Activation of protein kinase G Increases the expression of p21CIP1, p27KIP1, and histidine triad protein 1 through Sp1

Histidine Triad Nucleotide-Binding Protein 1 Improves Critical Limb Ischemia by Regulating Mitochondrial Homeostasis

Critical limb ischemia (CLI) is a common complication of diabetes mellitus that typically occurs in the later stages of the disease. Vascularization is indeed an important physiological process involving the formation of new blood vessels from existing ones. It occurs in response to various normal and pathophysiological conditions, and one of its critical roles is to compensate for inadequate oxygen supply, which is often seen in situations like chronic limb ischemia (CLI). Histidine triad nucleotide-binding protein 1 (Hint1) is a member of the Hint family that has been shown to attenuate cardiac hypertrophy, but its role in vascularization still needs to be clarified. In this study, we investigated the role of Hint1 in CLI. We found that Hint1 is significantly reduced in the muscle tissue of STZ-induced diabetic mice and high-glucose (HG)-treated endothelial cells (ECs). Hint1 deletion impaired blood flow recovery and vascularization, whereas Hint1 overexpression promoted these processes. In addition, our in vitro study showed that Hint1 deficiency aggravated mitochondrial dysfunction in ECs, as evidenced by impaired mitochondrial respiration, decreased mitochondrial membrane potential, and increased reactive oxygen species. Our findings suggest that Hint1 deficiency impairs blood perfusion by damaging mitochondrial function and that Hint1 may represent a potential therapeutic target for treating CLI.

Integrated analysis of ceRNA network reveals potential prognostic Hint1-related lncRNAs involved in hepatocellular carcinoma progression

Background

Hint1 is a novel tumor suppressor gene, and inactivation of its expression is closely associated with the carcinogenesis of a variety of malignancies. The effects of Hint1 deficiency on the competing endogenous RNA (ceRNA) regulatory network in the context of HCC remains to be fully characterized. This study aims to explore Hint1-related hub lncRNAs in HCC and to establish a reliable prognostic model for HCC patients based on these hub lncRNAs.

Methods

lncRNA + mRNA microarray was used to identify differentially expressed (DE) lncRNAs and mRNAs in Huh7 cells before and after Hint1 knockdown. A Hint1-related ceRNA network was mapped by bioinformation technology. The DEmRNAs in the network were analyzed via GO and KEGG enrichment analyses. Hub DElncRNAs associated with HCC patient prognosis were then detected through univariate and multivariate Cox regression analyses and were incorporated into a prognostic model. The prognostic value of this model was then assessed through the use of Kaplan-Meier curves, time-related ROC analyses, and nomograms. We also utilized Kaplan-Meier curves to validate the relationship between hub lncRNAs and the overall survival (OS) of HCC patients. Finally, A Hint1-related core ceRNA network based on the hub DElncRNAs and DEmRNAs was mapped.

Results

We identified 417 differentially expressed DElncRNAs and 2096 DEmRNAs in Huh7 cells before and after Hint1 knockdown. Three hub DElncRNAs (LINC00324, SNHG3, and DIO3OS) in the Hint1-associated ceRNA network were screened out using univariate and multivariate Cox regression analyses. A hepatocellular carcinoma (HCC) prognostic risk-scoring model and nomogram were constructed using these three hub lncRNAs, and it was confirmed that the risk score of the model could be used as an independent predictor of HCC prognosis. A Hint1-related core ceRNA network based on the hub DElncRNAs and DEmRNAs was also mapped.

Conclusion

We constructed a reliable prognostic model for HCC patients based on three Hint1-related hub lncRNAs, and we believe these three hub lncRNAs may play critical roles in hepatocarcinogenesis, and progression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12957-022-02535-z.

Impaired intestinal stem cell activity in ETEC infection: enterotoxins, cyclic nucleotides, and Wnt signaling

Enterotoxigenic Escherichia coli (ETEC) in humans and animals colonizes the intestine and thereafter secrets heat-stable enterotoxin (ST) with or without heat-labile enterotoxin (LT), which triggers massive fluid and electrolyte secretion into the gut lumen. The crosstalk between the cyclic nucleotide-dependent protein kinase/cystic fibrosis transmembrane conductance regulator (cAMP or cGMP/CFTR) pathway involved in ETEC-induced diarrhea channels, and the canonical Wnt/β-catenin signaling pathway leads to changes in intestinal stem cell (ISC) fates, which are strongly associated with developmental disorders caused by diarrhea. We review how alterations in enterotoxin-activated ion channel pathways and the canonical Wnt/β-catenin signaling pathway can explain inhibited intestinal epithelial activity, characterize alterations in the crosstalk of cyclic nucleotides, and predict harmful effects on ISCs in targeted therapy. Besides, we discuss current deficits in the understanding of enterotoxin-intestinal epithelial cell activity relationships that should be considered when interpreting sequelae of diarrhea.

HINT1 (Histidine Triad Nucleotide-Binding Protein 1) Attenuates Cardiac Hypertrophy Via Suppressing HOXA5 (Homeobox A5) Expression

BACKGROUND

Cardiac hypertrophy is an important prepathology of, and will ultimately lead to, heart failure. However, the mechanisms underlying pathological cardiac hypertrophy remain largely unknown. This study aims to elucidate the effects and mechanisms of HINT1 (histidine triad nucleotide-binding protein 1) in cardiac hypertrophy and heart failure.

METHODS

HINT1 was downregulated in human hypertrophic heart samples compared with nonhypertrophic samples by mass spectrometry analysis. Hint1 knockout mice were challenged with transverse aortic constriction surgery. Cardiac-specific overexpression of HINT1 mice by intravenous injection of adeno-associated virus 9 (AAV9)-encoding Hint1 under the cTnT (cardiac troponin T) promoter were subjected to transverse aortic construction. Unbiased transcriptional analyses were used to identify the downstream targets of HINT1. AAV9 bearing shRNA against Hoxa5 (homeobox A5) was administrated to investigate whether the effects of HINT1 on cardiac hypertrophy were HOXA5-dependent. RNA sequencing analysis was performed to recapitulate possible changes in transcriptome profile.Coimmunoprecipitation assays and cellular fractionation analyses were conducted to examine the mechanism by which HINT1 regulates the expression of HOXA5.

