post-translational modifications. In embryonic stem cells (ESCs), the chromatin organizers SATB2 and SATB1 balance pluripotency and differentiation by activating and repressing pluripotency genes, respectively. Here, we show that conditional Satb2 gene inactivation weakens ESC pluripotency, and we identify SUMO2 modification of SATB2 by the E3 ligase ZFP451 as a potential driver of ESC differentiation. Mutations of two SUMO-acceptor lysines of Satb2 (Satb2K →R ) or knockout of Zfp451 impair the ability of ESCs to silence pluripotency genes and activate differentiation-associated genes in response to retinoic acid (RA) treatment. Notably, the forced expression of a SUMO2-SATB2 fusion protein in either Satb2K →R or Zfp451-/- ESCs rescues, in part, their impaired differentiation potential and enhances the down-regulation of Nanog The differentiation defect of Satb2K →R ESCs correlates with altered higher-order chromatin interactions relative to Satb2wt ESCs. Upon RA treatment of Satb2wt ESCs, SATB2 interacts with ZFP451 and the LSD1/CoREST complex and gains binding at differentiation genes, which is not observed in RA-treated Satb2K →R cells. Thus, SATB2 SUMOylation may contribute to the rewiring of transcriptional networks and the chromatin interactome of ESCs in the transition of pluripotency to differentiation.

Chromatin accessibility and pioneer factor FOXA1 restrict glucocorticoid receptor action in prostate cancer.

Treatment of prostate cancer relies predominantly on the inhibition of androgen receptor (AR) signaling. Despite the initial effectiveness of the antiandrogen therapies, the cancer often develops resistance to the AR blockade. One mechanism of the resistance is glucocorticoid receptor (GR)-mediated replacement of AR function. Nevertheless, the mechanistic ways and means how the GR-mediated antiandrogen resistance occurs have remained elusive. Here, we have discovered several crucial features of GR action in prostate cancer cells through genome-wide techniques. We detected that the replacement of AR by GR in enzalutamide-exposed prostate cancer cells occurs almost exclusively at pre-accessible chromatin sites displaying FOXA1 occupancy. Counterintuitively to the classical pioneer factor model, silencing of FOXA1 potentiated the chromatin binding and transcriptional activity of GR. This was attributed to FOXA1-mediated repression of the NR3C1 (gene encoding GR) expression via the corepressor TLE3. Moreover, the small-molecule inhibition of coactivator p300's enzymatic activity efficiently restricted GR-mediated gene regulation and cell proliferation. Overall, we identified chromatin pre-accessibility and FOXA1-mediated repression as important regulators of GR action in prostate cancer, pointing out new avenues to oppose steroid receptor-mediated antiandrogen resistance.

Central role of SUMOylation in the regulation of chromatin interactions and transcriptional outputs of the androgen receptor in prostate cancer cells.

The androgen receptor (AR) is pivotal in prostate cancer (PCa) progression and represents a critical therapeutic target. AR-mediated gene regulation involves intricate interactions with nuclear proteins, with many mediating and undergoing post-translational modifications that present alternative therapeutic avenues. Through chromatin proteomics in PCa cells, we identified SUMO ligases together with nuclear receptor coregulators and pioneer transcription factors within the AR's protein network. Intriguingly, this network displayed a significant association with SUMO2/3. To elucidate the influence of SUMOylation on AR chromatin interactions and subsequent gene regulation, we inhibited SUMOylation using ML-792 (SUMOi). While androgens generally facilitated the co-occupancy of SUMO2/3 and AR on chromatin, SUMOi induced divergent effects dependent on the type of AR-binding site (ARB). SUMOi augmented AR's pioneer-like binding on inaccessible chromatin regions abundant in androgen response elements (AREs) and diminished its interaction with accessible chromatin regions sparse in AREs yet rich in pioneer transcription factor motifs. The SUMOi-impacted ARBs divergently influenced AR-regulated genes; those associated with AR-mediated activation played roles in negative regulation of cell proliferation, while those with AR-mediated repression were involved in pattern formation. In conclusion, our findings underscore the pervasive influence of SUMOylation in shaping AR's role in PCa cells, potentially unveiling new therapeutic strategies.

Reprogramming of glucocorticoid receptor function by hypoxia.

