Long Noncoding RNA p53-Stabilizing and Activating RNA Promotes p53 Signaling by Inhibiting Heterogeneous Nuclear Ribonucleoprotein K deSUMOylation and Suppresses Hepatocellular Carcinoma.

To identify hepatocellular carcinoma (HCC)-implicated long noncoding RNAs (lncRNAs), we performed an integrative omics analysis by integrating mRNA and lncRNA expression profiles in HCC tissues. We identified a collection of candidate HCC-implicated lncRNAs. Among them, we demonstrated that an lncRNA, which is named as p53-stabilizing and activating RNA (PSTAR), inhibits HCC cell proliferation and tumorigenicity through inducing p53-mediated cell cycle arrest. We further revealed that PSTAR can bind to heterogeneous nuclear ribonucleoprotein K (hnRNP K) and enhance its SUMOylation and thereby strengthen the interaction between hnRNP K and p53, which ultimately leads to the accumulation and transactivation of p53. PSTAR is down-regulated in HCC tissues, and the low PSTAR expression predicts poor prognosis in patients with HCC, especially those with wild-type p53. Conclusion: This study sheds light on the tumor suppressor role of lncRNA PSTAR, a modulator of the p53 pathway, in HCC.

Mechanistic Characterization of RASGRP1 Variants Identifies an hnRNP-K-Regulated Transcriptional Enhancer Contributing to SLE Susceptibility.

Systemic lupus erythematosus (SLE) is an autoimmune disease with a strong genetic component. We recently identified a novel SLE susceptibility locus near RASGRP1, which governs the ERK/MAPK kinase cascade and B-/T-cell differentiation and development. However, precise causal RASGRP1 functional variant(s) and their mechanisms of action in SLE pathogenesis remain undefined. Our goal was to fine-map this locus, prioritize genetic variants likely to be functional, experimentally validate their biochemical mechanisms, and determine the contribution of these SNPs to SLE risk. We performed a meta-analysis across six Asian and European cohorts (9,529 cases; 22,462 controls), followed by in silico bioinformatic and epigenetic analyses to prioritize potentially functional SNPs. We experimentally validated the functional significance and mechanism of action of three SNPs in cultured T-cells. Meta-analysis identified 18 genome-wide significant (p < 5 × 10-8) SNPs, mostly concentrated in two haplotype blocks, one intronic and the other intergenic. Epigenetic fine-mapping, allelic, eQTL, and imbalance analyses predicted three transcriptional regulatory regions with four SNPs (rs7170151, rs11631591-rs7173565, and rs9920715) prioritized for functional validation. Luciferase reporter assays indicated significant allele-specific enhancer activity for intronic rs7170151 and rs11631591-rs7173565 in T-lymphoid (Jurkat) cells, but not in HEK293 cells. Following up with EMSA, mass spectrometry, and ChIP-qPCR, we detected allele-dependent interactions between heterogeneous nuclear ribonucleoprotein K (hnRNP-K) and rs11631591. Furthermore, inhibition of hnRNP-K in Jurkat and primary T-cells downregulated RASGRP1 and ERK/MAPK signaling. Comprehensive association, bioinformatics, and epigenetic analyses yielded putative functional variants of RASGRP1, which were experimentally validated. Notably, intronic variant (rs11631591) is located in a cell type-specific enhancer sequence, where its risk allele binds to the hnRNP-K protein and modulates RASGRP1 expression in Jurkat and primary T-cells. As risk allele dosage of rs11631591 correlates with increased RASGRP1 expression and ERK activity, we suggest that this SNP may underlie SLE risk at this locus.

A novel oncogenic BTK isoform is overexpressed in colon cancers and required for RAS-mediated transformation.