RESULTS

The reduction of HINT1 expression was observed in the hearts of hypertrophic patients and pressure overloaded-induced hypertrophic mice, respectively. In Hint1-deficient mice, cardiac hypertrophy deteriorated after transverse aortic construction. Conversely, cardiac-specific overexpression of HINT1 alleviated cardiac hypertrophy and dysfunction. Unbiased profiler polymerase chain reaction array showed HOXA5 is 1 target for HINT1, and the cardioprotective role of HINT1 was abolished by HOXA5 knockdown in vivo. Hoxa5 was identified to affect hypertrophy through the TGF-β (transforming growth factor β) signal pathway. Mechanically, HINT1 inhibited PKCβ1 (protein kinase C β type 1) membrane translocation and phosphorylation via direct interaction, attenuating the MEK/ERK/YY1 (mitogen-activated protein kinase/extracellular signal-regulated kinase kinase/yin yang 1) signal pathway, downregulating HOXA5 expression, and eventually attenuating cardiac hypertrophy.

CONCLUSIONS

HINT1 protects against cardiac hypertrophy through suppressing HOXA5 expression. These findings indicate that HINT1 may be a potential target for therapeutic interventions in cardiac hypertrophy and heart failure.

Propofol Exposure Disturbs the Differentiation of Rodent Neural Stem Cells via an miR-124-3p/Sp1/Cdkn1b Axis

Accumulating studies have indicated that propofol may lead to neurotoxicity and its effect on neural stem cells (NSCs) may play pivotal role in propofol-related neurotoxicity. Previously, we found that propofol could promote NSCs proliferation and could regulate several microRNA expressions. However, the underlying mechanism between microRNAs and NSCs development after propofol exposure is still unclear. Our data first observed that rat primary neural stem cells exposed to propofol exhibited a cell cycle arrest status and an inclination to differentiate into GFAP⁺ or S100β⁺ cells. This phenomenon was accompanying with a lower miR-124-3p expression and could be reversed via overexpression miR-124-3p in NSCs. Using bioinformatic predictions and luciferase assay we confirmed that Sp1 (Specificity Protein 1) is the target gene of miR-124-3p, indicating that miR-124-3p may regulate NSCs development through Sp1. Further, knockdown of Sp1 rescue the effect of propofol on NSCs differentiation. Finally, we demonstrated that Sp1 could bind cdkn1b promoter region through chromatin immunoprecipitation assay, indicating that Sp1 affect NSC's cell cycle through cdkn1b directly. Overall, our study highlights the miR-124-3p/Sp1/cdkn1b axis to be important in propofol interfering the differentiation of NSCs.

The Guanylate Cyclase C-cGMP Signaling Axis Opposes Intestinal Epithelial Injury and Neoplasia

Guanylate cyclase C (GUCY2C) is a transmembrane receptor expressed on the luminal aspect of the intestinal epithelium. Its ligands include bacterial heat-stable enterotoxins responsible for traveler's diarrhea, the endogenous peptide hormones uroguanylin and guanylin, and the synthetic agents, linaclotide, plecanatide, and dolcanatide. Ligand-activated GUCY2C catalyzes the synthesis of intracellular cyclic GMP (cGMP), initiating signaling cascades underlying homeostasis of the intestinal epithelium. Mouse models of GUCY2C ablation, and recently, human populations harboring GUCY2C mutations, have revealed the diverse contributions of this signaling axis to epithelial health, including regulating fluid secretion, microbiome composition, intestinal barrier integrity, epithelial renewal, cell cycle progression, responses to DNA damage, epithelial-mesenchymal cross-talk, cell migration, and cellular metabolic status. Because of these wide-ranging roles, dysregulation of the GUCY2C-cGMP signaling axis has been implicated in the pathogenesis of bowel transit disorders, inflammatory bowel disease, and colorectal cancer. This review explores the current understanding of cGMP signaling in the intestinal epithelium and mechanisms by which it opposes intestinal injury. Particular focus will be applied to its emerging role in tumor suppression. In colorectal tumors, endogenous GUCY2C ligand expression is lost by a yet undefined mechanism conserved in mice and humans. Further, reconstitution of GUCY2C signaling through genetic or oral ligand replacement opposes tumorigenesis in mice. Taken together, these findings suggest an intriguing hypothesis that colorectal cancer arises in a microenvironment of functional GUCY2C inactivation, which can be repaired by oral ligand replacement. Hence, the GUCY2C signaling axis represents a novel therapeutic target for preventing colorectal cancer.

β-catenin nuclear translocation in colorectal cancer cells is suppressed by PDE10A inhibition, cGMP elevation, and activation of PKG

Phosphodiesterase 10A (PDE10) is a cGMP and cAMP degrading PDE isozyme that is highly expressed in the brain striatum where it appears to play an important role in cognition and psychomotor activity. PDE10 inhibitors are being developed for the treatment of schizophrenia and Huntington's disease and are generally well tolerated, possibly because of low expression levels in most peripheral tissues. We recently reported high levels of PDE10 in colon tumors and that genetic silencing of PDE10 by siRNA or inhibition with small molecule inhibitors can suppress colon tumor cell growth with a high degree of selectivity over normal colonocytes (Li et al., Oncogene 2015). These observations suggest PDE10 may have an unrecognized role in tumorigenesis. Here we report that the concentration range by which the highly specific PDE10 inhibitor, Pf-2545920 (MP-10), inhibits colon tumor cell growth parallels the concentration range required to increase cGMP and cAMP levels, and activates PKG and PKA, respectively. Moreover, PDE10 knockdown by shRNA reduces the sensitivity of colon tumor cells to the growth inhibitory activity of Pf-2545920. Pf-2545920 also inhibits the translocation of β-catenin to the nucleus, thereby reducing β-catenin mediated transcription of survivin, resulting in caspase activation and apoptosis. PDE10 mRNA was also found to be elevated in colon tumors compared with normal tissues. These findings suggest that PDE10 can be targeted for cancer therapy or prevention whereby inhibition results in cGMP elevation and PKG activation to reduce β-catenin-mediated transcription of survival proteins leading to the selective apoptosis of cancer cells.