Here, we investigate the impact of hypoxia on the hepatic response of glucocorticoid receptor (GR) to dexamethasone (DEX) in mice via RNA-sequencing. Hypoxia causes three types of reprogramming of GR: (i) much weaker induction of classical GR-responsive genes by DEX in hypoxia, (ii) a number of genes is induced by DEX specifically in hypoxia, and (iii) hypoxia induces a group of genes via activation of the hypothalamic-pituitary-adrenal (HPA) axis. Transcriptional profiles are reflected by changed GR DNA-binding as measured by ChIP sequencing. The HPA axis is induced by hypothalamic HIF1α and HIF2α activation and leads to GR-dependent lipolysis and ketogenesis. Acute inflammation, induced by lipopolysaccharide, is prevented by DEX in normoxia but not during hypoxia, and this is attributed to HPA axis activation by hypoxia. We unfold new physiological pathways that have consequences for patients suffering from GC resistance.

The androgen receptor depends on ligand-binding domain dimerization for transcriptional activation.

Whereas dimerization of the DNA-binding domain of the androgen receptor (AR) plays an evident role in recognizing bipartite response elements, the contribution of the dimerization of the ligand-binding domain (LBD) to the correct functioning of the AR remains unclear. Here, we describe a mouse model with disrupted dimerization of the AR LBD (ARLmon/Y ). The disruptive effect of the mutation is demonstrated by the feminized phenotype, absence of male accessory sex glands, and strongly affected spermatogenesis, despite high circulating levels of testosterone. Testosterone replacement studies in orchidectomized mice demonstrate that androgen-regulated transcriptomes in ARLmon/Y mice are completely lost. The mutated AR still translocates to the nucleus and binds chromatin, but does not bind to specific AR binding sites. In vitro studies reveal that the mutation in the LBD dimer interface also affects other AR functions such as DNA binding, ligand binding, and co-regulator binding. In conclusion, LBD dimerization is crucial for the development of AR-dependent tissues through its role in transcriptional regulation in vivo. Our findings identify AR LBD dimerization as a possible target for AR inhibition.

SUMOylation regulates the protein network and chromatin accessibility at glucocorticoid receptor-binding sites.

Glucocorticoid receptor (GR) is an essential transcription factor (TF), controlling metabolism, development and immune responses. SUMOylation regulates chromatin occupancy and target gene expression of GR in a locus-selective manner, but the mechanism of regulation has remained elusive. Here, we identify the protein network around chromatin-bound GR by using selective isolation of chromatin-associated proteins and show that the network is affected by receptor SUMOylation, with several nuclear receptor coregulators and chromatin modifiers preferring interaction with SUMOylation-deficient GR and proteins implicated in transcriptional repression preferring interaction with SUMOylation-competent GR. This difference is reflected in our chromatin binding, chromatin accessibility and gene expression data, showing that the SUMOylation-deficient GR is more potent in binding and opening chromatin at glucocorticoid-regulated enhancers and inducing expression of target loci. Blockage of SUMOylation by a SUMO-activating enzyme inhibitor (ML-792) phenocopied to a large extent the consequences of GR SUMOylation deficiency on chromatin binding and target gene expression. Our results thus show that SUMOylation modulates the specificity of GR by regulating its chromatin protein network and accessibility at GR-bound enhancers. We speculate that many other SUMOylated TFs utilize a similar regulatory mechanism.

TNF-α inhibits glucocorticoid receptor-induced gene expression by reshaping the GR nuclear cofactor profile.

Glucocorticoid resistance (GCR) is defined as an unresponsiveness to the therapeutic effects, including the antiinflammatory ones of glucocorticoids (GCs) and their receptor, the glucocorticoid receptor (GR). It is a problem in the management of inflammatory diseases and can be congenital as well as acquired. The strong proinflammatory cytokine TNF-alpha (TNF) induces an acute form of GCR, not only in mice, but also in several cell lines: e.g., in the hepatoma cell line BWTG3, as evidenced by impaired Dexamethasone (Dex)-stimulated direct GR-dependent gene up- and down-regulation. We report that TNF has a significant and broad impact on this transcriptional performance of GR, but no impact on nuclear translocation, dimerization, or DNA binding capacity of GR. Proteome-wide proximity-mapping (BioID), however, revealed that the GR interactome was strongly modulated by TNF. One GR cofactor that interacted significantly less with the receptor under GCR conditions is p300. NFκB activation and p300 knockdown both reduced direct transcriptional output of GR whereas p300 overexpression and NFκB inhibition reverted TNF-induced GCR, which is in support of a cofactor reshuffle model. This hypothesis was supported by FRET studies. This mechanism of GCR opens avenues for therapeutic interventions in GCR diseases.

