Mutation at the acidic residue-rich domain of eukaryotic initiation factor 2 (eIF2alpha)-associated glycoprotein p67 increases the protection of eIF2alpha phosphorylation during heat shock

Methionine aminopeptidase‑2 is a pivotal regulator of vasculogenic mimicry

Vasculogenic mimicry (VM) is the formation of a blood supply system that confers aggressive and metastatic properties to tumors and correlates with a poor prognosis in cancer patients. Thus, the inhibition of VM is considered an effective approach for cancer treatment, although such a mechanism remains poorly described. In the present study, we examined methionine aminopeptidase-2 (MetAP2), a key factor of angiogenesis, and demonstrated that it is pivotal for VM, using pharmacological and genetic approaches. Fumagillin and TNP-470, angiogenesis inhibitors that target MetAP2, significantly suppressed VM in various human cancer cell lines. We established MetAP2-knockout (KO) human fibrosarcoma HT1080 cells using the CRISPR/Cas9 system and found that VM was attenuated in these cells. Furthermore, re-expression of wild-type MetAP2 restored VM in the MetAP2-KO HT1080 cells, but the substitution of D251, a conserved amino acid in MetAP2, failed to rescue the VM. Collectively, our results demonstrate that MetAP2 is critical for VM in human cancer cells and suggest fumagillin and TNP-470 as potent VM-suppressing agents.

Roles of P67/MetAP2 as a tumor suppressor

A precise balance between growth promoting signals and growth inhibitory signals plays important roles in the maintenance of healthy mammalian cells. Any deregulation of this critical balance converts normal cells into abnormal or cancerous cells. Several macromolecules are being identified and characterized that are involved in the regulation of cell signaling pathways that connect to the cell cycle and thus they play roles as tumor promoters or tumor suppressors. In situ tumor formation needs active angiogenesis, a process that generates new blood vessels from existing ones either by splitting or sprouting. Several small molecule inhibitors and proteins have been identified as inhibitors of angiogenesis. One such protein, p67/MetAP2 also known as methionine aminopeptidase 2 (MetAP2), has been shown to bind covalently to fumagillin and its derivatives that have anti-angiogenic activity. In addition to fumagillin or its derivatives, several other small molecule inhibitors of p67/MetAP2 have been recently identified and some of these drugs are in phase III trials for cancer therapy. Although molecular details of actions toward tumor suppression by these drugs are largely unknown, a significant progress has been made to understand the structure-function relationship of p67/MetAP2 and its roles in the maintenance of the levels of phosphorylation of the proportional, variant-subunit of eukaryotic initiation factor 2 (eIF2 proportional, variant) and extracellular signal-regulated kinases 1 and 2 (ERK1/2). In this article, roles of p67/MetAP2 in the suppression of cancer development are also discussed.

Impact of protein kinase PKR in cell biology: from antiviral to antiproliferative action

The double-stranded RNA-dependent protein kinase PKR is a critical mediator of the antiproliferative and antiviral effects exerted by interferons. Not only is PKR an effector molecule on the cellular response to double-stranded RNA, but it also integrates signals in response to Toll-like receptor activation, growth factors, and diverse cellular stresses. In this review, we provide a detailed picture on how signaling downstream of PKR unfolds and what are the ultimate consequences for the cell fate. PKR activation affects both transcription and translation. PKR phosphorylation of the alpha subunit of eukaryotic initiation factor 2 results in a blockade on translation initiation. However, PKR cannot avoid the translation of some cellular and viral mRNAs bearing special features in their 5' untranslated regions. In addition, PKR affects diverse transcriptional factors such as interferon regulatory factor 1, STATs, p53, activating transcription factor 3, and NF-kappaB. In particular, how PKR triggers a cascade of events involving IKK phosphorylation of IkappaB and NF-kappaB nuclear translocation has been intensively studied. At the cellular and organism levels PKR exerts antiproliferative effects, and it is a key antiviral agent. A point of convergence in both effects is that PKR activation results in apoptosis induction. The extent and strength of the antiviral action of PKR are clearly understood by the findings that unrelated viral proteins of animal viruses have evolved to inhibit PKR action by using diverse strategies. The case for the pathological consequences of the antiproliferative action of PKR is less understood, but therapeutic strategies aimed at targeting PKR are beginning to offer promising results.