Conserved genes and pathways in primary human fibroblast strains undergoing replicative and radiation induced senescence

Background

Cellular senescence is induced either internally, for example by replication exhaustion and cell division, or externally, for example by irradiation. In both cases, cellular damages accumulate which, if not successfully repaired, can result in senescence induction. Recently, we determined the transcriptional changes combined with the transition into replicative senescence in primary human fibroblast strains. Here, by γ-irradiation we induced premature cellular senescence in the fibroblast cell strains (HFF and MRC-5) and determined the corresponding transcriptional changes by high-throughput RNA sequencing.

Results

Comparing the transcriptomes, we found a high degree of similarity in differential gene expression in replicative as well as in irradiation induced senescence for both cell strains suggesting, in each cell strain, a common cellular response to error accumulation. On the functional pathway level, “Cell cycle” was the only pathway commonly down-regulated in replicative and irradiation-induced senescence in both fibroblast strains, confirming the tight link between DNA repair and cell cycle regulation. However, “DNA repair” and “replication” pathways were down-regulated more strongly in fibroblasts undergoing replicative exhaustion. We also retrieved genes and pathways in each of the cell strains specific for irradiation induced senescence.

Conclusion

We found the pathways associated with “DNA repair” and “replication” less stringently regulated in irradiation induced compared to replicative senescence. The strong regulation of these pathways in replicative senescence highlights the importance of replication errors for its induction.

Electronic supplementary material

The online version of this article (doi:10.1186/s40659-016-0095-2) contains supplementary material, which is available to authorized users.

Oxygen regulates molecular mechanisms of cancer progression and metastasis

Oxygen is the basic molecule which supports life and it truly is "god's gift to life." Despite its immense importance, research on "oxygen biology" has never received the light of the day and has been limited to physiological and biochemical studies. It seems that in modern day biology, oxygen research is summarized in one word "hypoxia." Scientists have focused on hypoxia-induced transcriptomics and molecular-cellular alterations exclusively in disease models. Interestingly, the potential of oxygen to control the basic principles of biology like homeostatic maintenance, transcription, replication, and protein folding among many others, at the molecular level, has been completely ignored. Here, we present a perspective on the crucial role played by oxygen in regulation of basic biological phenomena. Our conclusion highlights the importance of establishing novel research areas like oxygen biology, as there is great potential in this field for basic science discoveries and clinical benefits to the society.

Hydrogen sulfide mediates the cardioprotective effects of gene therapy with PKG-Iα

Cyclic GMP-dependent protein kinase (PKG) is a serine-threonine kinase that mediates the cardioprotective effect of ischemic and pharmacologic preconditioning. Since hydrogen sulfide (H2S) has been implicated in mediating the cardioprotective effects of the cGMP modulators tadalafil and cinaciguat, we tested the hypothesis that myocardial gene therapy with PKG exerts cardioprotection against ischemia/reperfusion (I/R) injury through a mechanism involving H2S. Adult rat cardiomyocytes were infected with adenoviral vector encoding PKGIα or inactive mutant PKGIαK390A (K390A) for 24 h. Necrosis and apoptosis (n = 6/group) were determined after 90 min of simulated ischemia and 1 or 18 h of reoxygenation, respectively. To study the effect of PKGIα in vivo, mice received intramyocardial injections of adenoviral PKGIα or K390A. Four days later, the hearts were subjected to 30 min of ischemia followed by reperfusion for 24 h. The inhibitor of H2S-producing enzyme, cystathionine-γ-lyase (CSE), dl-propargylglycine (PAG, 50 mg/kg, ip) was given 30 min before ischemia. PKGIα overexpression induced CSE expression, whereas cystathionine-β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase expression was not changed. PKGIα overexpression increased H2S in the heart and cardiomyocytes in relation to control and PKGIαK390A. Moreover, PAG abolished protection with PKGIα in vitro by increasing necrosis (35.2 ± 1.7%, P < 0.05) and apoptosis (23.5 ± 1.8 %, P < 0.05) as compared to PKGIα-overexpressing cells (necrosis: 17.2 ± 0.9% and apoptosis: 13.2 ± 0.8%). In vivo, PKGIα overexpression reduced infarct size and preserved left ventricular fractional shortening as compared with K390A (P < 0.05) and PAG abolished the cardioprotective effect of PKGIα. The protective effect of myocardial gene therapy with PKGIα against I/R injury is mediated through a mechanism involving H2S signaling.

The role of cyclic nucleotide signaling pathways in cancer: targets for prevention and treatment

For more than four decades, the cyclic nucleotides cyclic AMP (cAMP) and cyclic GMP (cGMP) have been recognized as important signaling molecules within cells. Under normal physiological conditions, cyclic nucleotides regulate a myriad of biological processes such as cell growth and adhesion, energy homeostasis, neuronal signaling, and muscle relaxation. In addition, altered cyclic nucleotide signaling has been observed in a number of pathophysiological conditions, including cancer. While the distinct molecular alterations responsible for these effects vary depending on the specific cancer type, several studies have demonstrated that activation of cyclic nucleotide signaling through one of three mechanisms-induction of cyclic nucleotide synthesis, inhibition of cyclic nucleotide degradation, or activation of cyclic nucleotide receptors-is sufficient to inhibit proliferation and activate apoptosis in many types of cancer cells. These findings suggest that targeting cyclic nucleotide signaling can provide a strategy for the discovery of novel agents for the prevention and/or treatment of selected cancers.

