Receptor-type protein tyrosine phosphatase zeta as a component of the signaling receptor complex for midkine-dependent survival of embryonic neurons

Identification of MDK as a Hypoxia- and Epithelial-Mesenchymal Transition-Related Gene Biomarker of Glioblastoma Based on a Novel Risk Model and In Vitro Experiments

BACKGROUND

Tumor cells are commonly exposed to a hypoxic environment, which can easily induce the epithelial-mesenchymal transition (EMT) of tumor cells, further affecting tumor proliferation, invasion, metastasis, and drug resistance. However, the predictive role of hypoxia and EMT-related genes in glioblastoma (GBM) has not been investigated.

METHODS

Intersection genes were identified by weighted correlation network analysis (WGCNA) and differential expression analyses, and a risk model was further constructed by LASSO and Cox analyses. Clinical, immune infiltration, tumor mutation, drug treatment, and enrichment profiles were analyzed based on the risk model. The expression level of the MDK gene was tested using RT-PCR, immunohistochemistry, and immunofluorescence. CCK8 and EdU were employed to determine the GBM cells' capacity for proliferation while the migration and invasion ability were detected by a wound healing assay and transwell assay, respectively.

RESULTS

Based on the GBM data of the TCGA and GTEx databases, 58 intersection genes were identified, and a risk model was constructed. The model was verified in the CGGA cohort, and its accuracy was confirmed by the ROC curve (AUC = 0.807). After combining clinical subgroups, univariate and multivariate Cox regression analyses showed that risk score and age were independent risk factors for GBM patients. Furthermore, our subsequent analysis of immune infiltration, tumor mutation, and drug treatment showed that risk score and high- and low-risk groups were associated with multiple immune cells, mutated genes, and drugs. Enrichment analysis indicated that the differences between high- and low-risk groups were manifested in tumor-related pathways, including the PI3K-AKT and JAK-STAT pathways. Finally, in vivo experiments proved that the hypoxia environment promoted the expression of MDK, and MDK knockdown reduced the proliferation, migration, and EMT of GBM cells induced by hypoxia.

CONCLUSIONS

Our novel prognostic correlation model provided more potential treatment strategies for GBM patients.

The role of midkine in health and disease

Midkine (MDK) is a neurotrophic growth factor highly expressed during embryogenesis with important functions related to growth, proliferation, survival, migration, angiogenesis, reproduction, and repair. Recent research has indicated that MDK functions as a key player in autoimmune disorders of the central nervous system (CNS), such as Multiple Sclerosis (MS) and is a promising therapeutic target for the treatment of brain tumors, acute injuries, and other CNS disorders. This review summarizes the modes of action and immunological functions of MDK both in the peripheral immune compartment and in the CNS, particularly in the context of traumatic brain injury, brain tumors, neuroinflammation, and neurodegeneration. Moreover, we discuss the role of MDK as a central mediator of neuro-immune crosstalk, focusing on the interactions between CNS-infiltrating and -resident cells such as astrocytes, microglia, and oligodendrocytes. Finally, we highlight the therapeutic potential of MDK and discuss potential therapeutic approaches for the treatment of neurological disorders.

Nasopharynx Battlefield: Cellular Immune Responses Mediated by Midkine in Nasopharyngeal Carcinoma and COVID-19

Clinical evidence suggests that the severe respiratory illness coronavirus disease 2019 (COVID-19) is often associated with a cytokine storm that results in dysregulated immune responses. Prolonged COVID-19 positivity is thought to disproportionately affect cancer patients. With COVID-19 disrupting the delivery of cancer care, it is crucial to gain momentum and awareness of the mechanistic intersection between these two diseases. This review discusses the role of the cytokine midkine (MK) as an immunomodulator in patients with COVID-19 and nasopharyngeal carcinoma (NPC), both of which affect the nasal cavity. We conducted a review and analysis of immunocellular similarities and differences based on clinical studies, research articles, and published transcriptomic datasets. We specifically focused on ligand-receptor pairs that could be used to infer intercellular communication, as well as the current medications used for each disease, including NPC patients who have contracted COVID-19. Based on our findings, we recommend close monitoring of the MK axis to maintain the desirable effects of therapeutic regimens in fighting both NPC and COVID-19 infections.

Multi-omics insights into the biological mechanisms underlying statistical gene-by-lifestyle interactions with smoking and alcohol consumption

Though both genetic and lifestyle factors are known to influence cardiometabolic outcomes, less attention has been given to whether lifestyle exposures can alter the association between a genetic variant and these outcomes. The Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Consortium's Gene-Lifestyle Interactions Working Group has recently published investigations of genome-wide gene-environment interactions in large multi-ancestry meta-analyses with a focus on cigarette smoking and alcohol consumption as lifestyle factors and blood pressure and serum lipids as outcomes. Further description of the biological mechanisms underlying these statistical interactions would represent a significant advance in our understanding of gene-environment interactions, yet accessing and harmonizing individual-level genetic and 'omics data is challenging. Here, we demonstrate the coordinated use of summary-level data for gene-lifestyle interaction associations on up to 600,000 individuals, differential methylation data, and gene expression data for the characterization and prioritization of loci for future follow-up analyses. Using this approach, we identify 48 genes for which there are multiple sources of functional support for the identified gene-lifestyle interaction. We also identified five genes for which differential expression was observed by the same lifestyle factor for which a gene-lifestyle interaction was found. For instance, in gene-lifestyle interaction analysis, the T allele of rs6490056 (ALDH2) was associated with higher systolic blood pressure, and a larger effect was observed in smokers compared to non-smokers. In gene expression studies, this allele is associated with decreased expression of ALDH2, which is part of a major oxidative pathway. Other results show increased expression of ALDH2 among smokers. Oxidative stress is known to contribute to worsening blood pressure. Together these data support the hypothesis that rs6490056 reduces expression of ALDH2, which raises oxidative stress, leading to an increase in blood pressure, with a stronger effect among smokers, in whom the burden of oxidative stress is greater. Other genes for which the aggregation of data types suggest a potential mechanism include: GCNT4×current smoking (HDL), PTPRZ1×ever-smoking (HDL), SYN2×current smoking (pulse pressure), and TMEM116×ever-smoking (mean arterial pressure). This work demonstrates the utility of careful curation of summary-level data from a variety of sources to prioritize gene-lifestyle interaction loci for follow-up analyses.

HCC biomarkers - state of the old and outlook to future promising biomarkers and their potential in everyday clinical practice

Hepatocellular carcinoma (HCC) is one of the most common and deadly tumors worldwide. Management of HCC depends on reliable biomarkers for screening, diagnosis, and monitoring of the disease, as well as predicting response towards therapy and safety. To date, imaging has been the established standard technique in the diagnosis and follow-up of HCC. However, imaging techniques have their limitations, especially in the early detection of HCC. Therefore, there is an urgent need for reliable, non/minimal invasive biomarkers. To date, alpha-fetoprotein (AFP) is the only serum biomarker used in clinical practice for the management of HCC. However, AFP is of relatively rather low quality in terms of specificity and sensitivity. Liquid biopsies as a source for biomarkers have become the focus of clinical research. Our review highlights alternative biomarkers derived from liquid biopsies, including circulating tumor cells, proteins, circulating nucleic acids, and exosomes, and their potential for clinical application. Using defined combinations of different biomarkers will open new perspectives for diagnosing, treating, and monitoring HCC.

