Midkine accumulated in nucleolus of HepG2 cells involved in rRNA transcription

Midkine (MDK) in Hepatocellular Carcinoma: More than a Biomarker

Midkine (MDK) is a multifunctional secreted protein that can act as a cytokine or growth factor regulating multiple signaling pathways and being implicated in fundamental cellular processes, such as survival, proliferation, and migration. Although its expression in normal adult tissues is barely detectable, MDK serum levels are found to be elevated in several types of cancer, including hepatocellular carcinoma (HCC). In this review, we summarize the findings of recent studies on the role of MDK in HCC diagnosis and progression. Overall, studies show that MDK is a powerful biomarker for HCC early diagnosis, as it can differentiate not only between HCC patients and normal individuals but also between HCC patients and patients with other liver pathologies. It is correlated with high recurrence rates and was shown to be valuable for the diagnosis of early-stage HCC, even in patients negative for α-fetoprotein (AFP), the most commonly used biomarker for HCC diagnosis. A comparison with AFP reveals that MDK is inferior to AFP with regard to specificity but significantly superior with regard to sensitivity, which further indicates the need for using both biomarkers for more effective HCC diagnosis.

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.

Unphosphorylated Form of the PAQosome Core Subunit RPAP3 Binds Ribosomal Preassembly Complexes to Modulate Ribosome Biogenesis

The PAQosome (particle for arrangement of quaternary structure) is a 12-subunit HSP90 co-chaperone involved in the biogenesis of several human protein complexes. Two mechanisms of client selection have previously been identified, namely, the selective recruitment of specific adaptors and the differential use of homologous core subunits. Here, we describe a third client selection mechanism by showing that RPAP3, one of the core PAQosome subunits, is phosphorylated at several Ser residues in HEK293 cells. Affinity purification coupled with mass spectrometry (AP-MS) using the expression of tagged RPAP3 with single phospho-null mutations at Ser116, Ser119, or Ser121 reveals binding of the unphosphorylated form to several proteins involved in ribosome biogenesis. In vitro phosphorylation assays indicate that the kinase CK2 phosphorylates these RPAP3 residues. This finding is supported by data showing that pharmacological inhibition of CK2 enhances the binding of RPAP3 to ribosome preassembly factors in AP-MS experiments. Moreover, the silencing of PAQosome subunits interferes with ribosomal assembly factors' interactome. Altogether, these results indicate that RPAP3 phosphate group addition/removal at specific residues modulates binding to subunits of preribosomal complexes and allows speculating that PAQosome posttranslational modification is a mechanism of client selection.

Midkine: A multifaceted driver of atherosclerosis

Atherosclerosis constitutes the pathological basis of life-threatening events, including heart attack and stroke. Midkine is a heparin-binding growth factor and forms a small protein family with pleiotrophin. Under inflammatory or hypoxic conditions, midkine expression is up-regulated. Upon binding to its receptors, midkine can activate multiple signal pathways to regulate cell survival and migration, epithelial-to-mesenchymal transition, and oncogenesis. Circulating midkine levels are significantly increased in patients with essential hypertension, obesity or severe peripheral artery disease. Importantly, midkine exerts a proatherogenic effect by altering multiple pathophysiological processes involving atherogenesis, including macrophage lipid accumulation, vascular inflammation, neointima formation, insulin resistance and macrophage apoptosis. Midkine represents a potential therapeutic target for atherosclerosis-associated diseases. This review described the structure characteristics, expression patterns and signal transduction pathways of midkine with an emphasis on its role in atherosclerosis.

Shedding more light on the role of Midkine in hepatocellular carcinoma: New perspectives on diagnosis and therapy

One of the most common malignant tumors is hepatocellular carcinoma (HCC). Progression of HCC mainly results from highly complex molecular and pathological pathways. Midkine (MDK) is a growth factor that impacts viability, migration, and other cell activities. Since MDK has been involved in the inflammatory responses, it has been claimed that MDK has a crucial role in HCC. MDK acts as an anti-apoptotic factor, which mediates tumor cell viability. In addition, MDK blocks anoikis to promote metastasis. There is also evidence that MDK is involved in angiogenesis. It has been shown that the application of anti-MDK approaches might be promising in the treatment of HCC. Besides, due to the elevated expression in HCC, MDK has been proposed as a biomarker in the prognosis and diagnosis of HCC. In this review, we will discuss the role of MDK in HCC. It is hoped that the development of new strategies concerning MDK-based therapies will be promising in HCC management.