Transcriptional activation of p21(WAF¹/CIP¹) is mediated by increased DNA binding activity and increased interaction between p53 and Sp1 via phosphorylation during replicative senescence of human embryonic fibroblasts

Although p21(WAF1/CIP1) is known to be elevated during replicative senescence of human embryonic fibroblasts (HEFs), the mechanism for p21 up-regulation has not been elucidated clearly. In order to explore the mechanism, we analyzed expression of p21 mRNA and protein and luciferase activity of full-length p21 promoter. The result demonstrated that p21 up-regulation was accomplished largely at transcription level. The promoter assay using serially-deleted p21 promoter constructs revealed that p53 binding site was the most important site and Sp1 binding sites were necessary but not sufficient for transcriptional activation of p21. In addition, p53 protein was shown to interact with Sp1 protein. The interaction was increased in aged fibroblasts and was regulated by phosphorylation of p53 and Sp1. DNA binding activity of p53 was significantly elevated in aged fibroblasts but that of Sp1 was not. DNA binding activities of p53 and Sp1 were also regulated by phosphorylation. Phosphorylation of p53 at serine-15 and of Sp1 at serines appears to be involved. Taken together, the result demonstrated that p21 transcription during replicative senescence of HEFs is up-regulated by increase in DNA binding activity and interaction between p53 and Sp1 via phosphorylation.

Intestinal cell proliferation and senescence are regulated by receptor guanylyl cyclase C and p21

Guanylyl cyclase C (GC-C) is expressed in intestinal epithelial cells and serves as the receptor for bacterial heat-stable enterotoxin (ST) peptides and the guanylin family of gastrointestinal hormones. Activation of GC-C elevates intracellular cGMP, which modulates intestinal fluid-ion homeostasis and differentiation of enterocytes along the crypt-villus axis. GC-C activity can regulate colonic cell proliferation by inducing cell cycle arrest, and mice lacking GC-C display increased cell proliferation in colonic crypts. Activation of GC-C by administration of ST to wild type, but not Gucy2c(-/-), mice resulted in a reduction in carcinogen-induced aberrant crypt foci formation. In p53-deficient human colorectal carcinoma cells, ST led to a transcriptional up-regulation of p21, the cell cycle inhibitor, via activation of the cGMP-responsive kinase PKGII and p38 MAPK. Prolonged treatment of human colonic carcinoma cells with ST led to nuclear accumulation of p21, resulting in cellular senescence and reduced tumorigenic potential. Our results, therefore, identify downstream effectors for GC-C that contribute to regulating intestinal cell proliferation. Thus, genomic responses to a bacterial toxin can influence intestinal neoplasia and senescence.

Sepiapterin ameliorates chemically induced murine colitis and azoxymethane-induced colon cancer

The effects of modulating tetrahydrobiopterin (BH4) levels with a metabolic precursor, sepiapterin (SP), on dextran sodium sulfate (DSS)-induced colitis and azoxymethane (AOM)-induced colorectal cancer were studied. SP in the drinking water blocks DSS-induced colitis measured as decreased disease activity index (DAI), morphologic criteria, and recovery of Ca(2+)-induced contractility responses lost as a consequence of DSS treatment. SP reduces inflammatory responses measured as the decreased number of infiltrating inflammatory macrophages and neutrophils and decreased expression of proinflammatory cytokines interleukin 1β (IL-1β), IL-6, and IL-17A. High-performance liquid chromatography analyses of colonic BH4 and its oxidized derivative 7,8-dihydrobiopterin (BH2) are inconclusive although there was a trend for lower BH4:BH2 with DSS treatment that was reversed with SP. Reduction of colonic cGMP levels by DSS was reversed with SP by a mechanism sensitive to 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a specific inhibitor of the NO-sensitive soluble guanylate cyclase (sGC). ODQ abrogates the protective effects of SP on colitis. This plus the finding that SP reduces DSS-enhanced protein Tyr nitration are consistent with DSS-induced uncoupling of NOS. The results agree with previous studies that demonstrated inactivation of sGC in DSS-treated animals as being important in recruitment of inflammatory cells and in altered cholinergic signaling and colon motility. SP also reduces the number of colon tumors in AOM/DSS-treated mice from 7 to 1 per unit colon length. Thus, pharmacologic modulation of BH4 with currently available drugs may provide a mechanism for alleviating some forms of colitis and potentially minimizing the potential for colorectal cancer in patients with colitis.