Midkine—A novel player in cardiovascular diseases

Midkine (MK) is a 13-kDa heparin-binding cytokine and growth factor with anti-apoptotic, pro-angiogenic, pro-inflammatory and anti-infective functions, that enable it to partake in a series of physiological and pathophysiological processes. In the past, research revolving around MK has concentrated on its roles in reproduction and development, tissue protection and repair as well as inflammatory and malignant processes. In the recent few years, MK's implication in a wide scope of cardiovascular diseases has been rigorously investigated. Nonetheless, there is still no broadly accepted consensus on whether MK exerts generally detrimental or favorable effects in cardiovascular diseases. The truth probably resides somewhere in-between and depends on the underlying physiological or pathophysiological condition. It is therefore crucial to thoroughly examine and appraise MK's participation in cardiovascular diseases. In this review, we introduce the MK gene and protein, its multiple receptors and signaling pathways along with its expression in the vascular system and its most substantial functions in cardiovascular biology. Further, we recapitulate the current evidence of MK's expression in cardiovascular diseases, addressing the various sources and modes of MK expression. Moreover, we summarize the most significant implications of MK in cardiovascular diseases with particular emphasis on MK's advantageous and injurious functions, highlighting its ample diagnostic and therapeutic potential. Also, we focus on conflicting roles of MK in a number of cardiovascular diseases and try to provide some clarity and guidance to MK's multifaceted roles. In summary, we aim to pave the way for MK-based diagnostics and therapies that could present promising tools in the diagnosis and treatment of cardiovascular diseases.

Midkine noncanonically suppresses AMPK activation through disrupting the LKB1-STRAD-Mo25 complex

Midkine (MDK), a secreted growth factor, regulates signal transduction and cancer progression by interacting with receptors, and it can be internalized into the cytoplasm by endocytosis. However, its intracellular function and signaling regulation remain unclear. Here, we show that intracellular MDK interacts with LKB1 and STRAD to disrupt the LKB1-STRAD-Mo25 complex. Consequently, MDK decreases the activity of LKB1 to dampen both the basal and stress-induced activation of AMPK by glucose starvation or treatment of 2-DG. We also found that MDK accelerates cancer cell proliferation by inhibiting the activation of the LKB1-AMPK axis. In human cancers, compared to other well-known growth factors, MDK expression is most significantly upregulated in cancers, especially in liver, kidney and breast cancers, correlating with clinical outcomes and inversely correlating with phosphorylated AMPK levels. Our study elucidates an inhibitory mechanism for AMPK activation, which is mediated by the intracellular MDK through disrupting the LKB1-STRAD-Mo25 complex.

Screening of novel Midkine binding protein by BioID2-based proximity labeling

Midkine (MK), a heparin-binding growth factor, is associated with the poor prognosis of the pediatric tumor, neuroblastoma. MK would be a druggable target as many studies showed inhibition of its function in various cancers suppressed tumor developments. To establish the therapy targeting MK, identification of its binding partners, and elucidation of its intracellular signaling are needed. It was reported that exogenous MK induced phosphorylation of ribosomal protein S6 (RPS6) downstream of mTOR signaling. Using RPS6 phosphorylation as a marker of MK response, we searched for MK reactive cell lines. We found that MK cell lines expressing less MK tended to respond better to MK. Next, using an MK reactive neuroblastoma cell line, MK-knocked down SH-SY5Y cells, we employed a proximity-dependent biotin identification method, which was invented to evaluate protein-protein interactions by biotinylation. We confirmed that secreted MK fused to the biotin ligase BioID2 (MK-BioID2) was able to biotinylate proteins from the cells. Biotinylated proteins were identified by liquid chromatography-mass spectrometry analyses. Twenty five proteins were found to be overlapped after three independent experiments, among which insulin-like growth binding protein 2 (IGFBP2) was further analyzed. IGFBP2 was indeed detected with immunoblotting after streptavidin pull down of MK-BioID2 labeled cell extract of MK-knocked down SH-SY5Y cells. Our study suggests that the BioID2 method is useful to identify binding partners of growth factors.

Midkine promotes glioblastoma progression via PI3K-Akt signaling

Background

Our previous bioinformatics-based study found that midkine (MDK) was associated with poor prognosis of glioblastoma (GBM). However, the mechanism of MDK in GBM remains elusive.

Methods

A public GBM-related dataset and GBM tissues from our center were used validate the aberrant expression of MDK in GBM at the RNA and protein levels. The relationship between MDK expression and survival of GBM patients was also explored through survival analysis. Subsequently, we identified MDK-related GBM-specific genes using differential expression analysis. Functional enrichment analyses were performed to reveal their potential biological functions. CCK-8, 5-ethynyl-2′-deoxyuridine, and Matrigel-transwell assays were performed in GBM cell lines in which MDK was knocked out or overexpressed in order assess the effects of MDK on proliferation, migration, and invasion of GBM cells. Western blotting was performed to detect candidate proteins.

Results

Our study showed MDK is a promising diagnostic and prognostic biomarker for GBM because it is highly expressed in the disease and it is associated with poor prognosis. MDK is involved in various cancer-related pathways, such as PI3K-Akt signaling, the cell cycle, and VEGF signaling. A comprehensive transcriptional regulatory network was constructed to show the potential pathways through which MDK may be involved in GBM. In vitro, Overexpression of MDK augmented proliferation, migration, and invasion of GBM cell lines, whereas suppression of MDK led to the opposite effects. Furthermore, our study confirmed that MDK promotes the progression of GBM by activating the PI3K-Akt signaling pathway.

Conclusions

Our present study proposes that MDK promotes GBM by activating the PI3K-Akt signaling pathway, and it describes a potential regulatory network involved.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-02212-3.

Characterization of Extracellular Vesicles Secreted in Lentiviral Producing HEK293SF Cell Cultures

Lentiviral vectors (LVs) are a powerful tool for gene and cell therapy and human embryonic kidney cells (HEK293) have been extensively used as a platform for production of these vectors. Like most cells and cellular tissues, HEK293 cells release extracellular vesicles (EVs). EVs released by cells share similar size, biophysical characteristics and even a biogenesis pathway with cell-produced enveloped viruses, making it a challenge to efficiently separate EVs from LVs. Thus, EVs co-purified with LVs during downstream processing, are considered “impurities” in the context of gene and cell therapy. A greater understanding of EVs co-purifying with LVs is needed to enable improved downstream processing. To that end, EVs from an inducible lentivirus producing cell line were studied under two conditions: non-induced and induced. EVs were identified in both conditions, with their presence confirmed by transmission electron microscopy and Western blot. EV cargos from each condition were then further characterized by a multi-omic approach. Nineteen proteins were identified by mass spectrometry as potential EV markers to differentiate EVs in LV preparations. Lipid composition of EV preparations before and after LV induction showed similar enrichment in phosphatidylserine. RNA cargos in EVs showed enrichment in transcripts involved in viral processes and binding functions. These findings provide insights on the product profile of lentiviral preparations and could support the development of improved separation strategies aimed at removing co-produced EVs.