A long hypoxia-inducible factor 3 isoform 2 is a transcription activator that regulates erythropoietin.

Hypoxia-inducible factor (HIF), an αß dimer, is the master regulator of oxygen homeostasis with hundreds of hypoxia-inducible target genes. Three HIF isoforms differing in the oxygen-sensitive α subunit exist in vertebrates. While HIF-1 and HIF-2 are known transcription activators, HIF-3 has been considered a negative regulator of the hypoxia response pathway. However, the human HIF3A mRNA is subject to complex alternative splicing. It was recently shown that the long HIF-3α variants can form αß dimers that possess transactivation capacity. Here, we show that overexpression of the long HIF-3α2 variant induces the expression of a subset of genes, including the erythropoietin (EPO) gene, while simultaneous downregulation of all HIF-3α variants by siRNA targeting a shared HIF3A region leads to downregulation of EPO and additional genes. EPO mRNA and protein levels correlated with HIF3A silencing and HIF-3α2 overexpression. Chromatin immunoprecipitation analyses showed that HIF-3α2 binding associated with canonical hypoxia response elements in the promoter regions of EPO. Luciferase reporter assays showed that the identified HIF-3α2 chromatin-binding regions were sufficient to promote transcription by all three HIF-α isoforms. Based on these data, HIF-3α2 is a transcription activator that directly regulates EPO expression.

A dysregulated acetyl/SUMO switch of FXR promotes hepatic inflammation in obesity.

Acetylation of transcriptional regulators is normally dynamically regulated by nutrient status but is often persistently elevated in nutrient-excessive obesity conditions. We investigated the functional consequences of such aberrantly elevated acetylation of the nuclear receptor FXR as a model. Proteomic studies identified K217 as the FXR acetylation site in diet-induced obese mice. In vivo studies utilizing acetylation-mimic and acetylation-defective K217 mutants and gene expression profiling revealed that FXR acetylation increased proinflammatory gene expression, macrophage infiltration, and liver cytokine and triglyceride levels, impaired insulin signaling, and increased glucose intolerance. Mechanistically, acetylation of FXR blocked its interaction with the SUMO ligase PIASy and inhibited SUMO2 modification at K277, resulting in activation of inflammatory genes. SUMOylation of agonist-activated FXR increased its interaction with NF-κB but blocked that with RXRα, so that SUMO2-modified FXR was selectively recruited to and trans-repressed inflammatory genes without affecting FXR/RXRα target genes. A dysregulated acetyl/SUMO switch of FXR in obesity may serve as a general mechanism for diminished anti-inflammatory response of other transcriptional regulators and provide potential therapeutic and diagnostic targets for obesity-related metabolic disorders.

Chromatin SUMOylation in heat stress: To protect, pause and organise?: SUMO stress response on chromatin.

Post-translational modifications, e.g. SUMO modifications (SUMOylation), provide a mechanism for swiftly changing a protein's activity. Various stress conditions trigger a SUMO stress response (SSR) - a stress-induced rapid change in the conjugation of SUMO to multiple proteins, which predominantly targets nuclear proteins. The SSR has been postulated to protect stressed cells by preserving the functionality of crucial proteins. However, it is unclear how it exerts its protective functions. Interestingly, heat stress (HS) increases SUMOylation of proteins at active promoters and enhancers. In promoters, HS-induced SUMOylation correlates with gene transcription and stress-induced RNA polymerase II (Pol2) pausing. Conversely, a disappearance of SUMOylation in HS occurs at chromatin anchor points that maintain chromatin-looping structures and the spatial organisation of chromatin. In reviewing the literature, we hypothesise that the SSR regulates Pol2 pausing by modulating the interactions of pausing-regulating proteins, whereas deSUMOylation alters the function of chromatin anchors.

Agonist-specific Protein Interactomes of Glucocorticoid and Androgen Receptor as Revealed by Proximity Mapping.