Kinetic mechanism of human histidine triad nucleotide binding protein 1

Human histidine triad nucleotide binding protein 1 (hHint1) is a member of a ubiquitous and ancient branch of the histidine triad protein superfamily. hHint1 is a homodimeric protein that catalyzes the hydrolysis of model substrates, phosphoramidate and acyl adenylate, with a high efficiency. Recently, catalytically inactive hHint1 has been identified as the cause of inherited peripheral neuropathy [Zimon, M., et al. (2012) Nat. Genet. 44, 1080-1083]. We have conducted the first detailed kinetic mechanistic studies of hHint1 and have found that the reaction mechanism is consistent with a double-displacement mechanism, in which the active site nucleophile His112 is first adenylylated by the substrate, followed by hydrolysis of the AMP-enzyme intermediate. A transient burst phase followed by a linear phase from the stopped-flow fluorescence assay indicated that enzyme adenylylation was faster than the subsequent intermediate hydrolysis and product release. Solvent viscosity experiments suggested that both chemical transformation and diffusion-sensitive events (product release or protein conformational change) limit the overall turnover. The catalytic trapping experiments and data simulation indicated that the true koff rate of the final product AMP is unlikely to control the overall kcat. Therefore, a protein conformational change associated with product release is likely rate-limiting. In addition, the rate of Hint1 adenylylation was found to be dependent on two residues with pKa values of 6.5 and 8, with the former pKa agreeing well with the nuclear magnetic resonance titration results for the pKa of the active site nucleophile His112. In comparison to the uncatalyzed rates, hHint1 was shown to enhance acyl-AMP and AMP phosphoramidate hydrolysis by 10(6)-10(8)-fold. Taken together, our analysis indicates that hHint1 catalyzes the hydrolysis of phosphoramidate and acyl adenylate with high efficiency, through a mechanism that relies on rapid adenylylation of the active residue, His112, while being partially rate-limited by intermediate hydrolysis and product release associated with a conformational change. Given the high degree of sequence homology of Hint proteins across all kingdoms of life, it is likely that their kinetic and catalytic mechanisms will be similar to those elucidated for hHint1.

TGF-β Signaling Regulates Pancreatic β-Cell Proliferation through Control of Cell Cycle Regulator p27 Expression

Proliferation of pancreatic β-cells is an important mechanism underlying β-cell mass adaptation to metabolic demands. Increasing β-cell mass by regeneration may ameliorate or correct both type 1 and type 2 diabetes, which both result from inadequate production of insulin by β-cells of the pancreatic islet. Transforming growth factor β (TGF-β) signaling is essential for fetal development and growth of pancreatic islets. In this study, we exposed HIT-T15, a clonal pancreatic β-cell line, to TGF-β signaling. We found that inhibition of TGF-β signaling promotes proliferation of the cells significantly, while TGF-β signaling stimulation inhibits proliferation of the cells remarkably. We confirmed that this proliferative regulation by TGF-β signaling is due to the changed expression of the cell cycle regulator p27. Furthermore, we demonstrated that there is no observed effect on transcriptional activity of p27 by TGF-β signaling. Our data show that TGF-β signaling mediates the cell-cycle progression of pancreatic β-cells by regulating the nuclear localization of CDK inhibitor, p27. Inhibition of TGF-β signaling reduces the nuclear accumulation of p27, and as a result this inhibition promotes proliferation of β-cells.

Human rpL3 induces G₁/S arrest or apoptosis by modulating p21 (waf1/cip1) levels in a p53-independent manner

It is now largely accepted that ribosomal proteins may be implicated in a variety of biological functions besides that of components of the translation machinery. Many evidences show that a subset of ribosomal proteins are involved in the regulation of the cell cycle and apoptosis through modulation of p53 activity. In addition, p53-independent mechanisms of cell cycle arrest in response to alterations of ribosomal proteins availability have been described. Here, we identify human rpL3 as a new regulator of cell cycle and apoptosis through positive regulation of p21 expression in a p53-independent system. We demonstrate that the rpL3-mediated p21 upregulation requires the specific interaction between rpL3 and Sp1. Furthermore, in our experimental system, p21 overexpression leads to a dual outcome, activating the G₁/S arrest of the cell cycle or the apoptotic pathway through mitochondria, depending on its intracellular levels. It is noteworthy that depletion of p21 abrogates both effects. Taken together, our findings unravel a novel extraribosomal function of rpL3 and reinforce the proapoptotic role of p21 in addition to its widely reported ability as an inhibitor of cell proliferation.

Stat6 cooperates with Sp1 in controlling breast cancer cell proliferation by modulating the expression of p21(Cip1/WAF1) and p27 (Kip1)

BACKGROUND

The signal transducer and activator of transcription 6 (Stat6), a member of the family of DNA-binding proteins, has been identified as a critical cell differentiation modulator in breast cancer cells. As of yet, the mechanisms underlying this function have remained largely unknown. To further elucidate the role of Stat6 in breast cancer development, we investigated the consequences of exogenous Stat6 expression.

METHODS

Proliferation assays and flow cytometry assays were conducted to evaluate the putative role of Stat6 on cell proliferation. To this end, we produced synchronized cells after a double thymidine block, as confirmed by FACS analysis. mRNA levels of Stat6 were measured by RNase protection analysis. To confirm the interaction among proteins, we employed GST pull-down assays and immunoprecipitation assays. Luciferase assays and ChIP assays were used to assess the transcriptional activity.

RESULTS

Compared to control breast cancer cells, we found that exogenous Stat6 expression plays a critical role in controlling cell proliferation. Also in different breast tumor cell lines, endogenous Stat6 expression was found to be positively related to a lower proliferation rate. Interestingly, in human breast cancer cells Stat6 functions in G1/S cell cycle progression, and the growth-inhibitory effect of Stat6 was shown to be mediated by induction of the G1 cyclin-dependent kinase inhibitors p21(Cip1/WAF1) (p21) and p27(Kip1) (p27). Simultaneously, G1-related cyclin/cyclin-dependent kinase activities and pRB phosphorylation were markedly reduced, and cell cycle progression was blocked in the G1 phase. Stat6 knockdown resulted in enhanced cell proliferation and a decrease in p21 and p27 mRNA levels in the steroid-responsive and non-responsive T-47D and MDA-MB-231 cell lines, respectively. In addition, the stimulatory effect of Stat6 on p21 and p27 gene transcription was found to be associated with interaction of Stat6 with the transcription factor Sp1 at the proximal Sp1-binding sites in their respective promoters.

CONCLUSIONS

Together, these results identify Stat6 as an important cell differentiation regulatory protein functioning, at least in part, by interacting with Sp1 to activate the p21 and p27 gene promoters in breast cancer cells.