Midkine is neuroprotective and influences glial reactivity and the formation of Müller glia-derived progenitor cells in chick and mouse retinas

Recent studies suggest midkine (MDK) is involved in the development and regeneration of the zebrafish retina. We investigate the expression patterns of MDK and related factors, roles in neuronal survival, and influence upon the formation of Müller glia-derived progenitor cells (MGPCs) in chick and mouse model systems. By using single-cell RNA-sequencing, we find that MDK and pleiotrophin (PTN), a MDK-related cytokine, are upregulated by Müller glia (MG) during later stages of development in chick. While PTN is downregulated, MDK is dramatically upregulated in mature MG after retinal damage or FGF2 and insulin treatment. By comparison, MDK and PTN are downregulated by MG in damaged mouse retinas. In both chick and mouse retinas, exogenous MDK induces expression of cFos and pS6 in MG. In the chick, MDK significantly decreases numbers dying neurons, reactive microglia, and proliferating MGPCs, whereas PTN has no effect. Inhibition of MDK-signaling with Na₃ VO₄ blocks neuroprotective effects with an increase in the number of dying cells and negates the pro-proliferative effects on MGPCs in damaged retinas. Inhibitors of PP2A and Pak1, which are associated with MDK-signaling through integrin β1, suppressed the formation of MGPCs in damaged chick retinas. In mice, MDK promotes a small but significant increase in proliferating MGPCs in damaged retinas and potently decreases the number of dying cells. We conclude that MDK expression is dynamically regulated in Müller glia during embryonic maturation, following retinal injury, and during reprogramming into MGPCs. MDK mediates glial activity, neuronal survival, and the re-programming of Müller glia into proliferating MGPCs.

Midkine: The Who, What, Where, and When of a Promising Neurotrophic Therapy for Perinatal Brain Injury

Midkine (MK) is a small secreted heparin-binding protein highly expressed during embryonic/fetal development which, through interactions with multiple cell surface receptors promotes growth through effects on cell proliferation, migration, and differentiation. MK is upregulated in the adult central nervous system (CNS) after multiple types of experimental injury and has neuroprotective and neuroregenerative properties. The potential for MK as a therapy for developmental brain injury is largely unknown. This review discusses what is known of MK's expression and actions in the developing brain, areas for future research, and the potential for using MK as a therapeutic agent to ameliorate the effects of brain damage caused by insults such as birth-related hypoxia and inflammation.

Relationship between cachexia and perineural invasion in pancreatic adenocarcinoma

BACKGROUND

Cachexia is responsible for the low quality of life in pancreatic adenocarcinoma (PDAC). The rapid disease progression and patient deterioration seems related to perineural invasion, but the relationship between cachexia and perineural invasion for the evolution of the disease has been rarely studied. As perineural invasion is difficult to be highlighted, a biomarker such as the neurotrophic factor Midkine (MK) which promotes the neuronal differentiation and the cell migration could be helpful. Also, Activin (ACV) has been described as cachexia related to PDAC. However, their role for assessing and predicting the disease course in daily practice is not known.

AIM

To assess the relationship between perineural invasion and cachexia and their biomarkers, MK and ACV, respectively, and their prognostic value.

METHODS

This study included prospectively enrolled patients with proven adenocarcinoma and a matched group of controls without any malignancies. Patients with other causes of malnutrition were excluded. The plasma levels of ACV and MK were analyzed using western blotting and were correlated with the clinicopathological features and survival data. These results were validated by immunohistochemical analyses of the pancreatic tumor tissue of the patients included in the study and a supplementary group of surgically resected specimens from patients with a benign disease.

RESULTS

The study comprised 114 patients with PDAC, 125 controls and a supplementary group of 14 benign pancreatic tissue samples. ACV and MK were both overexpressed more frequently in the plasma of patients with PDAC than in the controls (63% vs 32% for ACV, P < 0.001; 47% vs 16% for MK, P < 0.001), with similar levels in pancreatic tissue the MK protein expression was closely related to the advanced clinical stage (P = 0.006), the presence of metastasis (P = 0.04), perineural invasion (P = 0.03) and diabetes (P = 0.002), but with no influence on survival. No correlation between clinicopathological factors and ACV expression was noted. Cachexia, present in 19% of patients, was unrelated to ACV or MK level. Higher ACV expression was associated with a shorter survival (P = 0.008).

CONCLUSION

The MK was a biomarker of perineural invasion, associated with tumor stage and diabetes, but without prognostic value as ACV. Cachexia was unrelated to perineural invasion, ACV level or survival.

Expression of the heparin-binding growth factors Midkine and pleiotrophin during ocular development

Midkine (MDK) and Pleiotrophin (PTN) belong to a group of heparin-binding growth factors that has been shown to have pleiotropic functions in various biological processes during development and disease. Development of the vertebrate eye is a multistep process that involves coordinated interactions between neuronal and non-neuronal cells, but very little is known about the potential function of MDK and PTN in these processes. In this study, we demonstrate by section in situ hybridization, the spatiotemporal expression of MDK and PTN during ocular development in chick and mouse. We show that MDK and PTN are expressed in dynamic patterns that overlap in a few non-neuronal tissues in the anterior eye and in neuronal cell layers of the posterior eye. We show that the expression patterns of MDK and PTN are only conserved in a few tissues in chick and mouse but they overlap with the expression of some of their receptors LRP1, RPTPZ, ALK, NOTCH2, ITGβ1, SDC1, and SDC3. The dynamic expression patterns of MDK, PTN and their receptors suggest that they function together during the multistep process of ocular development and they may play important roles in cell proliferation, adhesion, and migration of neuronal and non-neuronal cells.

Midkine Is a Novel Regulator of Amphetamine-Induced Striatal Gliosis and Cognitive Impairment: Evidence for a Stimulus-Dependent Regulation of Neuroinflammation by Midkine

Midkine (MK) is a cytokine that modulates amphetamine-induced striatal astrogliosis, suggesting a possible role of MK in neuroinflammation induced by amphetamine. To test this hypothesis, we studied astrogliosis and microglial response induced by amphetamine (10 mg/kg i.p. four times, every 2 h) in different brain areas of MK−/− mice and wild type (WT) mice. We found that amphetamine-induced microgliosis and astrocytosis are enhanced in the striatum of MK−/− mice in a region-specific manner. Surprisingly, LPS-induced astrogliosis in the striatum was blocked in MK−/− mice. Since striatal neuroinflammation induced by amphetamine-type stimulants correlates with the cognitive deficits induced by these drugs, we also tested the long-term effects of periadolescent amphetamine treatment (3 mg/kg i.p. daily for 10 days) in a memory task in MK−/− and WT mice. Significant deficits in the Y-maze test were only observed in amphetamine-pretreated MK−/− mice. The data demonstrate for the first time that MK is a novel modulator of neuroinflammation depending on the inflammatory stimulus and the brain area considered. The data indicate that MK limits amphetamine-induced striatal neuroinflammation. In addition, our data demonstrate that periadolescent amphetamine treatment in mice results in transient disruption of learning and memory processes in absence of endogenous MK.

The midkine family of growth factors: diverse roles in nervous system formation and maintenance

Midkines are heparin-binding growth factors involved in a wide range of biological processes. Originally identified as retinoic acid inducible genes, midkines are widely expressed during embryogenesis with particularly high levels in the developing nervous system. During postnatal stages, midkine expression generally ceases but is often up-regulated under disease conditions, most notably those affecting the nervous system. Midkines are known as neurotrophic factors, as they promote neurite outgrowth and neuron survival in cell culture. Surprisingly, however, mouse embryos deficient for midkine (knockout mice) are phenotypically normal, which suggests functional redundancy by related growth factors. During adult stages, on the other hand, midkine knockout mice develop striking deficits in neuroprotection and regeneration after drug-induced neurotoxicity and injury. The detailed mechanisms by which midkine controls neuron formation, differentiation and maintenance remain unclear. Recent studies in zebrafish and chick have provided important insight into the role of midkine and its putative receptor, anaplastic lymphoma kinase, in cell cycle control in the central and peripheral nervous systems. A recent structural analysis of zebrafish midkine furthermore revealed essential protein domains required for biological activity that serve as promising novel targets for future drug designs. This review will summarize latest findings in the field that help to better understand the diverse roles of midkine in nervous system formation and maintenance.

