Modulation of the Interleukin-6 Receptor Subunit Glycoprotein 130 Complex and Its Signaling by LMO4 Interaction

Cytoplasmic LMO2-LDB1 Complex Activates STAT3 Signaling through Interaction with gp130-JAK in Glioma Stem Cells

The oncogenic role of nuclear LIM domain only 2 (LMO2) as a transcriptional regulator is well established, but its function in the cytoplasm is largely unknown. Here, we identified LMO2 as a cytoplasmic activator for signal transducer and activator of transcription 3 (STAT3) signaling in glioma stem cells (GSCs) through biochemical and bioinformatics analyses. LMO2 increases STAT3 phosphorylation by interacting with glycoprotein 130 (gp130) and Janus kinases (JAKs). LMO2-driven activation of STAT3 signaling requires the LDB1 protein and leads to increased expression of an inhibitor of differentiation 1 (ID1), a master regulator of cancer stemness. Our findings indicate that the cytoplasmic LMO2-LDB1 complex plays a crucial role in the activation of the GSC signaling cascade via interaction with gp130 and JAK1/2. Thus, LMO2-LDB1 is a bona fide oncogenic protein complex that activates either the JAK-STAT signaling cascade in the cytoplasm or direct transcriptional regulation in the nucleus.

Generation of resolving memory neutrophils through pharmacological training with 4-PBA or genetic deletion of TRAM

Neutrophils are the dominant leukocytes in circulation and the first responders to infection and inflammatory cues. While the roles of neutrophils in driving inflammation have been widely recognized, the contribution of neutrophils in facilitating inflammation resolution is under-studied. Here, through single-cell RNA sequencing analysis, we identified a subpopulation of neutrophils exhibiting pro-resolving characteristics with greater Cd200r and Cd86 expression at the resting state. We further discovered that 4-PBA, a peroxisomal stress-reducing agent, can potently train neutrophils into the resolving state with enhanced expression of CD200R, CD86, as well as soluble pro-resolving mediators Resolvin D1 and SerpinB1. Resolving neutrophils trained by 4-PBA manifest enhanced phagocytosis and bacterial-killing functions. Mechanistically, the generation of resolving neutrophils is mediated by the PPARγ/LMO4/STAT3 signaling circuit modulated by TLR4 adaptor molecule TRAM. We further demonstrated that genetic deletion of TRAM renders the constitutive expansion of resolving neutrophils, with an enhanced signaling circuitry of PPARγ/LMO4/STAT3. These findings may have profound implications for the effective training of resolving neutrophils with therapeutic potential in the treatment of both acute infection as well as chronic inflammatory diseases.

Lmo4 Deficiency Enhances Susceptibility to Cisplatin-Induced Cochlear Apoptosis and Hearing Loss

Cisplatin, a potent chemotherapeutic drug, induces ototoxicity, which limits its clinical utility. Cisplatin-induced oxidative stress plays a causal role in cochlear apoptosis while the consequent nitrative stress leads to the nitration of LIM domain only 4 (LMO4), a transcriptional regulator, and decreases its cochlear expression levels. Here, we show a direct link between cochlear LMO4 and cisplatin-induced hearing loss by employing a Lmo4 conditional knockout mouse model (Lmo4^lox/lox; Gfi1^Cre/+). Hair cell-specific deletion of Lmo4 did not alter cochlear morphology or affect hearing thresholds and otoacoustic emissions, in the absence of apoptotic stimuli. Cisplatin treatment significantly elevated the auditory brainstem response thresholds of conditional knockouts, across all frequencies. Moreover, deletion of Lmo4 compromised the activation of STAT3, a downstream target that regulates anti-apoptotic machinery. Immunostaining indicated that the expression of phosphorylated STAT3 was significantly decreased while the expression of activated caspase 3 was significantly increased in Lmo4 deficient hair cells, post-cisplatin treatment. These findings suggest an otoprotective role of LMO4 as cisplatin-induced decrease in cochlear LMO4 could compromise the LMO4/STAT3 cellular defense mechanism to induce ototoxicity.