Cinaciguat, a novel activator of soluble guanylate cyclase, protects against ischemia/reperfusion injury: role of hydrogen sulfide

Cinaciguat (BAY 58-2667) is a novel nitric oxide (NO)-independent activator of soluble guanylate cyclase (sGC), which induces cGMP-generation and vasodilation in diseased vessels. We tested the hypothesis that cinaciguat might trigger protection against ischemia/reperfusion (I/R) in the heart and adult cardiomyocytes through cGMP/protein kinase G (PKG)-dependent generation of hydrogen sulfide (H(2)S). Adult New Zealand White rabbits were pretreated with 1 or 10 μg/kg cinaciguat (iv) or 10% DMSO (vehicle) 15 min before I/R or with 10 μg/kg cinaciguat (iv) at reperfusion. Additionally, adult male ICR mice were treated with either cinaciguat (10 μg/kg ip) or vehicle 30 min before I/R or at the onset of reperfusion (10 μg/kg iv). The PKG inhibitor KT5283 (KT; 1 mg/kg ip) or dl-propargylglycine (PAG; 50 mg/kg ip) the inhibitor of the H(2)S-producing enzyme cystathionine-γ-lyase (CSE) were given 10 and 30 min before cinaciguat. Cardiac function and infarct size were assessed by echocardiography and tetrazolium staining, respectively. Primary adult mouse cardiomyocytes were isolated and treated with cinaciguat before simulated ischemia/reoxygenation. Cinaciguat caused 63 and 41% reduction of infarct size when given before I/R and at reperfusion in rabbits, respectively. In mice, cinaciguat pretreatment caused a more robust 80% reduction in infarct size vs. 63% reduction when given at reperfusion and preserved cardiac function following I/R, which were blocked by KT and PAG. Cinaciguat also caused an increase in myocardial PKG activity and CSE expression. In cardiomyocytes, cinaciguat (50 nM) reduced necrosis and apoptosis and increased H(2)S levels, which was abrogated by KT. Cinaciguat is a novel molecule to induce H(2)S generation and a powerful protection against I/R injury in heart.

cGMP-dependent protein kinases as potential targets for colon cancer prevention and treatment

In recent years, several antitumor signaling pathways mediated by the cGMP-dependent protein kinases have been identified in colon cancer cells. This review aims to present the mounting evidence in favor of cGMP/protein kinase G (PKG) signaling as a therapeutic strategy in colon cancer. The homeostatic and tumor suppressive effects of cGMP in the intestine are uncontested, but the signaling details are not understood. PKG is the central cGMP effector, and can block proliferation and tumor angiogenesis by inhibiting β-catenin/TCF and SOX9 signaling. Therapeutic activation of cGMP/PKG offers a promising avenue for the prevention and treatment of colon cancer, but additional preclinical studies are needed to fully understand the potential of this system.

Cyclic 3',5'-guanosine monophosphate-dependent protein kinase inhibits colon cancer cell adaptation to hypoxia

BACKGROUND

Type 1 cyclic 3',5'-guanosine monophosphate-dependent protein kinase (PKG) has recently been reported to inhibit tumor growth and angiogenesis. These effects suggest that PKG activation may have therapeutic value for colon cancer treatment, but the signaling downstream of this enzyme is poorly understood. The present study examined the mechanism underlying the inhibition of angiogenesis by PKG.

METHODS

The effect of ectopically expressed PKG on colon cancer cell adaptation to a 1% O(2) (hypoxic) environment was examined in vitro by measuring hypoxic markers, cell death/viability, and hypoxia inducible factor (HIF) activity.

RESULTS

Ectopic PKG inhibited angiogenesis in SW620 xenografts and significantly attenuated hypoxia-induced increases in vascular endothelial growth factor at both the mRNA and protein levels. PKG activation also blocked hypoxia-induced hexokinase 2 expression, which corresponded with reduced cellular adenosine triphosphate levels. Moreover, PKG expression significantly reduced cell viability and promoted necrotic cell death after 2 days in a hypoxic environment. To gain some mechanistic insight, the effect of PKG on HIF activation was determined using luciferase reporter assays. PKG activation inhibited HIF transcriptional activity in several colon cancer cell lines, including SW620, HCT116, and HT29. The mechanism by which PKG can inhibit HIF activity is not known, but it does not affect HIF-1α protein accumulation or nuclear translocation.

CONCLUSIONS

These findings demonstrate for the first time that PKG can block the adaptation of colon cancer cells to hypoxia and highlights this enzyme for further evaluation as a potential target for colon cancer treatment.

Transforming growth factor-β-inducible early response gene 1 is a novel substrate for atypical protein kinase Cs

The protein kinase C (PKC) family of serine/threonine kinases consists of ten different isoforms grouped into three subfamilies, denoted classical, novel and atypical PKCs (aPKCs). The aPKCs, PKCι/λ and PKCζ serve important roles during development and in processes subverted in cancer such as cell and tissue polarity, cell proliferation, differentiation and apoptosis. In an effort to identify novel interaction partners for aPKCs, we performed a yeast two-hybrid screen with the regulatory domain of PKCι/λ as bait and identified the Krüppel-like factors family protein TIEG1 as a putative interaction partner for PKCι/λ. We confirmed the interaction of both aPKCs with TIEG1 in vitro and in cells, and found that both aPKCs phosphorylate the DNA-binding domain of TIEG1 on two critical residues. Interestingly, the aPKC-mediated phosphorylation of TIEG1 affected its DNA-binding activity, subnuclear localization and transactivation potential.