LINKED ARTICLES

This article is part of a themed section on Midkine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-4.

Genetic inactivation of midkine modulates behavioural responses to ethanol possibly by enhancing GABA(A) receptor sensitivity to GABA(A) acting drugs

Midkine (MK) is a cytokine with important functions in dopaminergic neurons that is found upregulated in the prefrontal cortex of alcoholics. We have studied the behavioural effects of ethanol in MK genetically deficient (MK-/-) and wild type (MK+/+) mice. A low dose of ethanol (1.0g/kg), unable to cause conditioned place preference (CPP) in MK+/+ mice, induced a significant CPP in MK-/- mice, suggesting that MK prevents the rewarding effects of low doses of ethanol. However, this difference between genotypes is lost when a higher, rewarding, dose of ethanol (2.0g/kg) is used. Accordingly, the anxiolytic effects of 1.0mg/kg diazepam, other GABA(A) acting drug, were significantly enhanced in MK-/- mice compared to MK+/+ mice; however, 2.0mg/kg diazepam caused increased anxiolytic effects in MK+/+ mice. In addition, MK-/- mice showed a significant delayed recovery from ethanol (2.0g/kg)-induced ataxia whereas the sedative effects induced by ethanol (3.6g/kg), tested in a loss of righting reflex paradigm, were found to be similar in MK-/- and MK+/+ mice. The data indicate that MK differentially regulates the behavioural responses to ethanol. The results suggest that differences in the sensitivity of GABA(A) receptors to GABA(A) acting drugs caused by genetic inactivation of MK could underlie the different behavioural responses to ethanol in MK-/- mice. Overall, these results suggest that MK may be a novel genetic factor of importance in alcohol use disorders, and that potentiation of MK signalling pathway may be a promising therapeutic strategy in the treatment of these disorders.

Midkine promotes perineural invasion in human pancreatic cancer

AIM: To investigate midkine (MK) and syndecan-3 protein expression in pancreatic cancer by immunohistochemistry, and to analyze their correlation with clinicopathological features, perineural invasion, and prognosis.

METHODS: Pancreatic cancer tissues (including adequately sized tumor tissue samples and tissue samples taken from areas less than 2.0 cm around the tumor) were taken from 42 patients who were undergoing a partial duodenopancreatectomy. MK and syndecan-3 proteins were detected by immunohistochemistry using a standardized streptavidin-peroxidase method, and analyzed for their correlation with clinicopathological features, perineural invasion, and prognosis. Associations of neural invasion with aggressive characteristics of pancreatic cancer and the presence of perineural invasion were assessed by two independent observers blinded to the patient status.

RESULTS: MK and syndecan-3 were found in 26 (61.9%) and 24 (57.1%) specimens, respectively. MK and syndecan-3 expression was associated with perineural invasion (P = 0.018 and 0.031, respectively). High MK expression was closely associated with advanced tumor, node and metastasis stage (P = 0.008), lymph node metastasis (P = 0.042), and decreased postoperative survival at 3 years (51.0% vs 21.8%, P = 0.001). Syndecan-3 levels were correlated with tumor size (P = 0.028). Patients who were syndecan-3 negative had a higher cumulative survival rate than those who were positive, but the difference was not significant (44.0% vs 23.0%, P > 0.05).

CONCLUSION: MK and syndecan-3 are frequently expressed in pancreatic cancer and associated with perineural invasion. High expression of MK and syndecan-3 may contribute to the highly perineural invasion and poor prognosis of human pancreatic cancer.

Midkine-deficient mice delayed degeneration and regeneration after skeletal muscle injury

Midkine (MK), a heparin-binding growth factor, was previously found to be expressed in the rat myotube-forming stage. We investigated MK gene-deficient (Mdk(-/-)) mice in terms of skeletal muscle degeneration and regeneration after injury by bupivacaine injection into the tibialis anterior muscle. Injured muscles showed intense inflammatory cell infiltration. Myotubes, myofibers with centrally located nuclei in their cytoplasm, were significantly smaller in Mdk(-/-) mice than in wild type (Mdk(+/+)) mice 7 days after injury (p=0.02). The distribution of myotube sizes showed quantitative differences between the two groups at 5 and 7 days, but not at 14 days. Many small myotubes were found in the regenerative area of Mdk(-/-) mice compared with that of Mdk(+/+)mice 5 and 7 days after injury. The expression of Iba1, a macrophage marker, was significantly lower in Mdk(-/-) mice 3 days after injury (p=0.01). The number of desmin-positive cells like myoblasts in Mdk(-/-) mice was significantly fewer than that in Mdk(+/+) mice 3 days after injury. Our results suggested that deletion of MK results in a delay in regeneration, preceded by decelerated migration of macrophages to the damaged area, and that MK has a role in cell differentiation and maturation after skeletal muscle injury.

Structure and function of midkine as the basis of its pharmacological effects

Midkine (MK) is a heparin-binding growth factor or cytokine and forms a small protein family, the other member of which is pleiotrophin. MK enhances survival, migration, cytokine expression, differentiation and other activities of target cells. MK is involved in various physiological processes, such as development, reproduction and repair, and also plays important roles in the pathogenesis of inflammatory and malignant diseases. MK is largely composed of two domains, namely a more N-terminally located N-domain and a more C-terminally located C-domain. Both domains are basically composed of three antiparallel β-sheets. In addition, there are short tails in the N-terminal and C-terminal sides and a hinge connecting the two domains. Several membrane proteins have been identified as MK receptors: receptor protein tyrosine phosphatase Z1 (PTPζ), low-density lipoprotein receptor-related protein, integrins, neuroglycan C, anaplastic lymphoma kinase and Notch-2. Among them, the most established one is PTPζ. It is a transmembrane tyrosine phophatase with chondroitin sulfate, which is essential for high-affinity binding with MK. PI3K and MAPK play important roles in the downstream signalling system of MK, while transcription factors affected by MK signalling include NF-κB, Hes-1 and STATs. Because of the involvement of MK in various physiological and pathological processes, MK itself as well as pharmaceuticals targeting MK and its signalling system are expected to be valuable for the treatment of numerous diseases.

LINKED ARTICLES

This article is part of a themed section on Midkine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-4.