Deciphering GRINA/Lifeguard1: Nuclear Location, Ca2+ Homeostasis and Vesicle Transport

The Glutamate Receptor Ionotropic NMDA-Associated Protein 1 (GRINA) belongs to the Lifeguard family and is involved in calcium homeostasis, which governs key processes, such as cell survival or the release of neurotransmitters. GRINA is mainly associated with membranes of the endoplasmic reticulum, Golgi, endosome, and the cell surface, but its presence in the nucleus has not been explained yet. Here we dissect, with the help of different software tools, the potential roles of GRINA in the cell and how they may be altered in diseases, such as schizophrenia or celiac disease. We describe for the first time that the cytoplasmic N-terminal half of GRINA (which spans a Proline-rich domain) contains a potential DNA-binding sequence, in addition to cleavage target sites and probable PY-nuclear localization sequences, that may enable it to be released from the rest of the protein and enter the nucleus under suitable conditions, where it could participate in the transcription, alternative splicing, and mRNA export of a subset of genes likely involved in lipid and sterol synthesis, ribosome biogenesis, or cell cycle progression. To support these findings, we include additional evidence based on an exhaustive review of the literature and our preliminary data of the protein–protein interaction network of GRINA.

Midkine's Role in Cardiac Pathology

Midkine (MDK) is a heparin-binding growth factor that is normally expressed in mid-gestational development mediating mesenchymal and epithelial interactions. As organisms age, expression of MDK diminishes; however, in adults, MDK expression is associated with acute and chronic pathologic conditions such as myocardial infarction and heart failure (HF). The role of MDK is not clear in cardiovascular disease and currently there is no consensus if it plays a beneficial or detrimental role in HF. The lack of clarity in the literature is exacerbated by differing roles that circulating and myocardial MDK play in signaling pathways in cardiomyocytes (some of which have yet to be elucidated). Of particular interest, serum MDK is elevated in adults with chronic heart failure and higher circulating MDK is associated with worse cardiac function. In addition, pediatric HF patients have higher levels of myocardial MDK. This review focuses on what is known about the effect of exogenous versus myocardial MDK in various cardiac disease models in an effort to better clarify the role of midkine in HF.

MPA Modulates Tight Junctions' Permeability via Midkine/PI3K Pathway in Caco-2 Cells: A Possible Mechanism of Leak-Flux Diarrhea in Organ Transplanted Patients

Mycophenolic acid (MPA) is prescribed to prevent allograft rejection in organ transplanted patients. However, its use is sporadically linked to leak flux diarrhea and other gastrointestinal (GI) disturbances in around 75% of patients through yet unknown mechanisms. Recently, we identified Midkine as a modulator of tight junctions (TJs) permeability in MPA treated Caco-2 monolayer. In the present study, we investigated the possible involvement of Midkine dependent PI3K pathway in alteration of TJs under MPA treatment. Caco-2 cells were grown as monolayer to develop TJs and were treated for 72 h with DMSO (control) or MPA in presence and absence of Midkine inhibitor (iMDK) or PI3K inhibitors (LY/AMG). Caco-2 monolayer integrity was assessed by transepithelial electrical resistance (TEER) and FITC-dextran assays. Our functional assays showed that PI3K inhibitors (LY/AMG) can significantly inhibit the compromised TJs integrity of MPA-treated Caco-2 cells monolayer. Chromatin immunoprecipitation analyses showed a significant epigenetic activation of Midkine, PI3K, Cdx-2, and Cldn-2 genes and epigenetic repression of Cldn-1 gene after MPA treatment. The MPA-induced epigenetic alterations were further confirmed by mRNA and protein expression analysis. Collectively, our data shows that PI3K pathway as the downstream target of Midkine which in turn modulates p38MAPK and pAKT signaling to alter TJs permeability in Caco-2 cell monolayers treated with MPA. These results highlight the possible use of either Midkine or PI3K inhibitors as therapeutic agents to prevent MPA induced GI disturbances.

Active and Repressive Chromatin-Associated Proteome after MPA Treatment and the Role of Midkine in Epithelial Monolayer Permeability

Mycophenolic acid (MPA) is prescribed to maintain allografts in organ-transplanted patients. However, gastrointestinal (GI) complications, particularly diarrhea, are frequently observed as a side effect following MPA therapy. We recently reported that MPA altered the tight junction (TJ)-mediated barrier function in a Caco-2 cell monolayer model system. This study investigates whether MPA induces epigenetic changes which lead to GI complications, especially diarrhea.

METHODS

We employed a Chromatin Immunoprecipitation-O-Proteomics (ChIP-O-Proteomics) approach to identify proteins associated with active (H3K4me3) as well as repressive (H3K27me3) chromatin histone modifications in MPA-treated cells, and further characterized the role of midkine, a H3K4me3-associated protein, in the context of epithelial monolayer permeability.