Sumoylation controls retinal progenitor proliferation by repressing cell cycle exit in Xenopus laevis

Precisely controlled progenitor proliferation is essential for normal development. However, molecular mechanisms, which control the correct timing of cell cycle withdrawal during development, have been poorly understood. We show here that ubc9, a sumo-conjugating enzyme, controls the cell cycle exit of retinal progenitors. We found that ubc9 is highly expressed in retinal progenitors and stem cells in Xenopus embryos. Ubc9 physically and functionally associates with Xenopus hmgb3, which is required for retinal cell proliferation, and prolonged expression of ubc9 and hmgb3 results in suppression of the cell cycle exit of retinal progenitors in a sumoylation-dependent manner. Overexpression of ubc9 and hmgb3 decreased expression of the cell-cycle inhibitor p27(Xic1). Furthermore, progenitor proliferation is regulated, at least in part, by sumoylation of transcription factor Sp1. These results suggest a significant role of sumoylation for cell cycle regulation in retinal progenitors.

Examination of the expanding pathways for the regulation of p21 expression and activity

p21(Waf1/Cip1/Sdi1) was originally identified as an inhibitor of cyclin-dependent kinases, a mediator of p53 in growth suppression and a marker of cellular senescence. p21 is required for proper cell cycle progression and plays a role in cell death, DNA repair, senescence and aging, and induced pluripotent stem cell reprogramming. Although transcriptional regulation is considered to be the initial control point for p21 expression, there is growing evidence that post-transcriptional and post-translational regulations play a critical role in p21 expression and activity. This review will briefly discuss the activity of p21 and focus on current knowledge of the determinants that control p21 transcription, mRNA stability and translation, and protein stability and activity.

PKG inhibits TCF signaling in colon cancer cells by blocking beta-catenin expression and activating FOXO4

Activation of cGMP-dependent protein kinase (PKG) has anti-tumor effects in colon cancer cells but the mechanisms are not fully understood. This study has examined the regulation of beta-catenin/TCF signaling, as this pathway has been highlighted as central to the anti-tumor effects of PKG. We show that PKG activation in SW620 cells results in reduced beta-catenin expression and a dramatic inhibition of TCF-dependent transcription. PKG did not affect protein stability, nor did it increase phosphorylation of the amino-terminal Ser33/37/Thr41 residues that are known to target beta-catenin for degradation. However, we found that PKG potently inhibited transcription from a luciferase reporter driven by the human CTNNB1 promoter, and this corresponded to reduced beta-catenin mRNA levels. Although PKG was able to inhibit transcription from both the CTNNB1 and TCF reporters, the effect on protein levels was less consistent. Ectopic PKG had a marginal effect on beta-catenin protein levels in SW480 and HCT116 but was able to inhibit TCF-reporter activity by over 80%. Investigation of alternative mechanisms revealed that cJun-N-terminal kinase (JNK) activation was required for the PKG-dependent regulation of TCF activity. PKG activation caused beta-catenin to bind to FOXO4 in colon cancer cells, and this required JNK. Activation of PKG was also found to increase the nuclear content of FOXO4 and increase the expression of the FOXO target genes MnSOD and catalase. FOXO4 activation was required for the inhibition of TCF activity as FOXO4-specific short-interfering RNA completely blocked the inhibitory effect of PKG. These data illustrate a dual-inhibitory effect of PKG on TCF activity in colon cancer cells that involves reduced expression of beta-catenin at the transcriptional level, and also beta-catenin sequestration by FOXO4 activation.

Phosphodiesterase-5 inhibitor, tadalafil, protects against myocardial ischemia/reperfusion through protein-kinase g-dependent generation of hydrogen sulfide

BACKGROUND

Tadalafil is a novel long-acting inhibitor of phosphodiesterase-5. Because cGMP-dependent protein kinase (PKG) signaling plays a key role in cardioprotection, we hypothesized that PKG activation with tadalafil would limit myocardial ischemia/reperfusion (I/R) injury and dysfunction. Additionally, we contemplated that cardioprotection with tadalafil is mediated by hydrogen sulfide (H(2)S) signaling in a PKG-dependent fashion.

METHODS AND RESULTS

After baseline transthoracic echocardiography (TTE), adult ICR mice were injected i.p. with vehicle (10% DMSO) or tadalafil (1 mg/kg) with or without KT5823 (KT, PKG blocker, 1 mg/kg) or dl-propargylglycine (PAG, Cystathionine-gamma-lyase [CSE, H(2)S-producing enzyme] blocker; 50 mg/kg) 1 hour before coronary artery ligation for 30 minutes and reperfusion for 24 hours, whereas C57BL wild-type and CSE-knockout mice were treated with either vehicle or tadalafil. After reperfusion, TTE was performed and hearts were collected for infarct size (IS) measurement using TTC staining. Survival was increased with tadalafil (95%) compared with control (65%, P<0.05). Infarct size was reduced with tadalafil (13.2+/-1.7%) compared to vehicle (40.6+/-2.5%; P<0.05). KT and PAG abolished tadalafil-induced protection (IS: 39.2+/-1% and 51.2+/-2.4%, respectively) similar to genetic deletion of CSE (47.2+/-5.1%). Moreover, tadalafil preserved fractional shortening (FS: 31+/-1.5%) compared to control (FS: 22+/-4.8%, P<0.05). Baseline FS was 44+/-1.7%. KT and PAG abrogated the preservation of LV function with tadalafil by decline in FS to 17+/-1% and 23+/-3%, respectively. Compared to vehicle, myocardial H(2)S production was significantly increased with tadalafil and was abolished with KT.

CONCLUSIONS

PKG activation with tadalafil limits myocardial infarction and preserves LV function through H(2)S signaling.