Phosphoproteomic analysis of the striatum from pleiotrophin knockout and midkine knockout mice treated with cocaine reveals regulation of oxidative stress-related proteins potentially underlying cocaine-induced neurotoxicity and neurodegeneration

The neurotrophic factors pleiotrophin (PTN) and midkine (MK) are highly upregulated in different brain areas relevant to drug addiction after administrations of different drugs of abuse, including psychostimulants. We have previously demonstrated that PTN and MK modulate amphetamine-induced neurotoxicity and that PTN prevents cocaine-induced cytotoxicity in NG108-15 and PC12 cells. In an effort to dissect the different mechanisms of action triggered by PTN and MK to exert their protective roles against psychostimulant neurotoxicity, we have now used a proteomic approach to study protein phosphorylation, in which we combined phosphoprotein enrichment, by immobilized metal affinity chromatography (IMAC), with two-dimensional gel electrophoresis and mass spectrometry, in order to identify the phosphoproteins regulated in the striatum of PTN knockout, MK knockout and wild type mice treated with a single dose of cocaine (15mg/kg, i.p.). We identified 7 differentially expressed phosphoproteins: 5'(3')-deoxyribonucleotidase, endoplasmic reticulum resident protein 60 (ERP60), peroxiredoxin-6 (PRDX6), glutamate dehydrogenase 1 (GLUD1), aconitase and two subunits of hemoglobin. Most of these proteins are related to neurodegeneration processes and oxidative stress and their variations specially affect the PTN knockout mice, suggesting a protective role of endogenous PTN against cocaine-induced neural alterations. Further studies are needed to validate these proteins as possible targets against neural alterations induced by cocaine.

Differential phosphoproteome of the striatum from pleiotrophin knockout and midkine knockout mice treated with amphetamine: correlations with amphetamine-induced neurotoxicity

The neurotrophic factors pleiotrophin (PTN) and midkine (MK) have been shown to modulate amphetamine-induced neurotoxicity. Accordingly, PTN-/- and MK-/- mice show an increased vulnerability to amphetamine-induced neurotoxic effects. In an effort to uncover new pharmacological targets to prevent amphetamine neurotoxic effects, we have now used a proteomic approach to study protein phosphorylation, in which we combined phosphoprotein enrichment, by immobilized metal affinity chromatography (IMAC), with two-dimensional gel electrophoresis and mass spectrometry, in order to identify the phosphoproteins regulated in the striatum of PTN-/-, MK-/- and wild type (WT) mice treated with amphetamine. We identified 13 differentially expressed phosphoproteins that are judged to be relevant in the neuroprotective roles of PTN and MK against amphetamine-induced neurotoxicity. It is very interesting to note that 4 of these phosphoproteins, annexin A7 (ANXA7), COP9 signalosome subunit 5 (COPS5), aldehyde dehydrogenase family 1 member A1 (ALDH1A1) and creatine kinase U-type (CKMT1), are known to be involved in Parkinson's disease, a result of significant importance since PTN and MK have been also demonstrated to limit Parkinson's disease (PD) progress and have been suggested to be among the important genetic factors possibly preventing the development of PD in methamphetamine abusers. The data identify phosphoproteins differentially regulated by amphetamine treatment and/or the presence of endogenous PTN/MK which may be relevant mediators of PTN/MK neuroprotective effects against amphetamine-induced neurotoxicity. The data support further studies to validate the phosphoproteins here identified as possible new pharmacological targets to prevent amphetamine neurotoxic effects.

ALK receptor activation, ligands and therapeutic targeting in glioblastoma and in other cancers

The intracellular anaplastic lymphoma kinase (ALK) fragment shows striking homology with members of the insulin receptor family and was initially identified as an oncogenic fusion protein resulting from a translocation in lymphoma and more recently in a range of cancers. The full-length ALK transmembrane receptor of ~220 kDa was identified based on this initial work. This tyrosine kinase receptor and its ligands, the growth factors pleiotrophin (PTN) and midkine (MK) are highly expressed during development of the nervous system and other organs. Each of these genes has been implicated in malignant progression of different tumor types and shown to alter phenotypes as well as signal transduction in cultured normal and tumor cells. Beyond its role in cancer, the ALK receptor pathway is thought to contribute to nervous system development, function, and repair, as well as metabolic homeostasis and the maintenance of tissue regeneration. ALK receptor activity in cancer can be up-regulated by amplification, overexpression, ligand binding, mutations in the intracellular domain of the receptor and by activity of the receptor tyrosine phosphatase PTPRz. Here we discuss the evidence for ligand control of ALK activity as well as the potential prognostic and therapeutic implications from gene expression and functional studies. An analysis of 18 published gene expression data sets from different cancers shows that overexpression of ALK, its smaller homolog LTK (leukocyte tyrosine kinase) and the ligands PTN and MK in cancer tissues from patients correlate significantly with worse course and outcome of the disease. This observation together with preclinical functional studies suggests that this pathway could be a valid therapeutic target for which complementary targeting strategies with small molecule kinase inhibitors as well as antibodies to ligands or the receptors may be used.

Midkine promotes selective expansion of the nephrogenic mesenchyme during kidney organogenesis

During kidney development, the growth and development of the stromal and nephrogenic mesenchyme cell populations and the ureteric bud epithelium is tightly coupled through intricate reciprocal signaling mechanisms between these three tissue compartments. Midkine, a target gene activated by retinoid signaling in the metanephros, encodes a secreted polypeptide with mitogenic and anti-apoptotic activities in a wide variety of cell types. Using immmunohistochemical methods we demonstrated that Midkine is found in the uninduced mesenchyme at the earliest stages of metanephric kidney development and only subsequently concentrated in the ureteric bud epithelium and basement membrane. The biological effects of purified recombinant Midkine were analyzed in metanephric organ culture experiments carried out in serum-free defined media. These studies revealed that Midkine selectively promoted the overgrowth of the Pax-2 and N-CAM positive nephrogenic mesenchymal cells, failed to stimulate expansion of the stromal compartment and suppressed branching morphogenesis of the ureteric bud. Midkine suppressed apoptosis and stimulated cellular proliferation of the nephrogenic mesenchymal cells, and was capable of maintaining the viability of isolated mesenchymes cultured in the absence of the ureteric bud. These results suggest that Midkine may regulate the balance of epithelial and stromal progenitor cell populations of the metanephric mesenchyme during renal organogenesis.

Midkine in the pathology of cancer, neural disease, and inflammation

Midkine (MK) is a heparin-binding growth factor involved in various cellular processes such as cellular proliferation, survival, and migration. In addition to these typical growth factor activities, MK exhibits several other activities related to fibrinolysis, blood pressure, host defense and other processes. Many cell-surface receptors have been identified to account for the multiple biological activities of MK. The expression of MK is frequently upregulated in many types of human carcinoma. Moreover, blood MK levels are closely correlated with patient outcome. Knockdown and blockade of MK suppress tumorigenesis and tumor development. Thus, MK serves as a tumor marker and a molecular target for cancer therapy. Furthermore, there is growing evidence that MK plays pivotal roles in neural and inflammatory diseases. Understanding of the mechanisms of action of MK is expected to create new therapeutic options for several human diseases.