Glucocorticoid receptor (GR) and androgen receptor (AR) are steroid-inducible transcription factors (TFs). The GR and the AR are central regulators of various metabolic, homeostatic and differentiation processes and hence important therapeutic targets, especially in inflammation and prostate cancer, respectively. Hormone binding to these steroid receptors (SRs) leads to DNA binding and activation or repression of their target genes with the aid of interacting proteins, coregulators. However, protein interactomes of these important drug targets have remained poorly defined. We used proximity-dependent biotin identification to map the protein interaction landscapes of GR and AR in the presence and absence of their cognate agonist (dexamethasone, 5α-dihydrotestosterone) and antagonist (RU486, enzalutamide) in intact human cells. We reproducibly identified more than 30 proteins that interacted with the GR in an agonist-specific manner and whose interactions were significantly influenced by the DNA-binding function of the receptor. Interestingly, the agonist-dependent interactome of the GR overlapped considerably with that of the AR. In addition to known coactivators, corepressors and components of BAF (SWI/SNF) chromatin-remodeling complex, we identified a number of proteins, including lysine methyltransferases and demethylases that have not been previously linked to glucocorticoid or androgen signaling. A substantial number of these novel agonist-dependent GR/AR-interacting proteins, e.g. BCOR, IRF2BP2, RCOR1, and TLE3, have previously been implicated in transcription repression. This together with our data on the effect of BCOR, IRF2BP2, and RCOR1 on GR target gene expression suggests multifaceted functions and roles for SR coregulators. These first high confidence SR interactomes will aid in therapeutic targeting of the GR and the AR.

Crosstalk between androgen and pro-inflammatory signaling remodels androgen receptor and NF-κB cistrome to reprogram the prostate cancer cell transcriptome.

Inflammatory processes and androgen signaling are critical for the growth of prostate cancer (PC), the most common cancer among males in Western countries. To understand the importance of potential interplay between pro-inflammatory and androgen signaling for gene regulation, we have interrogated the crosstalk between androgen receptor (AR) and NF-κB, a key transcriptional mediator of inflammatory responses, by utilizing genome-wide chromatin immunoprecipitation sequencing and global run-on sequencing in PC cells. Co-stimulation of LNCaP cells with androgen and pro-inflammatory cytokine TNFα invoked a transcriptome which was very distinct from that induced by either stimulation alone. The altered transcriptome that included gene programs linked to cell migration and invasiveness was orchestrated by significant remodeling of NF-κB and AR cistrome and enhancer landscape. Although androgen multiplied the NF-κB cistrome and TNFα restrained the AR cistrome, there was no general reciprocal tethering of the AR to the NF-κB on chromatin. Instead, redistribution of FOXA1, PIAS1 and PIAS2 contributed to the exposure of latent NF-κB chromatin-binding sites and masking of AR chromatin-binding sites. Taken together, concomitant androgen and pro-inflammatory signaling significantly remodels especially the NF-κB cistrome, reprogramming the PC cell transcriptome in fashion that may contribute to the progression of PC.

ZNHIT3 is defective in PEHO syndrome, a severe encephalopathy with cerebellar granule neuron loss.

Progressive encephalopathy with oedema, hypsarrhythmia, and optic atrophy (PEHO) syndrome is an early childhood onset, severe autosomal recessive encephalopathy characterized by extreme cerebellar atrophy due to almost total granule neuron loss. By combining homozygosity mapping in Finnish families with Sanger sequencing of positional candidate genes and with exome sequencing a homozygous missense substitution of leucine for serine at codon 31 in ZNHIT3 was identified as the primary cause of PEHO syndrome. ZNHIT3 encodes a nuclear zinc finger protein previously implicated in transcriptional regulation and in small nucleolar ribonucleoprotein particle assembly and thus possibly to pre-ribosomal RNA processing. The identified mutation affects a highly conserved amino acid residue in the zinc finger domain of ZNHIT3. Both knockdown and genome editing of znhit3 in zebrafish embryos recapitulate the patients' cerebellar defects, microcephaly and oedema. These phenotypes are rescued by wild-type, but not mutant human ZNHIT3 mRNA, suggesting that the patient missense substitution causes disease through a loss-of-function mechanism. Transfection of cell lines with ZNHIT3 expression vectors showed that the PEHO syndrome mutant protein is unstable. Immunohistochemical analysis of mouse cerebellar tissue demonstrated ZNHIT3 to be expressed in proliferating granule cell precursors, in proliferating and post-mitotic granule cells, and in Purkinje cells. Knockdown of Znhit3 in cultured mouse granule neurons and ex vivo cerebellar slices indicate that ZNHIT3 is indispensable for granule neuron survival and migration, consistent with the zebrafish findings and patient neuropathology. These results suggest that loss-of-function of a nuclear regulator protein underlies PEHO syndrome and imply that establishment of its spatiotemporal interaction targets will be the basis for developing therapeutic approaches and for improved understanding of cerebellar development.