Bruton's tyrosine kinase (BTK) is essential for B-cell proliferation/differentiation and it is generally believed that its expression and function are limited to bone marrow-derived cells. Here, we report the identification and characterization of p65BTK, a novel isoform abundantly expressed in colon carcinoma cell lines and tumour tissue samples. p65BTK protein is expressed, through heterogeneous nuclear ribonucleoprotein K (hnRNPK)-dependent and internal ribosome entry site-driven translation, from a transcript containing an alternative first exon in the 5'-untranslated region, and is post-transcriptionally regulated, via hnRNPK, by the mitogen-activated protein kinase (MAPK) pathway. p65BTK is endowed with strong transforming activity that depends on active signal-regulated protein kinases-1/2 (ERK1/2) and its inhibition abolishes RAS transforming activity. Accordingly, p65BTK overexpression in colon cancer tissues correlates with ERK1/2 activation. Moreover, p65BTK inhibition affects growth and survival of colon cancer cells. Our data reveal that BTK, via p65BTK expression, is a novel and powerful oncogene acting downstream of the RAS/MAPK pathway and suggest that its targeting may be a promising therapeutic approach.

Heterogeneous nuclear ribonucleoprotein K, an RNA-binding protein, is required for optic axon regeneration in Xenopus laevis.

Axotomized optic axons of Xenopus laevis, in contrast to those of mammals, retain their ability to regenerate throughout life. To better understand the molecular basis for this successful regeneration, we focused on the role of an RNA-binding protein, heterogeneous nuclear ribonucleoprotein (hnRNP) K, because it is required for axonogenesis during development and because several of its RNA targets are under strong post-transcriptional control during regeneration. At 11 d after optic nerve crush, hnRNP K underwent significant translocation into the nucleus of retinal ganglion cells (RGCs), indicating that the protein became activated during regeneration. To suppress its expression, we intravitreously injected an antisense Vivo-Morpholino oligonucleotide targeting hnRNP K. In uninjured eyes, it efficiently knocked down hnRNP K expression in only the RGCs, without inducing either an axotomy response or axon degeneration. After optic nerve crush, staining for multiple markers of regenerating axons showed no regrowth of axons beyond the lesion site with hnRNP K knockdown. RGCs nonetheless responded to the injury by increasing expression of multiple growth-associated RNAs and experienced no additional neurodegeneration above that normally seen with optic nerve injury. At the molecular level, hnRNP K knockdown during regeneration inhibited protein, but not mRNA, expression of several known hnRNP K RNA targets (NF-M, GAP-43) by compromising their efficient nuclear transport and disrupting their loading onto polysomes for translation. Our study therefore provides evidence of a novel post-transcriptional regulatory pathway orchestrated by hnRNP K that is essential for successful CNS axon regeneration.

Translation of myelin basic protein mRNA in oligodendrocytes is regulated by integrin activation and hnRNP-K.

Myelination in the central nervous system provides a unique example of how cells establish asymmetry. The myelinating cell, the oligodendrocyte, extends processes to and wraps multiple axons of different diameter, keeping the number of wraps proportional to the axon diameter. Local regulation of protein synthesis represents one mechanism used to control the different requirements for myelin sheath at each axo-glia interaction. Prior work has established that ß1-integrins are involved in the axoglial interactions that initiate myelination. Here, we show that integrin activation regulates translation of a key sheath protein, myelin basic protein (MBP), by reversing the inhibitory effect of the mRNA 3'UTR. During oligodendrocyte differentiation and myelination α6ß1-integrin interacts with hnRNP-K, an mRNA-binding protein, which binds to MBP mRNA and translocates from the nucleus to the myelin sheath. Furthermore, knockdown of hnRNP-K inhibits MBP protein synthesis during myelination. Together, these results identify a novel pathway by which axoglial adhesion molecules coordinate MBP synthesis with myelin sheath formation.

Androgen receptor and heterogeneous nuclear ribonucleoprotein K colocalize in the nucleoplasm and are modulated by bicalutamide and 4-hydroxy-tamoxifen in prostatic cancer cell lines.