The heparin binding growth factors midkine and pleiotrophin regulate the antinociceptive effects of morphine through α(2)-adrenergic independent mechanisms

Genetic deletion of pleiotrophin (PTN) impairs spinal nociceptive transmission suggesting that this heparin binding growth factor could play roles in acute pain processing. Despite the high functional redundancy between PTN and midkine (MK), the only other member of this family of growth factors, we now demonstrate that genetic inactivation of MK does not alter acute nociceptive transmission since pain responses of female MK genetically deficient (MK-/-) and wild type (WT+/+) mice were found to be similar in the hot-plate and tail-immersion tests. It has also been shown that morphine administration significantly regulates MK levels within the brain, suggesting that MK could play a role in morphine-induced antinociceptive effects. To test this hypothesis, we have now studied morphine-induced antinociceptive effects in female MK-/- and WT+/+ mice. We did not find differences among genotypes using different doses of morphine (2.5, 5 and 10 mg/kg) in the hot-plate test. In contrast, we found that morphine significantly delayed pain responses in MK-/- mice compared to WT+/+ mice in the tail-immersion test. In confirmation of previous results from our group, we also found significantly enhanced morphine-induced antinociceptive effects in PTN-/- mice in the tail-immersion test. In addition, we now demonstrate that enhanced morphine analgesic effects in PTN-/- and MK-/- mice are not caused by a different contribution of descending noradrenergic inhibitory pathways since the α(2)-adrenergic antagonist yohimbine failed to alter morphine-induced analgesia in all genotypes. The data demonstrate that MK is an endogenous modulator of morphine antinociceptive effects, identify significant differences between PTN and MK in the control of pain processing at the spinal level, and support the hypothesis that inhibitors of the PTN/MK signaling pathway could potentiate opioid analgesia which may be relevant in opioid-refractory pain cases.

The cytokine midkine and its receptor RPTPζ regulate B cell survival in a pathway induced by CD74

Lasting B cell persistence depends on survival signals that are transduced by cell surface receptors. In this study, we describe a novel biological mechanism essential for survival and homeostasis of normal peripheral mature B cells and chronic lymphocytic leukemia cells, regulated by the heparin-binding cytokine, midkine (MK), and its proteoglycan receptor, the receptor-type tyrosine phosphatase ζ (RPTPζ). We demonstrate that MK initiates a signaling cascade leading to B cell survival by binding to RPTPζ. In mice lacking PTPRZ, the proportion and number of the mature B cell population are reduced. Our results emphasize a unique and critical function for MK signaling in the previously described MIF/CD74-induced survival pathway. Stimulation of CD74 with MIF leads to c-Met activation, resulting in elevation of MK expression in both normal mouse splenic B and chronic lymphocytic leukemia cells. Our results indicate that MK and RPTPζ are important regulators of the B cell repertoire. These findings could pave the way toward understanding the mechanisms shaping B cell survival and suggest novel therapeutic strategies based on the blockade of the MK/RPTPζ-dependent survival pathway.

Midkine in inflammation

The 13 kDa heparin-binding growth factor midkine (MK) was originally identified as a molecule involved in the orchestration of embryonic development. Recent studies provided evidence for a new role of MK in acute and chronic inflammatory processes. Accordingly, several inflammatory diseases including nephritis, arthritis, atherosclerosis, colitis, and autoimmune encephalitis have been shown to be alleviated in the absence of MK in animal models. Reduced leukocyte recruitment to the sites of inflammation was found to be one important mechanism attenuating chronic inflammation when MK was absent. Furthermore, MK was found to modulate expression of proinflammatory cytokines and the expansion of regulatory T-cells. Here, we review the current understanding of the role of MK in different inflammatory disorders and summarize the knowledge of MK biology.

Midkine-deficiency increases the anabolic response of cortical bone to mechanical loading

The adaptive response of bone to load is dependent on molecular factors, including growth factor signaling, which is involved in the regulation of proliferation, differentiation and function of osteoblasts and osteoclasts. Based on a recent study, which has shown that the deficiency of growth factor midkine (Mdk) in mice at 12 and 18 months of age resulted in increased trabecular bone formation, we hypothesized that mechanically-induced bone remodeling may, at least in part, be dependent on Mdk expression. To investigate this, we loaded the ulnae of Mdk-deficient mice and appropriate wild-type mice at the age of 12 months using the in vivo ulna loading model. Histomorphometric quantification of the periosteal bone demonstrated an increased mineralizing surface, mineral apposition rate, and bone formation rate in ulnae of Mdk-deficient mice compared to wild-type mice in response to loading. Because Mdk has been shown to bind to a complex of receptor-type protein tyrosine phosphatase zeta (Ptprz) and low density lipoprotein receptor-related protein-6 (Lrp-6) together with the α4β1- and α6β1-integrins, we performed in vitro studies using osteoblastic cells, transiently over-expressing Mdk, Wnt-3a, and Ptprz to evaluate whether Mdk has a role in regulating bone formation by modulating Wnt signaling. We observed a negative effect of Mdk on Wnt signaling, the extent of which appeared to be dependent on Ptprz expression. Moreover, we performed in vitro loading studies with osteoblasts treated with recombinant Mdk and observed a negative effect on the expression of Wnt target genes, which play a critical role in osteoblast proliferation. In summary, our data demonstrate that Mdk-deficiency in mice has an anabolic effect on mechanically induced cortical bone formation. This could be due to an improved osteoblast function based on an enhancement of β-catenin-dependent Wnt signaling by both Mdk-deficiency and mechanical loading.

Midkine: a promising molecule for drug development to treat diseases of the central nervous system

Midkine (MK) is a heparin-binding cytokine, and promotes growth, survival, migration and other activities of target cells. After describing the general properties of MK, this review focuses on MK and MK inhibitors as therapeutics for diseases in the central nervous system. MK is strongly expressed during embryogenesis especially at the midgestation period, but is expressed only at restricted sites in adults. MK expression is induced upon tissue injury such as ischemic brain damage. Since exogenously administered MK or the gene transfer of MK suppresses neuronal cell death in experimental systems, MK has the potential to treat cerebral infarction. MK might become important also in the treatment of neurodegenerative diseases such as Alzheimer's disease. MK is involved in inflammatory diseases by enhancing migration of leukocytes, inducing chemokine production and suppressing regulatory T cells. Since an aptamer to MK suppresses experimental autoimmune encephalitis, MK inhibitors are promising for the treatment of multiple sclerosis. MK is overexpressed in most malignant tumors including glioblastoma, and is involved in tumor invasion. MK inhibitors may be of value in the treatment of glioblastoma. Furthermore, an oncolytic adenovirus, whose replication is under the control of the MK promoter, inhibits the growth of glioblastoma xenografts. MK inhibitors under development include antibodies, aptamers, glycosaminoglycans, peptides and low molecular weight compounds. siRNA and antisense oligoDNA have proved effective against malignant tumors and inflammatory diseases in experimental systems. Practical information concerning the development of MK and MK inhibitors as therapeutics is described in the final part of the review.

Receptor tyrosine phosphatase PTPγ is a regulator of spinal cord neurogenesis

During spinal cord development the proliferation, migration and survival of neural progenitors and precursors is tightly controlled, generating the fine spatial organisation of the cord. In order to understand better the control of these processes, we have examined the function of an orphan receptor protein tyrosine phosphatase (RPTP) PTPγ, in the developing chick spinal cord. Widespread expression of PTPγ occurs post-embryonic day 3 in the early cord and is consistent with a potential role in either neurogenesis or neuronal maturation. Using gain-of-function and loss-of-function approaches in ovo, we show that PTPγ perturbation significantly reduces progenitor proliferation rates and neuronal precursor numbers, resulting in hypoplasia of the neuroepithelium. PTPγ gain-of-function causes widespread suppression of Wnt/β-catenin-driven TCF signalling. One potential target of PTPγ may therefore be β-catenin itself, since PTPγ can dephosphorylate it in vitro, but alternative targets are also likely. PTPγ loss-of-function is not sufficient to alter TCF signalling. Instead, loss-of-function leads to increased apoptosis and defective cell–cell adhesion in progenitors and precursors. Furthermore, motor neuron precursor migration is specifically defective. PTPγ therefore regulates neurogenesis during a window of spinal cord development, with molecular targets most likely related to Wnt/β-catenin signalling, cell survival and cell adhesion.