Inhibition of protein nitration prevents cisplatin-induced inactivation of STAT3 and promotes anti-apoptotic signaling in organ of Corti cells

JAK/STAT pathway is one among the several oxidative stress-responsive signaling pathways that play a critical role in facilitating cisplatin-induced ototoxicity. Cisplatin treatment decreases the levels of cochlear LMO4, which acts as a scaffold for IL6-GP130 protein complex. Cisplatin-induced nitration and degradation of LMO4 could destabilize this protein complex, which in turn could compromise the downstream STAT3-mediated cellular defense mechanism. Here, we investigated the link between cisplatin-induced nitrative stress and STAT3-mediated apoptosis by using organ of Corti cell cultures. SRI110, a peroxynitrite decomposition catalyst that prevented cisplatin-induced decrease in LMO4 levels and ototoxicity, was used to inhibit nitrative stress. Immunoblotting and immunostaining indicated that cisplatin treatment decreased the expression levels, phosphorylation, and nuclear localization of STAT3 in UB/OC1 cells. Inhibition of nitration by SRI110 co-treatment prevented cisplatin-induced inactivation of STAT3 and promoted its nuclear localization. SRI110 co-treatment reversed the cisplatin-induced changes in the expression levels of Bcl2l1, Ccnd1, Jak2, Jak3, and Src and significantly attenuated the changes in the expression levels of Cdkn1a, Egfr, Fas, Il6st, Jak1, Stat3, and Tyk2. Collectively, these results suggest that the inhibition of cisplatin-induced nitration prevents the inactivation of STAT3, which in turn enables the transcription of anti-apoptotic genes and thereby helps to mitigate cisplatin-induced toxicity.

CRISPR/Cas9-mediated knockout of Lim-domain only four retards organ of Corti cell growth

Lim-domain only 4 (LMO4) plays a critical role in mediating the ototoxic side-effects of cisplatin, a highly effective anti-cancer drug. However, the signaling mechanism by which cochlear LMO4 mediates otopathology is yet to be fully understood. Knockout cell culture models are useful tools for investigating the functional roles of novel genes and delineating associated signaling pathways. Therefore, LMO4 knockout organ of Corti cells were generated by using the CRISPR (clustered regularly interspersed short palindromic repeats)/Cas9 (CRISPR-associated protein 9) system. Successful knockout of LMO4 in UB/OC1 cells was verified by the absence of LMO4 protein bands in immunoblots. Though the Knockout of LMO4 retarded the growth rate and the migratory potential of the cells it did not inhibit their long-term viability as the LMO4 knockout UB/OC1 cells were able to survive, proliferate, and form colonies. In addition, the knockout of LMO4 did not alter the expression of myosin VIIa, a biomarker of hair cells, suggesting that the knockout cells retain important characteristic features of cochlear sensory receptor cells. Thus, the findings of this study indicate that CRISPR/Cas9 system is a simple and versatile method for knocking out genes of interest in organ of Corti cells and that LMO4 knockout UB/OC1 cells are viable experimental models for studying the functional role of LMO4 in ototoxicity.

Targeting nitrative stress for attenuating cisplatin-induced downregulation of cochlear LIM domain only 4 and ototoxicity

Cisplatin-induced ototoxicity remains a primary dose-limiting adverse effect of this highly effective anticancer drug. The clinical utility of cisplatin could be enhanced if the signaling pathways that regulate the toxic side-effects are delineated. In previous studies, we reported cisplatin-induced nitration of cochlear proteins and provided the first evidence for nitration and downregulation of cochlear LIM domain only 4 (LMO4) in cisplatin ototoxicity. Here, we extend these findings to define the critical role of nitrative stress in cisplatin-induced downregulation of LMO4 and its consequent ototoxic effects in UBOC1 cell cultures derived from sensory epithelial cells of the inner ear and in CBA/J mice. Cisplatin treatment increased the levels of nitrotyrosine and active caspase 3 in UBOC1 cells, which was detected by immunocytochemical and flow cytometry analysis, respectively. The cisplatin-induced nitrative stress and apoptosis were attenuated by co-treatment with SRI110, a peroxynitrite decomposition catalyst (PNDC), which also attenuated the cisplatin-induced downregulation of LMO4 in a dose-dependent manner. Furthermore, transient overexpression of LMO4 in UBOC1 cells prevented cisplatin-induced cytotoxicity while repression of LMO4 exacerbated cisplatin-induced cell death, indicating a direct link between LMO4 protein levels and cisplatin ototoxicity. Finally, auditory brainstem responses (ABR) recorded from CBA/J mice indicated that co-treatment with SRI110 mitigated cisplatin-induced hearing loss. Together, these results suggest that cisplatin-induced nitrative stress leads to a decrease in the levels of LMO4, downregulation of LMO4 is a critical determinant in cisplatin-induced ototoxicity, and targeting peroxynitrite could be a promising strategy for mitigating cisplatin-induced hearing loss.