SUMO ligase PIAS1 functions as a target gene selective androgen receptor coregulator on prostate cancer cell chromatin.

Androgen receptor (AR) is a ligand-activated transcription factor that plays a central role in the development and growth of prostate carcinoma. PIAS1 is an AR- and SUMO-interacting protein and a putative transcriptional coregulator overexpressed in prostate cancer. To study the importance of PIAS1 for the androgen-regulated transcriptome of VCaP prostate cancer cells, we silenced its expression by RNAi. Transcriptome analyses revealed that a subset of the AR-regulated genes is significantly influenced, either activated or repressed, by PIAS1 depletion. Interestingly, PIAS1 depletion also exposed a new set of genes to androgen regulation, suggesting that PIAS1 can mask distinct genomic loci from AR access. In keeping with gene expression data, silencing of PIAS1 attenuated VCaP cell proliferation. ChIP-seq analyses showed that PIAS1 interacts with AR at chromatin sites harboring also SUMO2/3 and surrounded by H3K4me2; androgen exposure increased the number of PIAS1-occupying sites, resulting in nearly complete overlap with AR chromatin binding events. PIAS1 interacted also with the pioneer factor FOXA1. Of note, PIAS1 depletion affected AR chromatin occupancy at binding sites enriched for HOXD13 and GATA motifs. Taken together, PIAS1 is a genuine chromatin-bound AR coregulator that functions in a target gene selective fashion to regulate prostate cancer cell growth.

SUMOylation modulates the transcriptional activity of androgen receptor in a target gene and pathway selective manner.

Androgen receptor (AR) plays an important regulatory role in prostate cancer. AR's transcriptional activity is regulated by androgenic ligands, but also by post-translational modifications, such as SUMOylation. To study the role of AR SUMOylation in genuine chromatin environment, we compared androgen-regulated gene expression and AR chromatin occupancy in PC-3 prostate cancer cell lines stably expressing wild-type (wt) or doubly SUMOylation site-mutated AR (AR-K386R,K520R). Our genome-wide gene expression analyses reveal that the SUMOylation modulates the AR function in a target gene and pathway selective manner. The transcripts that are differentially regulated by androgen and SUMOylation are linked to cellular movement, cell death, cellular proliferation, cellular development and cell cycle. Fittingly, SUMOylation mutant AR cells proliferate faster and are more sensitive to apoptosis. Moreover, ChIP-seq analyses show that the SUMOylation can modulate the chromatin occupancy of AR on many loci in a fashion that parallels their differential androgen-regulated expression. De novo motif analyses reveal that FOXA1, C/EBP and AP-1 motifs are differentially enriched at the wtAR- and the AR-K386R,K520R-preferred genomic binding positions. Taken together, our data indicate that SUMOylation does not simply repress the AR activity, but it regulates AR's interaction with the chromatin and the receptor's target gene selection.

SUMOylation regulates the chromatin occupancy and anti-proliferative gene programs of glucocorticoid receptor.

In addition to the glucocorticoids, the glucocorticoid receptor (GR) is regulated by post-translational modifications, including SUMOylation. We have analyzed how SUMOylation influences the activity of endogenous GR target genes and the receptor chromatin binding by using isogenic HEK293 cells expressing wild-type GR (wtGR) or SUMOylation-defective GR (GR3KR). Gene expression profiling revealed that both dexamethasone up- and downregulated genes are affected by the GR SUMOylation and that the affected genes are significantly associated with pathways of cellular proliferation and survival. The GR3KR-expressing cells proliferated more rapidly, and their anti-proliferative response to dexamethasone was less pronounced than in the wtGR-expressing cells. ChIP-seq analyses indicated that the SUMOylation modulates the chromatin occupancy of GR on several loci associated with cellular growth in a fashion that parallels with their differential dexamethasone-regulated expression between the two cell lines. Moreover, chromatin SUMO-2/3 marks, which were associated with active GR-binding sites, showed markedly higher overlap with the wtGR cistrome than with the GR3KR cistrome. In sum, our results indicate that the SUMOylation does not simply repress the GR activity, but regulates the activity of the receptor in a target locus selective fashion, playing an important role in controlling the GR activity on genes influencing cell growth.