Midkine: a novel prognostic biomarker for cancer

Since diagnosis at an early stage still remains a key issue for modern oncology and is crucial for successful cancer therapy, development of sensitive, specific, and non-invasive tumor markers, especially, in serum, is urgently needed. Midkine (MK), a plasma secreted protein, was initially identified in embryonal carcinoma cells at early stages of retinoic acid-induced differentiation. Multiple studies have reported that MK plays important roles in tumor progression, and is highly expressed in various malignant tumors. Because increased serum MK concentrations also have been reported in patients with various tumors, serum MK may have the potential to become a very useful tumor marker. Here, we review and discuss the possibility and usefulness of MK as a novel tumor marker.

Effect of single-chain antibody targeting of the ligand-binding domain in the anaplastic lymphoma kinase receptor

The tyrosine kinase receptor anaplastic lymphoma kinase (ALK) and its ligand, the growth factor pleiotrophin (PTN), are highly expressed during the development of the nervous system and have been implicated in the malignant progression of different tumor types. Here, we describe human single-chain variable fragment (scFv) antibodies that target the ligand-binding domain (LBD) in ALK and show the effect in vitro and in vivo. The ALK LBD was used as a bait in a yeast two-hybdrid system to select human scFv from a library with randomized complementarity-determining region 3 domains. Surface plasmon resonance showed high-affinity binding of the selected scFv. The anti-ALK scFv competed for binding of PTN to ALK in intact cells and inhibited PTN-dependent signal transduction through endogenous ALK. Invasion of an intact endothelial cell monolayer by U87MG human glioblastoma cells was inhibited by the anti-ALK scFv. In addition, the growth of established tumor xenografts in mice was reversed after the induction of the conditional expression of the anti-ALK scFv. In archival malignant brain tumors expression levels of ALK and PTN were found elevated and appear correlated with poor patient survival. This suggests a rate-limiting function of the PTN/ALK interaction that may be exploited therapeutically.

Stable isotope labeling by amino acids in cell culture (SILAC) and quantitative comparison of the membrane proteomes of self-renewing and differentiating human embryonic stem cells

Stable isotope labeling by amino acids in cell culture (SILAC) is a powerful quantitative proteomics platform for comprehensive characterization of complex biological systems. However, the potential of SILAC-based approaches has not been fully utilized in human embryonic stem cell (hESC) research mainly because of the complex nature of hESC culture conditions. Here we describe complete SILAC labeling of hESCs with fully preserved pluripotency, self-renewal capabilities, and overall proteome status that was quantitatively analyzed to a depth of 1556 proteins and 527 phosphorylation events. SILAC-labeled hESCs appear to be perfectly suitable for functional studies, and we exploited a SILAC-based proteomics strategy for discovery of hESC-specific surface markers. We determined and quantitatively compared the membrane proteomes of the self-renewing versus differentiating cells of two distinct human embryonic stem cell lines. Of the 811 identified membrane proteins, six displayed significantly higher expression levels in the undifferentiated state compared with differentiating cells. This group includes the established marker CD133/Prominin-1 as well as novel candidates for hESC surface markers: Glypican-4, Neuroligin-4, ErbB2, receptor-type tyrosine-protein phosphatase zeta (PTPRZ), and Glycoprotein M6B. Our study also revealed 17 potential markers of hESC differentiation as their corresponding protein expression levels displayed a dramatic increase in differentiated embryonic stem cell populations.

Midkine positively regulates the proliferation of human gastric cancer cells

Midkine (MDK), a heparin-binding growth factor, modulates the proliferation and migration of various cells, is often highly expressed in many malignant tumors, and may act as an oncoprotein. We found that MDK is overexpressed in clinical human gastric cancer tissues relative to its expression in adjacent noncancerous tissues. To further investigate the biological activities of MDK in gastric cancer, we introduced the MDK gene into human SGC7901 gastric cancer cells, where it contributed to the proliferation of SGC7901 cells in vitro and in vivo. Conversely, the knockdown of MDK expression by siRNA resulted in significantly reduced proliferation of BGC823 cells. Our study also shows that MDK activates both the Akt and ERK1/2 pathways and upregulates the expression of several cell-cycle-related proteins, including cyclin A, cyclin D1, Cdk2, Cdk4, and Cdk6, which in part explains the contribution of MDK to gastric cancer cell survival and growth. These results demonstrate that MDK contributes to gastric cancer cell proliferation and suggest that it plays an important role in the development of human gastric cancer.

Behavioral and neurochemical alterations in mice deficient in anaplastic lymphoma kinase suggest therapeutic potential for psychiatric indications

The receptor tyrosine kinase product of the anaplastic lymphoma kinase (ALK) gene has been implicated in oncogenesis as a product of several chromosomal translocations, although its endogeneous role in the hematopoietic and neural systems has remained poorly understood. We describe that the generation of animals homozygous for a deletion of the ALK tyrosine kinase domain leads to alterations in adult brain function. Evaluation of adult ALK homozygotes (HOs) revealed an age-dependent increase in basal hippocampal progenitor proliferation and alterations in behavioral tests consistent with a role for this receptor in the adult brain. ALK HO animals displayed an increased struggle time in the tail suspension test and the Porsolt swim test and enhanced performance in a novel object-recognition test. Neurochemical analysis demonstrates an increase in basal dopaminergic signalling selectively within the frontal cortex. Altogether, these results suggest that ALK functions in the adult brain to regulate the function of the frontal cortex and hippocampus and identifies ALK as a new target for psychiatric indications, such as schizophrenia and depression, with an underlying deregulated monoaminergic signalling.

Midkine secretion protects Hep3B cells from cadmium induced cellular damage

AIM: To evaluate role of midkine secretion during Cadmium (Cd) exposure in the human hepatocyte cell line Hep3B cells.

METHODS: Different dosages of Cd (0.5-1-5-10 &mgr;g/mL) were applied to Hep3B cells and their effects to apoptosis, lactate dehydrogenase (LDH) leakage and midkine secretion were evaluated as time dependent manner. Same experiments were repeated with exogenously applied midkine (250-5000 pg/mL) and/or 5 &mgr;g/mL Cd.

RESULTS: Cd exposure induced prominent apoptosis and LDH leakage beginning from lower dosages at the 48^th h. Cd induced midkine secretion with higher dosages (P < 0.001), (control, Cd 0.5-1-5-10 &mgr;g/mL respectively: 1123 ± 73, 1157 ± 63, 1242 ± 90, 1886 ± 175, 1712 ± 166 pg/mL). Exogenous 500-5000 pg/mL midkine application during 5 &mgr;g/mL Cd toxicity prevented caspase-3 activation (control, Cd toxicity, 250, 500, 1000, 2500, 5000 pg/mL midkine+ Cd toxicity, respectively: 374 ± 64, 1786 ± 156, 1545 ± 179, 1203 ± 113, 974 ± 116, 646 ± 56, 556 ± 63 cfu) LDH leakage and cell death in Hep3B cells (P < 0.001).

CONCLUSION: Our results showed that midkine secretion from Hep3B cells during Cd exposure protects liver cells from Cd induced cellular damage. Midkine has anti-apoptotic and cytoprotective role during Cd toxicity. Further studies are needed to explain the mechanism of midkine secretion and cytoprotective role of midkine during Cd exposure. Midkine may be a promising therapeutic agent in different toxic hepatic diseases.