BACKGROUND: Bicalutamide (BIC) is widely used in prostate cancer therapy. The dose and schedule employed are well tolerated, but about 50% of patients develop gynecomastia. Several studies have shown a significant reduction of the troublesome effects when Tamoxifen is concomitantly administered with BIC. However, the results reported in the literature seem to be preliminary and possible interferences could be present. In order to clarify the molecular mechanisms of the combination of the two drugs, we have investigated whether the expression of the proteins belonging to nuclear matrix (NM), one modulator of hormone action, is altered by BIC and/or 4-hydroxy-tamoxifen (4OHT) in LNCaP cells. We focused above all on heterogeneous nuclear ribonucleoprotein K (hnRNP K) a NM protein with a key role in prostate carcinoma. METHODS: NM proteins were analyzed by two-dimensional gel electrophoresis. Modulation and compartmentalization of the androgen receptor and the hnRNP K were studied by Western blotting, confocal microscopy, and immunoprecipitation. RESULTS: Proteomic analysis revealed that there is a similarity in the changes of the NM proteins elicited by drugs alone but that their combination does not result in a simple additive effect. Moreover, we found that in the nucleoplasm the androgen receptor and the hnRNP K colocalize in a complex that is highly proximal to DNA and that both proteins were synchronously modulated by BIC and/or 4OHT treatment. CONCLUSION: This study confirm the pivotal role of hnRNP K in prostate carcinoma and suggest that this role might be played by the interaction with the androgen receptor.

Role of multi-hnRNP nuclear complex in regulation of tumor suppressor ANXA7 in prostate cancer cells.

Annexin-A7 (ANXA7) tumor suppressor role has been shown in various tumors, and ANXA7 expression has been particularly lost in androgen-resistant prostate cancers. In this study, we studied ANXA7 regulation in normal prostate versus androgen-sensitive and -resistant prostate cancer cells. Deletion mapping analysis showed lowest ANXA7-promoter activities in androgen-sensitive LNCaP prostate cancer cells. Genomatix analysis of ANXA7 promoter identified a cluster of steroid nuclear hormone receptor elements, including V$GREF (V$GRE.02/ARE.02). Gelshift analysis clearly indicated distinct nuclear protein occupancy at this ANXA7-promoter site (-1086/-890) in prostate cancer (LNCaP, DU145, and PC3) versus normal prostate (PrEC) cells. In matrix-assisted laser desorption time-of-flight mass spectrometry-based search for ANXA7 nuclear regulators, we identified several heterogeneous nuclear ribonucleoproteins (hnRNPs) (A1, A2/B1 and K) attached to the steroid-associated ANXA7-promoter site in the androgen-resistant PC3 prostate cancer cells with high ANXA7 gene copy number, but not in PrEC. The hnPNP role in ANXA7 regulation (that was validated by hnRNPA2/B1 antibody interference) resulted in multiple ANXA7 cDNA and protein products in PC3, but not in PrEC. Ingenuity pathways analysis showed plausible molecular paths between ANXA7 and the hnRNP-associated network in prostate cancer progression. Thus, a multi-hnRNP complex can be responsible for aberrant ANXA7 transcription and splicing, thereby affecting ANXA7 expression pattern and tumor suppressor function in prostate cancer.

Heterogeneous nuclear ribonucleoprotein K is a novel regulator of androgen receptor translation.

The regulation of androgen receptor (AR) expression in prostate cancer is still poorly understood. The activation of the epidermal growth factor receptor (EGFR) in prostate cancer cells was previously shown to lower AR expression by a rapamycin-sensitive, posttranscriptional mechanism involving the AR mRNA 5'-untranslated region (5'-UTR). In a search for an intermediate within the EGFR/phosphoinositide 3-kinase/Akt/mammalian target of rapamycin pathway that regulates AR at this site, we identified the nucleic acid-binding protein, heterogeneous nuclear ribonucleoprotein K (hnRNP-K), by mass spectrometric analysis of Akt immune complexes from lipid raft-enriched subcellular fractions. We show here that hnRNP-K is a novel inhibitor of AR mRNA translation that regulates androgen-responsive gene expression and prostate cancer cell proliferation. A functional hnRNP-K binding site involved in down-regulating AR protein levels was identified in the AR mRNA 5'-UTR. Further analysis revealed that hnRNP-K is also able to inhibit AR translation in the absence of the 5'-UTR, consistent with the presence of additional predicted hnRNP-K binding sites within the AR open reading frame and in the 3'-UTR. Immunohistochemical analysis of a human prostate cancer tissue microarray revealed an inverse correlation between hnRNP-K expression and AR protein levels in organ-confined prostate tumors and a substantial decline in cytoplasmic hnRNP-K in metastases, despite an overall increase in hnRNP-K levels in metastatic tumors. These data suggest that translational inhibition of AR by hnRNP-K may occur in organ-confined tumors but possibly at a reduced level in metastases. HnRNP-K is the first protein identified that directly interacts with and regulates the AR translational apparatus.