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Cisplatin-induced nitrative stress leads to a decrease in the levels of LMO4.

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Downregulation of LMO4 is a critical factor in cisplatin-induced ototoxicity.

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SRI110 appears to be a promising candidate for preventing cisplatin ototoxicity.

ZnT4 (SLC30A4)-null ("lethal milk") mice have defects in mammary gland secretion and hallmarks of precocious involution during lactation

During lactation, highly specialized secretory mammary epithelial cells (MECs) produce and secrete huge quantities of nutrients and nonnutritive factors into breast milk. The zinc (Zn) transporter ZnT4 (SLC30A4) transports Zn into the trans-Golgi apparatus for lactose synthesis, and across the apical cell membrane for efflux from MECs into milk. This is consistent with observations in "lethal milk" (lm/lm) mice, which have a truncation mutation in SLC30A4, and present with not only low milk Zn concentration, but also smaller mammary glands, decreased milk volume, and lactation failure by lactation day 2. However, the molecular underpinnings of these defects are not understood. Here, we used lactating C57BL/6J(lm/lm) (ZnT4-null) mice to explore the consequences of a ZnT4-null phenotype on mammary gland function during early lactation. Lactating C57BL/6J(lm/lm) mice had significantly fewer, smaller, and collapsed alveoli comprising swollen, lipid-filled MECs during early lactation. These defects were associated with decreased Akt expression and STAT5 activation, indicative of defects in MEC secretion. In addition, increased expression of ZnT2, TNF-α, and cleaved e-cadherin concomitant with increased activation of STAT3 implicated the loss of ZnT4 in precocious activation of involution. Collectively, our study indicates that the loss of ZnT4 has profound consequences on MEC secretion and may promote tissue remodeling in the mammary gland during early lactation.

Downstream targets of Lmo4 are modulated by cisplatin in the inner ear of Wistar rats

Lmo4, a transcriptional regulator, appears to be a key player in mediating the cochlear pathology in cisplatin ototoxicity, as it controls cellular responses by modulating the formation of transcriptional complexes. We provided the first evidence of in vivo nitration of Lmo4 in cisplatin ototoxicity. Our data suggested that nitration of Lmo4 and associated decrease in its cochlear expression has the potential to play a pivotal role in cisplatin ototoxicity. However, the Lmo4 interactomes that signal the downstream events in the cochlea are poorly understood. Therefore, custom-made gene arrays were employed to evaluate the modulation of known binding partners or targets of Lmo4, in Wistar rats treated with 16 mg/kg cisplatin. RT-PCR analysis, 3 days post cisplatin treatment, indicated that cisplatin induced up/down regulation of multiple cochlear genes associated with Lmo4 signaling. The cochlear expression of Esr1 was significantly up-regulated by cisplatin treatment, while the expression of Stat3 was down-regulated. Co-treatment with Trolox, an otoprotective antioxidant, attenuated the cisplatin-induced modulation of 5 genes in the cochlea. Consistent with the changes observed at the gene level, immunoblots with anti-Stat3 indicated that cisplatin-induced decrease in cochlear protein levels were attenuated by Trolox co-treatment. These results suggest that cisplatin-induced decreases in the cochlear Lmo4 upon nitration, and associated modulation in the cochlear expression of its binding partners Esr1 and Jak1, probably facilitates the repression of Stat3, a downstream target of Lmo4 implicated in drug mediated apoptosis. Collectively, these findings provide insights on Lmo4 downstream events and indicate a potential role of Jak/Stat transcriptional machinery in relaying the Lmo4 protein signaling in cisplatin-induced ototoxicity.