Midkine, a heparin-binding growth factor, is expressed in neural precursor cells and promotes their growth

Midkine, a heparin-binding growth factor, was found to be expressed in neural precursor cells, which consist of neural stem cells and the progenitor cells. When embryonic brain cells were allowed to form neurospheres enriched in neural precursor cells, numbers were significantly smaller from the midkine-deficient brain than from the wild-type brain. Dissociated neurosphere cells yielded nestin-positive neural precursor cells and differentiated neuronal cells upon culture on a substratum. Neural precursor cells from the midkine-deficient brain spread poorly and grew less effectively on a substratum coated with poly-l-lysine than the cells on midkine-coated substratum. Neural precursor cells from the wild-type brain spread and grew well on both the substrata. Differentiation to neurons and glia cells was not affected by the absence of midkine. Heparitinase digestion of dissociated neurosphere cells resulted in poor growth of neural precursor cells, while chondroitinase digestion had no effect. These results indicate that midkine is involved in the growth of neural precursor cells and suggest that the interaction with heparan sulfate proteoglycans is important in midkine action to these cells.

Midkine and its receptor in regenerating rat skeletal muscle after bupivacaine injection

Midkine (MK) is a multifunctional cytokine and heparin-binding growth factor with neurotrophic activity. MK and its receptor were examined for up to 14 days in a chemically injured rat muscle regeneration process caused by the injection of bupivacaine using immunohistochemical and Western blot analysis. Although MK immunoreactivity was not detectable in the mature uninjured skeletal muscle, MK was strongly detected in the regenerating muscle cells. MK immunoreactivity was observed in the myoblast-like cells and myotubes, which were desmin-positive cells, whereas it was not detectable in the surviving normal muscle fibers. Most myotubes labeling for desmin showed MK immunoreactivity 5-7days after the injury. However, MK immunoreactivity was not detected 14 days after the injury. Immunoreactivity of low-density lipoprotein receptor-related protein (LRP), a cell membrane receptor of MK, was detected in the regenerating muscle cells, whereas it was not detected in the normal adult skeletal muscle and surviving muscle. These findings suggested that MK was involved. MK may have a role for differentiation during skeletal muscle regeneration and may be taken up in an autocrine fashion with LRP.

Neuroglycan C Is a Novel Midkine Receptor Involved in Process Elongation of Oligodendroglial Precursor-like Cells

Midkine is a heparin-binding growth factor that promotes cell attachment and process extension in undifferentiated bipolar CG-4 cells, an oligodendroglial precursor cell line. We found that CG-4 cells expressed a non-proteoglycan form of neuroglycan C, known as a part-time transmembrane proteoglycan. We demonstrated that neuroglycan C before or after chondroitinase ABC treatment bound to a midkine affinity column. Neuroglycan C lacking chondroitin sulfate chains was eluted with 0.5 m NaCl as a major fraction from the column. We confirmed that CG-4 cells expressed two isoforms of neuroglycan C, I, and III, by isolating cDNA. Among three functional domains of the extracellular part of neuroglycan C, the chondroitin sulfate attachment domain and acidic amino acid cluster box domain showed affinity for midkine, but the epidermal growth factor domain did not. Furthermore, cell surface neuroglycan C could be cross-linked with soluble midkine. Process extension on midkine-coated dishes was inhibited by either a monoclonal anti-neuroglycan C antibody C1 or a glutathione S-transferase-neuroglycan C fusion protein. Finally, stable transfectants of B104 neuroblastoma cells overexpressing neuroglycan C-I or neuroglycan C-III attached to the midkine substrate, spread well, and gave rise to cytoskeletal changes. Based on these results, we conclude that neuroglycan C is a novel component of midkine receptors involved in process elongation.

Inhibitors of protein tyrosine phosphatases: next-generation drugs?

The protein tyrosine phosphatases (PTPs) constitute a family of closely related key regulatory enzymes that dephosphorylate phosphotyrosine residues in their protein substrates. Malfunctions in PTP activity are linked to various diseases, ranging from cancer to neurological disorders and diabetes. Consequently, PTPs have emerged as promising targets for therapeutic intervention in recent years. In this review, general aspects of PTPs and the development of small-molecule inhibitors of PTPs by both academic research groups and pharmaceutical companies are discussed. Different strategies have been successfully applied to identify potent and selective inhibitors. These studies constitute the basis for the future development of PTP inhibitors as drugs.

Altered gene expression profiles by sodium/iodide symporter gene transfection in a human anaplastic thyroid carcinoma cell line using a radioactive complementary DNA microarray

BACKGROUND

The sodium/iodide symporter (NIS) is a membrane glycoprotein that mediates active 131I uptake during the treatment of cancer of the thyroid gland and extrathyroidal tissues. NIS gene transfection, a gene-therapy modality, has been introduced in many types of cancer, such as prostate cancer and breast cancer, and has demonstrated a high potential for the treatment of non-thyroidal cancers.

AIM

To investigate the pattern of NIS gene expression and provide evidence of its beneficial effects in human anaplastic cancer ARO cells by using a radioactive complementary DNA (cDNA) microarray.

METHODS

For cDNA microarray data analysis, superimposed images and clustergrams were prepared from basic radioactivity data obtained using a phosphoimager system. Gene expression profiles were constructed using the Z-transformed values of genes related to cancer biology.

RESULTS

Radioactive cDNA microarray studies showed that 11 genes were upregulated (Z ratio > 1.5) and 31 genes were downregulated (Z ratio < -1.5) in response to NIS gene transfection. Of these differentially expressed genes, 33% were related to cell proliferation and apoptosis. Moreover, NIS gene transfection into an anaplastic thyroid cancer cell line affected the expression of the protein tyrosine phosphatase (PTP) family and Ras oncogene family, including Ras, Rac and Rab.

CONCLUSION

The identification of changes in the patterns of gene expression may provide a better understanding of the response of molecular mechanisms to NIS gene transfection.

Identification of midkine as a mediator for intercellular transfer of drug resistance

Resistance to cytotoxic agents is a major limitation for their clinical use to treat human cancers. Tumors become resistant to chemotherapy when a subset of cells undergoes molecular changes leading to overexpression of drug transport proteins, alterations in drug-target interactions or reduced ability to commit apoptosis. However, such changes may not be sufficient to explain why both resistant and nonresistant cells survive drug's action in tumors that ultimately become drug resistant. We hypothesized that, in such tumors, a cytoprotective relationship may exist between drug-resistant and neighboring drug-sensitive cells. The present study addresses the possibility that drug-resistant cells secrete in their culture medium factors able to protect sensitive cells from drug toxicity. A survival molecule, midkine, was identified by cDNA array to be expressed only in drug-resistant cells. Midkine-enriched fractions obtained by affinity chromatography exert a significant cytoprotective effect against doxorubicin in the wild-type drug-sensitive cells. Moreover, transfection of these cells with the midkine gene caused a decreased response to doxorubicin. The underlying mechanism of this cytoprotection appeared to imply activation of the Akt pathway and inhibition of drug-induced proliferation arrest as well as apoptotic cell death. These findings provide evidence for the existence of intercellular cytoprotective signals such as the one mediated by midkine, originating from cells with acquired drug resistance to protect neighboring drug-sensitive cells and thus contribute to development of resistance to chemotherapy.