hnRNP K interacts with RNA binding motif protein 42 and functions in the maintenance of cellular ATP level during stress conditions.

Heterogeneous nuclear ribonucleoprotein K (hnRNP K) is a conserved RNA-binding protein that is involved in multiple processes of gene expression, including chromatin remodeling, transcription, RNA splicing, mRNA stability and translation, together with diverse groups of molecular partners. Here we identified a previously uncharacterized protein RNA binding motif protein 42 (RBM42) as hnRNP K-binding protein. RBM42 directly bound to hnRNP K in vivo and in vitro. RBM42 also directly bound to the 3' untranslated region of p21 mRNA, one of the target mRNAs for hnRNP K. RBM42 predominantly localized within the nucleus and co-localized with hnRNP K there. When cells were treated with agents, puromycin, sorbitol or arsenite, which induced the formation of stress granules (SGs), cytoplasmic aggregates of stalled translational pre-initiation complexes, both hnRNP K and RBM42 localized at SGs. Depletion of hnRNP K by RNA interference decreased cellular ATP level following release from stress conditions. Simultaneous depletion of RBM42 with hnRNP K enhanced the effect of the hnRNP K depletion. Our results indicate that hnRNP K and RBM42 are components of SGs and suggest that hnRNP K and RBM42 have a role in the maintenance of cellular ATP level in the stress conditions possibly through protecting their target mRNAs.

Heterogeneous ribonucleoprotein k and thymidine phosphorylase are independent prognostic and therapeutic markers for nasopharyngeal carcinoma.

PURPOSE: Heterogeneous ribonucleoprotein K (hnRNP K) regulates thymidine phosphorylase (TP) mRNA stability. The aim of the present study was to analyze hnRNP K and TP expression in nasopharyngeal carcinoma (NPC) and to evaluate the prognostic and therapeutic potential of these two markers. EXPERIMENTAL DESIGN: We analyzed hnRNP K and TP expression immunohistochemically in 121 clinically proven NPC cases. Statistical analyses were applied to correlate cytoplasmic hnRNP K with elevated TP expression and determine the prognostic significance of these parameters. The therapeutic implication of elevated TP expression was determined by measuring sensitivity of NPC cells to the TP-targeting drug, 5-fluoro-5'-deoxyuridine (5'-DFUR). RESULTS: There was a high correlation between cytoplasmic hnRNP K and high TP (P < 0.001). Both cytoplasmic hnRNP K and high TP were associated with poor overall survival (OS; P = 0.007 and P < 0.001, respectively) and distant metastasis-free survival (P = 0.003 and 0.001, respectively) of NPC patients. A multivariate analysis confirmed that both cytoplasmic hnRNP K and high TP are independent prognostic predictors for OS (P = 0.020 and 0.010, respectively). NPC cells expressing high TP were more sensitive to treatment with the TP-targeting drug, 5'-DFUR. CONCLUSIONS: Cytoplasmic hnRNP K and high TP are associated with shorter OS and distant metastasis-free survival in NPC patients. In vitro experiments suggest that NPC tumors with high TP expression may be sensitive to 5'-DFUR treatment. Cytoplasmic hnRNP K and high TP may be potential prognostic and therapeutic markers for NPC, but additional validation studies are warranted.

Renal regional proteomes in young Dahl salt-sensitive rats.