Axon growth inhibition by RhoA/ROCK in the central nervous system

Rho kinase (ROCK) is a serine/threonine kinase and a downstream target of the small GTPase Rho. The RhoA/ROCK pathway is associated with various neuronal functions such as migration, dendrite development, and axonal extension. Evidence from animal studies reveals that RhoA/ROCK signaling is involved in various central nervous system (CNS) diseases, including optic nerve and spinal cord injuries, stroke, and neurodegenerative diseases. Given that RhoA/ROCK plays a critical role in the pathophysiology of CNS diseases, the development of therapeutic agents targeting this pathway is expected to contribute to the treatment of CNS diseases. The RhoA/ROCK pathway mediates the effects of myelin-associated axon growth inhibitors—Nogo, myelin-associated glycoprotein (MAG), oligodendrocyte-myelin glycoprotein (OMgp), and repulsive guidance molecule (RGM). Blocking RhoA/ROCK signaling can reverse the inhibitory effects of these molecules on axon outgrowth, and promotes axonal sprouting and functional recovery in animal models of CNS injury. To date, several RhoA/ROCK inhibitors have been under development or in clinical trials as therapeutic agents for neurological disorders. In this review, we focus on the RhoA/ROCK signaling pathway in neurological disorders. We also discuss the potential therapeutic approaches of RhoA/ROCK inhibitors for various neurological disorders.

The LIM domain only 4 protein is a metabolic responsive inhibitor of protein tyrosine phosphatase 1B that controls hypothalamic leptin signaling

Protein tyrosine phosphatase 1B (PTP1B) counteracts leptin signaling and is a therapeutic target for obesity and diabetes. Here we found that LIM domain only 4 (LMO4) inhibits PTP1B activity by increasing the oxidized inactive form of PTP1B. Mice with neuronal ablation of LMO4 have elevated PTP1B activity and impaired hypothalamic leptin signaling, and a PTP1B inhibitor normalized PTP1B activity and restored leptin control of circulating insulin levels. LMO4 is palmitoylated at its C-terminal cysteine, and deletion of this residue prevented palmitoylation and retention of LMO4 at the endoplasmic reticulum and abolished its inhibitory effect on PTP1B. Importantly, LMO4 palmitoylation is sensitive to metabolic stress; mice challenged with a brief high-fat diet or acute intracerebroventricular infusion of saturated fatty acid had less palmitoylated LMO4, less oxidized PTP1B, and increased PTP1B activity in the hypothalamus. Thus, unleashed PTP1B activity attributable to loss of LMO4 palmitoylation may account for rapid loss of central leptin signaling after acute exposure to saturated fat.

Pharmacological activation of AMPK suppresses inflammatory response evoked by IL-6 signalling in mouse liver and in human hepatocytes

Interleukin-6 (IL-6) induces inflammatory signalling in liver, leading to impaired insulin action in hepatocytes. In this study, we demonstrate that pharmacological activation of AMP-activated protein kinase (AMPK) represses IL-6-stimulated expression of proinflammatory markers serum amyloid A (Saa) as well as suppressor of cytokine signalling 3 (Socs3) in mouse liver. Further studies using the human hepatocellular carcinoma cell line HepG2 suggest that AMPK inhibits IL-6 signalling by repressing IL-6-stimulated phosphorylation of several downstream components of the pathway such as Janus kinase 1 (JAK1), SH2-domain containing protein tyrosine phosphatase 2 (SHP2) and signal transducer and activator of transcription 3 (STAT3). In summary, inhibition of IL-6 signalling cascade in liver by the metabolic master switch of the body, AMPK, supports the role of this kinase as a crucial point of convergence of metabolic and inflammatory pathways in hepatocytes.

LMO4 modulates proliferation and differentiation of 3T3-L1 preadipocytes

Previous microarray analyses revealed that LMO4 is expressed in 3T3-L1 preadipocytes, however, its roles in adipogenesis are unknown. In the present study, using RT-PCR sequencing and quantitative real-time RT-PCR, we confirmed that LMO4 gene is expressed in 3T3-L1 preadipocytes and its expression peaks at the early stage of 3T3-L1 preadipocyte differentiation. Further analyses showed that LMO4 knockdown decreased the proliferation of 3T3-L1 preadipocytes, and attenuated the differentiation of 3T3-L1 preadipocytes, as evidenced by reduced lipid accumulation and down-regulation of PPARγ gene expression. Collectively, our findings indicate that LMO4 is a novel modulator of adipogenesis.