RESULTS

We identified a total of 333 and 306 proteins associated with active and repressive histone modification marks, respectively. Among them, 241 proteins were common both in active and repressive chromatin, 92 proteins were associated exclusively with the active histone modification mark, while 65 proteins remained specific to repressive chromatin. Our results show that 45 proteins which bind to the active and seven proteins which bind to the repressive chromatin region exhibited significantly altered abundance in MPA-treated cells as compared to DMSO control cells. A number of novel proteins whose function is not known in bowel barrier regulation were among the identified proteins, including midkine. Our functional integrity assays on the Caco-2 cell monolayer showed that the inhibition of midkine expression prior to MPA treatment could completely block the MPA-mediated increase in barrier permeability.

CONCLUSIONS

The ChIP-O-Proteomics approach delivered a number of novel proteins with potential implications in MPA toxicity. Consequently, it can be proposed that midkine inhibition could be a potent therapeutic approach to prevent the MPA-mediated increase in TJ permeability and leak flux diarrhea in organ transplant patients.

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.

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.

Imatinib mesylate decreases the cytotoxic effect of roscovitine on human glioblastoma cells in vitro and the role of midkine

The purpose of the present study was to overcome resistance to imatinib (IM) by combining it with roscovitine (ROSC) and to investigate whether or not midkine (MK) had an effect on this combination in the treatment of glioblastoma (GBL). Human T98 GBL cells were used to evaluate the effects of IM (10 μM), ROSC (200 μM) and their combination on the cell proliferation index, apoptotic index, the apoptotic protein and anti-apoptotic protein levels, and ultrastructure. All applications decreased the cell proliferation index and increased the apoptotic index, but ROSC was the most efficient drug and the second most efficient drug was IM. Notably, ROSC increased anti-apoptotic proteins levels (PDGFR-α, AQP-4, hTERT), COX-1 activity and ribosome numbers. The effects of ROSC on hTERT, MK, AQP-4 and MRP-1 levels and COX-1 activity were reported for the first time. ROSC induced the highest increase in caspase-3 levels. Autophagy was not involved in the activity of ROSC in GBL spheroids. The combination of IM with ROSC showed an antagonist effect in the treatment of human GBL cells. The combination group decreased certain anti-apoptotic protein levels (PDGFR-α, EGFR, p-gp, MRP-1 and MK), cAMP levels, COX-1 activity and apoptotic protein levels (caspase-3). However, it induced the highest increase in hTERT levels and COX-2 activity. Ribosome numbers were much lower than those in the ROSC group and no autophagic vacuole was observed. In conclusion, more investigations are required to identify the key regulatory components that are responsible for this antagonism; however, the determination of this combination therapy as a failure therapy may be precautionary for oncologists in the treatment of GBL patients and potentially may contribute to the efficacy of new therapeutic regimens.

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.

Midkine, a heparin-binding cytokine with multiple roles in development, repair and diseases

Midkine is a heparin-binding cytokine or a growth factor with a molecular weight of 13 kDa. Midkine binds to oversulfated structures in heparan sulfate and chondroitin sulfate. The midkine receptor is a molecular complex containing proteoglycans. Midkine promotes migration, survival and other activities of target cells. Midkine has about 50% sequence identity with pleiotrophin. Mice deficient in both factors exhibit severe abnormalities including female infertility. In adults, midkine is expressed in damaged tissues and involved in the reparative process. It is also involved in inflammatory reactions by promoting the migration of leukocytes, induction of chemokines and suppression of regulatory T cells. Midkine is expressed in a variety of malignant tumors and promotes their growth and invasion. Midkine appears to be helpful for the treatment of injuries in the heart, brain, spinal cord and retina. Midkine inhibitors are expected to be effective in the treatment of malignancies, rheumatoid arthritis, multiple sclerosis, renal diseases, restenosis, hypertension and adhesion after surgery.

Midkine translocated to nucleoli and involved in carcinogenesis

Midkine (MK) is a heparin-binding growth factor with its gene first identified in embryonal carcinoma cells at early stages of retinoic acid-induced differentiation. MK is frequently and highly expressed in a variety of human carcinomas. Furthermore, the blood MK level is frequently elevated with advance of human carcinomas, decreased after surgical removal of the tumors. Thus, it is expected to become a promising marker for evaluating the progress of carcinomas. There is mounting evidence that MK plays a significant role in carcinogenesis-related activities, such as proliferation, migration, anti-apoptosis, mitogenesis, transforming, and angiogenesis. In addition, siRNA and anti-sense oligonucleotides for MK have yielded great effects in anti-tumor activities. Therefore, MK appears to be a potential candidate molecular target of therapy for human carcinomas. In this paper, we review MK targeting at nucleoli in different tumor cells and its role in carcinogenesis to deepen our understanding of the mechanism of MK involved in carcinogenesis.