We performed an extensive proteomic analysis of the Dahl model of salt-sensitive hypertension. The consomic SS-13(BN) rat, genetically similar to the Dahl salt-sensitive rat, while exhibiting a significant amelioration of salt-induced hypertension, was used as a control. Proteomic analysis, using differential in-gel electrophoresis and mass spectrometry techniques, was performed in the renal cortex and the renal medulla of 6-week-old SS and SS-13(BN) rats before significant differences in blood pressure were developed between the 2 strains of rat. Several dozen proteins were identified as differentially expressed between SS and SS-13(BN) rats fed the 0.4% NaCl diet or switched to the 4% NaCl diet for 3 days (n=4). The identified proteins were involved in cellular functions or structures including signal transduction, energy metabolism, and the cytoskeleton. The proteomic analysis and subsequent Western blotting indicated that heterogeneous nuclear ribonucleoprotein K in the renal medulla was upregulated by the 4% NaCl diet in SS-13(BN) rats but downregulated in SS rats. The level of angiotensinogen mRNA in the renal medulla was regulated in an opposite manner. Silencing of heterogeneous nuclear ribonucleoprotein K resulted in an upregulation of angiotensinogen in cultured human kidney cells. In summary, we identified significant differences in kidney regional proteomic profiles between SS and SS-13(BN) rats and demonstrated a potential role of heterogeneous nuclear ribonucleoprotein K in the regulation of angiotensinogen expression in the renal medulla.

Heterogeneous nuclear ribonucleoprotein K contributes to angiotensin II stimulation of vascular endothelial growth factor mRNA translation.

ANG II rapidly increases VEGF synthesis in proximal tubular epithelial cells through mRNA translation. The role of heterogeneous nuclear ribonucleoprotein K (hnRNP K) in ANG II regulation of VEGF mRNA translation initiation was examined. ANG II activated hnRNP K as judged by binding to poly(C)- and poly(U)-agarose. ANG II increased hnRNP K binding to VEGF mRNA at the same time as it stimulated its translation, suggesting that hnRNP K contributes to VEGF mRNA translation. Inhibition of hnRNP K expression by RNA interference significantly reduced ANG II stimulation of VEGF synthesis. ANG II increased hnRNP K phosphorylation on both tyrosine and serine residues with distinct time courses; only Ser302 phosphorylation paralleled binding to VEGF mRNA. Src inhibition using PP2 or RNA interference inhibited PKCdelta activity and prevented hnRNP K phosphorylation on both tyrosine and serine residues and its binding to VEGF mRNA. Under these conditions, ANG II-induced VEGF synthesis was inhibited. ANG II treatment induced redistribution of both VEGF mRNA and hnRNP K protein from light to heavy polysomal fractions, suggesting increased binding of hnRNP K to VEGF mRNA that is targeted for increased translation. This study shows that hnRNP K augments efficiency of VEGF mRNA translation stimulated by ANG II.

Heterogeneous nuclear ribonucleoprotein K modulates angiotensinogen gene expression in kidney cells.

The present studies aimed to identify the 70-kDa nuclear protein that binds to an insulin-responsive element in the rat angiotensinogen gene promoter and to define its action on angiotensinogen gene expression. Nuclear proteins were isolated from rat kidney proximal tubular cells and subjected to two-dimensional electrophoresis. The 70-kDa nuclear protein was detected by Southwestern blotting and subsequently identified by mass spectrometry, which revealed that it was identical to 65-kDa heterogeneous nuclear ribonucleoprotein K (hnRNP K). hnRNP K bound to the insulin-responsive element of the rat angiotensinogen gene was revealed by a gel mobility shift assay and chromatin immunoprecipitation assay. hnRNP K inhibited angiotensinogen mRNA expression and promoter activity. In contrast, hnRNP K down-expression by small interference RNA enhanced angiotensinogen mRNA expression. Moreover, hnRNP K interacted with hnRNP F in pulldown and co-immunoprecipitation assays. Co-transfection of hnRNP K and hnRNP F further suppressed angiotensinogen mRNA expression. Finally, in vitro and in vivo studies demonstrated that high glucose increases and insulin inhibits hnRNP K expression in rat kidney proximal tubular cells. In conclusion, our experiments revealed that hnRNP K is a nuclear protein that binds to the insulin-responsive element of the rat angiotensinogen gene promoter and modulates angiotensinogen gene transcription in the kidney.