Overexpression of Sp1 leads to p53-dependent apoptosis in cancer cells

Numerous studies have documented that Sp1 expression level were elevated in various human cancers. However, the promoters of many pro-apoptotic genes have been found to contain the Sp1 binding elements and are activated by Sp1 overexpression. To better understand the role and the mechanism of increased Sp1 levels on apoptosis, we used adenovirus to ectopically express GFP-Sp1 protein in various cancer cell lines. First, in HeLa and A549 cells, we found that Sp1 overexpression suppressed the cell growth and increased the detection of sub-G1 fraction, caspase-3 cleavage, and annexin-V signal revealed that apoptosis occurred. Furthermore, when cells entered the mitotic stage, the cell apoptosis was induced by Sp1 overexpression through affecting mitotic chromatin packaging. We also verified that p53 protein was accumulated and activated the p53-dependent apoptotic pathways in the wild-type p53 cells but not in the p53-mutated or p53-deleted cell lines when these cells were infected with adeno-GFP-Sp1 virus. In addition, A549 (p53(+/+)) cells could be protected from apoptosis under Sp1 overexpression when p53 was knockdown by p53 shRNA. Finally, H1299 (p53(-/-)) cell viability was significantly inhibited by adeno-GFP-Sp1 virus infection in the expression of p53. In conclusion, p53 was an essential factor for Sp1 overexpression-induced apoptotic cell death in transforming cells.

Expression of constitutively active cGMP-dependent protein kinase inhibits glucose-induced vascular smooth muscle cell proliferation

Previously, we have demonstrated that cGMP-dependent protein kinase (PKG) activity is downregulated in vessels from diabetic animals or in vascular smooth muscle cells (VSMCs) exposed to high-glucose conditions, contributing to diabetes-associated vessel dysfunction. However, whether decreased PKG activity plays a role in hyperglycemia-induced proliferation of VSMCs is unknown. In this report, high-glucose-mediated decreased PKG activity in VSMCs was restored by transfection of cells with expression vector for the catalytic domain of PKG-I (PKG-CD, constitutive active PKG). The effect of glucose on cell proliferation was determined. Our data demonstrated that high glucose exposure stimulated VSMC proliferation and G1 to S phase progression of the cell cycle, which was inhibited by restoration of PKG activity. Expression of constitutively active PKG inhibited G1 phase exit in VSMCs under high glucose conditions, which was accompanied by an inhibition of retinoblastoma protein (Rb) phosphorylation (a key switch for G1 to S phase cell cycle progression). Glucose-induced cyclin E expression and cyclin E-cyclin-dependent kinase 2 activity was also reduced by expression of PKG-CD in VSMCs. Moreover, expression of PKG-CD suppressed glucose-induced p27 degradation. These data demonstrate that restoring the high-glucose-mediated decrease in PKG activity in VSMCs inhibits glucose-induced abnormal VSMC proliferation occurring upstream of Rb phosphorylation. Our work provides the first direct evidence linking decreased PKG activity to high glucose-induced proliferation and cell cycle progression in VSMCs, suggesting that strategies to increase PKG activity might be useful in preventing abnormal VSMC proliferation in diabetic patients and might provide treatments for diabetes-associated proliferative vascular diseases.

ROCK mediates phorbol ester-induced apoptosis in prostate cancer cells via p21Cip1 up-regulation and JNK

It is established that androgen-dependent prostate cancer cells undergo apoptosis upon treatment with phorbol esters and related analogs, an effect primarily mediated by PKCdelta. Treatment of LNCaP prostate cancer cells with phorbol 12-myristate 13-acetate (PMA) causes a strong and sustained activation of RhoA and its downstream effector ROCK (Rho kinase) as well as the formation of stress fibers. These effects are impaired in cells subjected to PKCdelta RNA interference depletion. Functional studies revealed that expression of a dominant negative RhoA mutant or treatment with the ROCK inhibitor Y-27632 inhibits the apoptotic effect of PMA in LNCaP cells. Remarkably, the cytoskeleton inhibitors cytochalasin B and blebbistatin blocked not only PMA-induced apoptosis but also the activation of JNK, a mediator of the cell death effect by the phorbol ester. In addition, we found that up-regulation of the cell cycle inhibitor p21(Cip1) is required for PMA-induced apoptosis and that inhibitors of ROCK or the cytoskeleton organization prevent p21(Cip1) induction. Real time PCR analysis and reporter gene assay revealed that PMA induces p21(Cip1) transcriptionally in a ROCK- and cytoskeleton-dependent manner. p21(Cip1) promoter analysis revealed that PMA induction is dependent on Sp1 elements in the p21(Cip1) promoter but independent of p53. Taken together, our studies implicate ROCK-mediated up-regulation of p21(Cip1) and the cytoskeleton in PKCdelta-dependent apoptosis in prostate cancer cells.

Cyclic GMP-dependent protein kinase II inhibits cell proliferation, Sox9 expression and Akt phosphorylation in human glioma cell lines

Earlier we used a glioma model to identify loci in the mouse genome, which were repeatedly targeted by platelet-derived growth factor (PDGF)-containing Moloney murine leukemia viruses. The gene Prkg2, encoding cyclic guanosine monophosphate (cGMP)-dependent protein kinase II, cGKII, was tagged by retroviral insertions in two brain tumors. The insertions were both situated upstream of the kinase domain and suggested creating a truncated form of the cGKII protein. We transfected different human glioma cell lines with Prkg2 and found an overall reduction in colony formation and cell proliferation compared with controls transfected with truncated Prkg2 (lacking the kinase domain) or empty vector. All glioma cells transfected with the cGKII phosphorylate vasodilator-stimulated phosphoprotein, VASP, after cGMP analog treatment. Glioma cell lines positive for the Sox9 transcription factor showed reduced Sox9 expression when Prkg2 was stably transfected. When cGKII was activated by cGMP analog treatment, Sox9 was phosphorylated, Sox9 protein expression was suppressed and the glioma cell lines displayed loss of cell adhesion, inhibition of Akt phosphorylation and G1 arrest. Sox9 repression by siRNA was similarly shown to reduce glioma cell proliferation. Expression analysis of stem and glial lineage cell markers also suggests that cGKII induces differentiation of glioma cell lines. These findings describe an anti-proliferative role of cGKII in human glioma biology and would further explain the retroviral tagging of the cGKII gene during brain tumor formation in PDGF-induced tumors.