LMO4 inhibits p53-mediated proliferative inhibition of breast cancer cells through interacting p53

AIMS

The LIM domain only proteins (LMOs) which consist of four members (LMO1-LMO4) are a family of nuclear transcription coregulators that are characterized by the exclusive presence of two tandem LIM domains and no other functional domains. They regulate gene transcription by functioning as "linker" or "scaffolding" proteins by virtue of their LIM domains and are involved in the formation of multiprotein complexes with several DNA-binding factors and transcriptional regulatory proteins. In the present study, we tried to find the physical interaction between p53 and LMO4, and the effect of LMO4 on p53-mediated proliferative inhibition of breast cancer cells.

MAIN METHODS

FCM analysis was developed to detect the apoptosis of breast cancer cells after adriamycin (ADR) treatment. RT-PCR and Western blot analysis were performed to detect the expression of LMO4 and p53-related genes and proteins. Immunoprecipitation assay was used to detect the interaction between LMO4 and p53. Colony formation assay was developed to detect the proliferation of breast cancer cells.

KEY FINDINGS

We found that p53 was induced, but LMO4 was down-regulated in response to ADR. We also found that enforced expression of p53 inhibited the expression of LMO4, suggesting that LMO4 is a direct transcriptional target of p53. Furthermore, LMO4 can interact with p53 and inhibit p53-mediated inhibition of colony formation of breast cancer MDA-MB-453 cells.

SIGNIFICANCE

The present study showed that LMO4 is a direct target of p53 and inhibits p53-mediated proliferative inhibition of breast cancer cells through interacting p53.

Ca2+-dependent transcriptional control of Ca2+ homeostasis

Intracellular free Ca(2+) ions regulate many cellular functions, and in turn, the cell devotes many genes/proteins to keep tight control of the level of intracellular free Ca(2+). Here, we review recent work on Ca(2+)-dependent mechanisms and effectors that regulate the transcription of genes encoding proteins involved in the maintenance of the homeostasis of Ca(2+) in the cell.

LIM domain only 4 (LMO4) regulates calcium-induced calcium release and synaptic plasticity in the hippocampus

The LIM domain only 4 (LMO4) transcription cofactor activates gene expression in neurons and regulates key aspects of network formation, but the mechanisms are poorly understood. Here, we show that LMO4 positively regulates ryanodine receptor type 2 (RyR2) expression, thereby suggesting that LMO4 regulates calcium-induced calcium release (CICR) in central neurons. We found that CICR modulation of the afterhyperpolarization in CA3 neurons from mice carrying a forebrain-specific deletion of LMO4 (LMO4 KO) was severely compromised but could be restored by single-cell overexpression of LMO4. In line with these findings, two-photon calcium imaging experiments showed that the potentiation of RyR-mediated calcium release from internal stores by caffeine was absent in LMO4 KO neurons. The overall facilitatory effect of CICR on glutamate release induced during trains of action potentials was likewise defective in LMO4 KO, confirming that CICR machinery is severely compromised in these neurons. Moreover, the magnitude of CA3-CA1 long-term potentiation was reduced in LMO4 KO mice, a defect that appears to be secondary to an overall reduced glutamate release probability. These cellular phenotypes in LMO4 KO mice were accompanied with deficits in hippocampus-dependent spatial learning as determined by the Morris water maze test. Thus, our results establish LMO4 as a key regulator of CICR in central neurons, providing a mechanism for LMO4 to modulate a wide range of neuronal functions and behavior.