Interaction of N-WASP with hnRNPK and its role in filopodia formation and cell spreading.

N-WASP is a member of the WASP family of proteins, which play essential roles in actin dynamics during cell adhesion and migration. hnRNPK is a member of the heterogeneous nuclear ribonucleoprotein complex, which has also been implicated in the regulation of cell spreading. Here, we identify a direct interaction between N-WASP and hnRNPK. We show that this interaction is mediated by the N-terminal WH1 domain of N-WASP and the segment of hnRNPK containing its K interaction (KI) domain. Furthermore, these two proteins are co-localized at the cell periphery in the spreading initiation center during the early stage of cell spreading. We found that co-expression of hnRNPK with N-WASP reverses the stimulation of cell spreading by N-WASP, and this effect is correlated with hnRNPK binding to N-WASP. Expression of hnRNPK does not affect subcellular localization of N-WASP protein. However, co-expression of hnRNPK with N-WASP reduced filopodia formation stimulated by N-WASP in spreading cells. Together, these results identify hnRNPK as a new negative regulator of N-WASP and suggest that hnRNPK may regulate the initial stage of cell spreading by direct association with N-WASP in the spreading initiation center.

The diverse involvement of heterogeneous nuclear ribonucleoprotein K in mitochondrial response to insulin.

Heterogeneous nuclear ribonucleoprotein K (hnRNP K protein) is an RNA/DNA-binding protein that acts in several compartments, including mitochondria. It integrates cellular signaling cascades with multiple processes of gene expression mechanisms. Our studies demonstrate that: (1) insulin activates the import of hnRNP K protein into mitochondria in vitro and in vivo; (2) overexpression of hnRNP K protein modulates insulin-activated mitochondrial gene expression; and (3) insulin treatment stimulates binding of hnRNP K protein to mitochondrial DNA. Based on these and our previously reported results we conclude that hnRNP K protein may be a mediator of mitochondrial response to insulin.

Editing of hnRNP K protein mRNA in colorectal adenocarcinoma and surrounding mucosa.

The heterogeneous nuclear ribonucleoprotein K (hnRNP K) protein is an RNA-binding protein involved in many processes that compose gene expression. K protein is upregulated in the malignant processes and has been shown to modulate the expression of genes involved in mitogenic responses and tumorigenesis. To explore the possibility that there are alternative isoforms of K protein expressed in colon cancer, we amplified and sequenced K protein mRNA that was isolated from colorectal cancers as well as from normal tissues surrounding the tumours. Sequencing revealed a single G-to-A base substitution at position 274 that was found in tumours and surrounding mucosa, but not in individuals that had no colorectal tumour. This substitution most likely reflects an RNA editing event because it was not found in the corresponding genomic DNAs. Sequencing of RNA from normal colonic mucosa of patients with prior resection of colorectal cancer revealed only the wild-type K protein transcript, indicating that G274A isoform is tumour related. To our knowledge, this is the first example of an RNA editing event in cancer and its surrounding tissue, a finding that may offer a new diagnostic and treatment marker.

hnRNP K: an HDM2 target and transcriptional coactivator of p53 in response to DNA damage.