Firefly luciferase inhibitor-conjugated peptide quenches bioluminescence: a versatile tool for real time monitoring cellular uptake of biomolecules.

In this paper, novel firefly luciferase-specific inhibitor compounds (FLICs) are evaluated as potential tools for cellular trafficking of transporter conjugates. As a proof-of-concept, we designed FLICs that were suitable for solid phase peptide synthesis and could be covalently conjugated to peptides via an amide bond. The spacer between inhibitor and peptide was optimized to gain efficient inhibition of recombinant firefly luciferase (FLuc) without compromising the activity of the model peptides. The hypothesis of using FLICs as tools for cellular trafficking studies was ensured with U87Fluc glioblastoma cells expressing firefly luciferase. Results show that cell penetrating peptide (penetratin) FLIC conjugate 9 inhibited FLuc penetrated cells efficiently (IC50 = 1.6 µM) and inhibited bioluminescence, without affecting the viability of the cells. Based on these results, peptide-FLIC conjugates can be used for the analysis of cellular uptake of biomolecules in a new way that can at the same time overcome some downsides seen with other methods. Thus, FLICs can be considered as versatile tools that broaden the plethora of methods that take advantage of the bioluminescence phenomena.

Serum androgen bioactivity is low in children with premature adrenarche.

BACKGROUND: Clinical findings in children with premature adrenarche (PA) correlate only partly with circulating levels of adrenal androgens. It is not known whether the prepubertal low circulating concentrations of testosterone (T) and dihydrotestosterone, together with those of adrenal androgens, are capable of activating the androgen receptor. METHODS: This cross-sectional study was performed at a university hospital. Circulating androgen bioactivity was measured in 67 prepubertal children with clinical signs of PA and 94 control children using a novel androgen bioassay. RESULTS: Circulating androgen bioactivity was low in the PA and control children. In the subgroup of children (n = 28) with serum T concentration over the assay sensitivity (0.35 nmol/l) and a signal in the androgen bioassay, we found a positive correlation between androgen bioactivity and serum T (r = 0.50; P < 0.01) and the free androgen index (r = 0.61; P < 0.01) and a negative correlation with serum sex hormone-binding globulin concentration (r = -0.41; P < 0.05). CONCLUSION: Peripheral metabolism of adrenal androgen precursors may be required for any androgenic effects in PA. However, the limitations in the sensitivity of the bioassay developed herein may hide some differences between the PA and control children.

Protein inhibitor of activated STAT3 (PIAS3) protein promotes SUMOylation and nuclear sequestration of the intracellular domain of ErbB4 protein.

ErbB4 is a receptor tyrosine kinase implicated in the development and homeostasis of the heart, central nervous system, and mammary gland. Cleavable isoforms of ErbB4 release a soluble intracellular domain (ICD) that can translocate to the nucleus and function as a transcriptional coregulator. In search of regulatory mechanisms of ErbB4 ICD function, we identified PIAS3 as a novel interaction partner of ErbB4 ICD. In keeping with the small ubiquitin-like modifier (SUMO) E3 ligase function of protein inhibitor of activated STAT (PIAS) proteins, we showed that the ErbB4 ICD is modified by SUMO, and that PIAS3 stimulates the SUMOylation. Upon overexpression of PIAS3, the ErbB4 ICD generated from the full-length receptor accumulated into the nucleus in a manner that was dependent on the functional nuclear localization signal of ErbB4. In the nucleus, ErbB4 colocalized with PIAS3 and SUMO-1 in promyelocytic leukemia nuclear bodies, nuclear domains involved in regulation of transcription. Accordingly, PIAS3 overexpression had an effect on the transcriptional coregulatory activity of ErbB4, repressing its ability to coactivate transcription with Yes-associated protein. Finally, knockdown of PIAS3 with siRNA partially rescued the inhibitory effect of the ErbB4 ICD on differentiation of MDA-MB-468 breast cancer and HC11 mammary epithelial cells. Our findings illustrate that PIAS3 is a novel regulator of ErbB4 receptor tyrosine kinase, controlling its nuclear sequestration and function.