Cisplatin-induced ototoxicity is mediated by nitroxidative modification of cochlear proteins characterized by nitration of Lmo4

Tyrosine nitration is an important sequel of cellular signaling induced by reactive oxygen species. Cisplatin is an anti-neoplastic agent that damages the inner ear through reactive oxygen species and by the formation of DNA adducts. This study reveals a correlation between cisplatin-mediated hearing loss and nitroxidative modification of cochlear proteins and is the first to report nitration of Lmo4. Cisplatin induced a dose-dependent increase in hearing loss in Wistar rats. A 10-15-dB decrease in distortion product amplitude and massive loss of outer hair cells at the basal turn of the cochlea was observed 3 days post-treatment after a 16 mg/kg dose. Cisplatin induced nitration of cellular proteins within the organ of Corti, spiral ganglion, and stria vascularis, which are known targets of cisplatin ototoxicity. Nitration of a 76-kDa cochlear protein correlated with cisplatin dose. The nitrated protein was identified as Lmo4 (LIM domain only 4) by MALDI-TOF (matrix-assisted laser desorption/ionization time of flight) mass spectrometry and confirmed by reciprocal immunoprecipitation and immunoblotting. Co-localization of nitrotyrosine and Lmo4 was particularly high in outer hair cell nuclei after cisplatin treatment. Cochlear levels of Lmo4 were decreased in rats treated with cisplatin. In vitro studies supported the repression of Lmo4 in nitroxidative conditions and the induction of apoptosis upon repression of Lmo4. Inhibition of cochlear protein nitration prevented cisplatin-induced hearing loss. As Lmo4 is a transcriptional regulator that controls the choice between cell survival and cell death, these results support the hypothesis that nitration of Lmo4 influences cisplatin-induced ototoxicity.

The LIM adaptor protein LMO4 is an essential regulator of neural crest development

The neural crest (NC) is a population of multipotent stem cell-like progenitors that arise at the neural plate border in vertebrates and migrate extensively before giving rise to diverse derivatives. A number of components of the neural crest gene regulatory network (NC-GRN) are used reiteratively to control multiple steps in the development of these cells. It is therefore important to understand the mechanisms that control the distinct function of reiteratively used factors in different cellular contexts, and an important strategy for doing so is to identify and characterize the regulatory factors they interact with. Here we report that the LIM adaptor protein, LMO4, is a Slug/Snail interacting protein that is essential for NC development. LMO4 is expressed in NC forming regions of the embryo, as well as in the central nervous system and the cranial placodes. LMO4 is necessary for normal NC development as morpholino-mediated knockdown of this factor leads to loss of NC precursor formation at the neural plate border. Misexpression of LMO4 leads to ectopic expression of some neural crest markers, but a reduction in the expression of others. LMO4 binds directly to Slug and Snail, but not to other components of the NC-GRN and can modulate Slug-mediated neural crest induction, suggesting a mechanistic link between these factors. Together these findings implicate LMO4 as a critical component of the NC-GRN and shed new light on the control of Snail family repressors.

Ablation of LMO4 in glutamatergic neurons impairs leptin control of fat metabolism

The LIM domain only 4 (LMO4) protein is expressed in the hypothalamus, but its function there is not known. Using mice with LMO4 ablated in postnatal glutamatergic neurons, including most neurons of the paraventricular (PVN) and ventromedial (VMH) hypothalamic nuclei where LMO4 is expressed, we asked whether LMO4 is required for metabolic homeostasis. LMO4 mutant mice exhibited early onset adiposity. These mice had reduced energy expenditure and impaired thermogenesis together with reduced sympathetic outflow to adipose tissues. The peptide hormone leptin, produced from adipocytes, activates Jak/Stat3 signaling at the hypothalamus to control food intake, energy expenditure, and fat metabolism. Intracerebroventricular infusion of leptin suppressed feeding similarly in LMO4 mutant and control mice. However, leptin-induced fat loss was impaired and activation of Stat3 in the VMH was blunted in these mice. Thus, our study identifies LMO4 as a novel modulator of leptin function in selective hypothalamic nuclei to regulate fat metabolism.