In response to DNA damage, mammalian cells trigger the p53-dependent transcriptional induction of factors that regulate DNA repair, cell-cycle progression, or cell survival. Through differential proteomics, we identify heterogeneous nuclear ribonucleoprotein K (hnRNP K) as being rapidly induced by DNA damage in a manner that requires the DNA-damage signaling kinases ATM or ATR. Induction of hnRNP K ensues through the inhibition of its ubiquitin-dependent proteasomal degradation mediated by the ubiquitin E3 ligase HDM2/MDM2. Strikingly, hnRNP K depletion abrogates transcriptional induction of p53 target genes and causes defects in DNA-damage-induced cell-cycle-checkpoint arrests. Furthermore, in response to DNA damage, p53 and hnRNP K are recruited to the promoters of p53-responsive genes in a mutually dependent manner. These findings establish hnRNP K as a new HDM2 target and show that, by serving as a cofactor for p53, hnRNP K plays key roles in coordinating transcriptional responses to DNA damage.

hnRNP K binds a core polypyrimidine element in the eukaryotic translation initiation factor 4E (eIF4E) promoter, and its regulation of eIF4E contributes to neoplastic transformation.

Translation initiation factor eukaryotic translation initiation factor 4E (eIF4E) plays a key role in regulation of cellular proliferation. Its effects on the m7GpppN mRNA cap are critical because overexpression of eIF4E transforms cells, and eIF4E function is rate-limiting for G1 passage. Although we identified eIF4E as a c-Myc target, little else is known about its transcriptional regulation. Previously, we described an element at position -25 (TTACCCCCCCTT) that was critical for eIF4E promoter function. Here we report that this sequence (named 4EBE, for eIF4E basal element) functions as a basal promoter element that binds hnRNP K. The 4EBE is sufficient to replace TATA sequences in a heterologous reporter construct. Interactions between 4EBE and upstream activator sites are position, distance, and sequence dependent. Using DNA affinity chromatography, we identified hnRNP K as a 4EBE-binding protein. Chromatin immunoprecipitation, siRNA interference, and hnRNP K overexpression demonstrate that hnRNP K can regulate eIF4E mRNA. Moreover, hnRNP K increased translation initiation, increased cell division, and promoted neoplastic transformation in an eIF4E-dependent manner. hnRNP K binds the TATA-binding protein, explaining how the 4EBE might replace TATA in the eIF4E promoter. hnRNP K is an unusually diverse regulator of multiple steps in growth regulation because it also directly regulates c-myc transcription, mRNA export, splicing, and translation initiation.

Heterogeneous nuclear ribonucleoprotein K enhances insulin-induced expression of mitochondrial UCP2 protein.

The uncoupling protein 2, UCP2, is a member of a family of inner mitochondrial membrane ion carriers involved in a host of metabolic processes. UCP2 protein is encoded by nuclear genome, but the protein is found exclusively in the mitochondria. The heterogeneous nuclear ribonucleoprotein K (hnRNPK) is an RNA-binding protein involved in many processes that compose gene expression, including mRNA processing and translation. The yeast three-hybrid screen revealed K protein bound to ucp2 mRNA through sites located in the 3'-untranslated region of the transcript. ucp2 mRNA-K protein complexes were associated with polysome-coated mitochondria. Expression of exogenous K protein augmented the insulin-induced mitochondrial level of UCP2 protein that was not accompanied by a corresponding increase in ucp2 mRNA. These results suggest the insulin stimulates translation of ucp2 mRNA in a process that involves K protein.

hnRNP K: one protein multiple processes.

Since its original identification as a component of the heterogeneous nuclear ribonucleoprotein (hnRNP) complex, K protein has been found not only in the nucleus but also in the cytoplasm and mitochondria and is implicated in chromatin remodeling, transcription, splicing and translation processes. K protein contains multiple modules that, on one hand, bind kinases while, on the other hand, recruit chromatin, transcription, splicing and translation factors. Moreover, the K- protein-mediated interactions are regulated by signaling cascades. These observations are consistent with K protein acting as a docking platform to integrate signaling cascades by facilitating cross-talk between kinases and factors that mediate nucleic-acid-directed processes. Comparison of K across species reveals that it is an essential factor in metazoans, but not in yeast. Although some of the K protein interactions and functions are conserved in eukaryotes from yeast to man, the mammalian protein seems to play a wider role. The greater diversity of mammalian K protein interactions and function may reflect gain of novel docking sites and expansion evolutionary of gene expression networks.