STAT3 mediates resistance to MEK inhibitor through microRNA miR-17

AZD6244 is a small molecule inhibitor of the MEK (MAP/ERK kinase) pathway currently in clinical trials. However, the mechanisms mediating intrinsic resistance to MEK inhibition are not fully characterized. To define molecular mechanisms of MEK inhibitor resistance, we analyzed responses of 38 lung cancer cell lines following AZD6244 treatment and their genome-wide gene expression profiles and identified a panel of genes correlated with sensitivity or resistance to AZD6244 treatment. In particular, ingenuity pathway analysis revealed that activation of the STAT3 pathway was associated with MEK inhibitor resistance. Inhibition of this pathway by JSI-124, a STAT3-specific small molecule inhibitor, or with STAT3-specific siRNA sensitized lung cancer cells to AZD6244 and induced apoptosis. Moreover, combining a STAT3 inhibitor with AZD6244 induced expression of BIM and PARP cleavage, whereas activation of the STAT3 pathway inhibited BIM expression and elicited resistance to MEK inhibitors. We found that the STAT3-regulated microRNA miR-17 played a critical role in MEK inhibitor resistance, such that miR-17 inhibition sensitized resistant cells to AZD6244 by inducing BIM and PARP cleavage. Together, these results indicated that STAT3-mediated overexpression of miR-17 blocked BIM expression and caused resistance to AZD6244. Our findings suggest novel approaches to overcome resistance to MEK inhibitors by combining AZD6244 with STAT3 or miR-17 inhibitors.

Aberrant expression of LMO4 induces centrosome amplification and mitotic spindle abnormalities in breast cancer cells

The LIM-only protein, LMO4, is a transcriptional modulator overexpressed in breast cancer. It is oncogenic in murine mammary epithelium and is required for G2/M progression of ErbB2-dependent cells as well as growth and invasion of other breast cancer cell types. However, the mechanisms underlying the oncogenic activity of LMO4 remain unclear. Herein, we show that LMO4 is expressed in all breast cancer subtypes examined and its expression level correlates with the degree of proliferation of such tumours. In addition, we have determined that LMO4 silencing induces G2/M arrest in cells from various breast cancer subtypes, suggesting that LMO4 action in the cell cycle is not restricted to a single breast cancer subtype. This arrest was accompanied by increased cell death, amplification of centrosomes, and formation of abnormal mitotic spindles. Consistent with its ability to positively and negatively regulate the formation of active transcription complexes, overexpression of LMO4 also resulted in an increase in centrosome number. Centrosome amplification has been shown to prolong the G2/M phase of the cell cycle and induce apoptosis; thus, we conclude that supernumerary centrosomes mediate the G2/M arrest and cell death in LMO4-deficient cells. Furthermore, the correlation of centrosome amplification with genomic instability suggests that the impact of dysregulated LMO4 on the centrosome cycle may promote LMO4-induced tumour formation.

LIM domain only 4 protein promotes granulocyte colony-stimulating factor-induced signaling in neurons

Granulocyte colony-stimulating factor (GCSF) is currently in clinical trials to treat neurodegenerative diseases and stroke. Here, we tested whether LIM domain only 4 protein (LMO4), a hypoxia-inducible gene that protects neurons from ischemic injury, could modulate the neuroprotective effect of GCSF. We showed that GCSF treatment acetylates and phosphorylates Stat3, activates expression of a Stat3-dependent anti-apoptotic gene, p27, and increases neuron survival from ischemic injury. LMO4 participates in Stat3 signaling in hepatocytes and associates with histone deacetylase 2 (HDAC2) in cancer cells. In the absence of LMO4, GCSF fails to rescue neurons from ischemic insults. In wild-type neurons, inhibition of HDAC promoted Stat3 acetylation and the antiapoptotic effect of GCSF. In LMO4 null cortical neurons, expression of wild-type but not HDAC-interaction-deficient LMO4 restored GCSF-induced Stat3 acetylation and p27 expression. Thus, our results indicate that LMO4 enhances GCSF-induced Stat3 signaling in neurons, in part by sequestering HDAC.

LIM-only protein 4 interacts directly with the repulsive guidance molecule A receptor Neogenin

Repulsive guidance molecule A (RGM A) was recently described as a potent inhibitor of neuroregeneration in a rat spinal cord injury model. The receptor mediating RGM A's repulsive activity was shown to be Neogenin, a member of the Deleted in Colorectal Cancer (DCC) family of netrin receptors. Binding of RGM A to Neogenin induces activation of the small GTPase RhoA and of its effector Rho-kinase by an unknown mechanism. Here we show, that the cytoplasmic tail of Neogenin interacts directly with the transcriptional coactivator LIM domain only 4 (LMO4) in human SH-SY5Y cells, human Ntera neurons, and in embryonic rat cortical neurons. RGM A binding to Neogenin but not binding of Netrin-1, induces release of LMO4 from Neogenin. Down-regulation of LMO4 neutralizes the repulsive activity of RGM A in neuronal cell lines and embryonic rat cortical neurons and prevents RhoA activation. These results show for the first time that an interaction of Neogenin with LMO4 is involved in the RGM A - Neogenin signal transduction pathway for RhoA activation.

sIL-6R: more than an agonist?

On target cells, interleukin-6 (IL-6) interacts with its receptor complex consisting of the membrane-bound IL-6 receptor (IL-6R) and the signal transducing protein gp130. IL-6R can exist as a soluble protein (sIL-6R), which binds the ligand IL-6. This soluble complex can bind to gp130 on cells that lack the membrane-bound IL-6R and initiate signaling. This process is named transsignaling. The significance of transsignaling via sIL-6R is underlined by different publications and exceeds very probably the significance of the membrane-bound IL-6R. It is the general assumption that sIL-6R acts as an agonist in combination with IL-6 resulting in an enhancement of the IL-6 effects. In this article, we suppose 'non-agonistic' properties. There are several publications that give reasons to speculate that sIL-6R (a) has IL-6-antagonistic effects, (b) has orphan properties and (c) interacts with yet unknown binding partners different from IL-6. Knowledge about additional properties of sIL-6R will enlarge the biologic understanding of this molecule and might give an explanation for the sometimes contrasting effects of the cytokine IL-6.

LMO4 mRNA stability is regulated by extracellular ATP in F11 cells

LIM only domain protein 4 (LMO4) interacts with many signaling and transcription factors to regulate cellular proliferation, differentiation and plasticity. In Drosophila, mutations in the 3' untranslated region (UTR) of the homologue dLMO cause a gain of function by increasing mRNA stability. LMO4 3'UTR contains several AU-rich elements (ARE) and is highly conserved among vertebrates, suggesting that RNA destabilizing mechanisms are evolutionarily conserved. Here, we found that extracellular ATP stabilized LMO4 mRNA in F11 cells. The LMO4 3'UTR added to a luciferase reporter markedly reduced reporter activity under basal conditions, but increased activity with ATP treatment. Two ARE motifs were characterized in the LMO4 3'UTR. ATP increased binding of HuD protein to ARE1. ARE1 conferred ATP and HuD-dependent mRNA stabilization. In contrast, sequences flanking ARE2 bound CUGBP1 and ATP destabilized this complex. Thus, our results suggest that ATP modulates recruitment of RNA-binding proteins to the 3'UTR to stabilize LMO4 mRNA.

Negative regulation of estrogen receptor alpha transactivation functions by LIM domain only 4 protein

LIM domain only 4 (LMO4), a member of the LIM-only family of transcriptional coregulatory proteins, consists of two LIM protein-protein interaction domains that enable it to function as a linker protein in multiprotein complexes. Here, we have identified estrogen receptor alpha (ERalpha) and its corepressor, metastasis tumor antigen 1 (MTA1), as two novel binding partners of LMO4. Interestingly, LMO4 exhibited binding with both ERalpha and MTA1 and existed as a complex with ERalpha, MTA1, and histone deacetylases (HDAC), implying that LMO4 was a component of the MTA1 corepressor complex. Consistent with this notion, LMO4 overexpression repressed ERalpha transactivation functions in an HDAC-dependent manner. Accordingly, silencing of endogenous LMO4 expression resulted in a significant increased recruitment of ERalpha to target gene chromatin, stimulation of ERalpha transactivation activity, and enhanced expression of ERalpha-regulated genes. These findings suggested that LMO4 was an integral part of the molecular machinery involved in the negative regulation of ERalpha transactivation function in breast cells. Because LMO4 is up-regulated in human breast cancers, repression of ERalpha transactivation functions by LMO4 might contribute to the process of breast cancer progression by allowing the development of ERalpha-negative phenotypes, leading to increased aggressiveness of breast cancer cells.