The binding between p67 and eukaryotic initiation factor 2 plays important roles in the protection of eIF2α from phosphorylation by kinases

Phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 is the major regulatory step in the initiation of protein synthesis in mammals. P67, a cellular glycoprotein, protects phosphorylation of eIF2alpha from kinases. Previously, we reported that the D6/2 mutant of p67 has higher levels of protection of eIF2alpha phosphorylation (POEP) activity. In this study, we report that the D6/2 mutant and its double mutants containing second-site alanine substitutions at the five conserved amino acid residues (D251, D262, H331, E364, and E459) show increased POEP activity in serum-starved rat tumor hepatoma cells. Serum-restoration to those cells did not abolish their increased POEP activity except the D6/2+H331A double mutant. The latter mutant shows slight inhibition of POEP activity during serum starvation and this inhibition increased significantly during serum restoration. KRC-7 cells constitutively expressing the D6/2 mutant showed slightly decreased levels of PKR phosphorylation and significantly low level of phosphorylation of ERKs 1 and 2. The D6/2 mutant also showed increased binding with eIF2alpha and eIF2gamma and almost similar binding with ERKs 1 and 2 as compared to wild type p67. Altogether, our data demonstrate that the increased binding of the D6/2 mutant with the subunits of eIF2 may be in part the cause for its high POEP activity.

Evidence of a dominant negative mutant of yeast methionine aminopeptidase type 2 in Saccharomyces cerevisiae

Eukaryotic methionine aminopeptidase type 2 (MetAP2, MetAP2 gene (MAP2)), together with eukaryotic MetAP1, cotranslationally hydrolyzes initiator methionine from nascent polypeptides when the side chain of the second residue is small and uncharged. In this report, we took advantage of the yeast (Saccharomyces cerevisiae) map1 null strain's reliance on MetAP2 activity for the growth and viability to provide evidence of the first dominant negative mutant of eukaryotic MetAP2. Replacement of the conserved His(174) with alanine within the C-terminal catalytic domain of yeast MetAP2 eliminated detectable catalytic activity against a peptide substrate in vitro. Overexpression of MetAP2 (H174A) under the strong GPD promoter in a yeast map1 null strain was lethal, whereas overexpression under the weaker GAL1 promoter slightly inhibited map1 null growth. Deletion mutants further revealed that the N-terminal region of MetAP2 (residues 2-57) is essential but not sufficient for MetAP2 (H174A) to fully interfere with map1 null growth. Together, these results indicate that catalytically inactive MetAP2 is a dominant negative mutant that requires its N-terminal region to interfere with wild-type MetAP2 function.

Regulation of the transcription factor, CTCF, by phosphorylation with protein kinase CK2

CTCF is a transcription factor involved in various aspects of gene regulation. We previously reported that CTCF function is modulated by protein kinase CK2. In this report we investigate further the role of CK2 in regulating the transcriptional properties of CTCF. We demonstrate that coexpression of CTCF with CK2 switches function of CTCF from repressor to activator. The non-phosphorylatable mutant increases repression by CTCF and potentiates the growth-suppressive ability of the protein, whereas the phospho-mimetic mutant behaves in the opposite fashion. Mutation of the individual serines reveals that Serine 612 is a critical residue in regulation of CTCF by CK2.

Treatment of cells with the angiogenic inhibitor fumagillin results in increased stability of eukaryotic initiation factor 2-associated glycoprotein, p67, and reduced phosphorylation of extracellular signal-regulated kinases

Fumagillin, an angiogenic inhibitor, binds to methionine aminopeptidase 2, which is the same as eukaryotic initiation factor 2-associated glycoprotein, p67. p67 protects eIF2alpha from phosphorylation by its kinases. To understand the importance of fumagillin binding to p67, we measured the level of p67 in mouse C2C12 myoblasts treated with fumagillin. We show that fumagillin increases the stability of p67 by decreasing its turnover rate. The increased levels of p67 result in inhibition of phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERKs 1 and 2). p67 binds to these ERKs, and the 108-480 amino acid segment is sufficient for this binding. p67's affinity to ERKs 1 and 2 also increases in fumagillin-treated myoblasts while its affinity for eIF2alpha remains unchanged. A mutant at the conserved amino acid residue D251A increases the phosphorylation of ERKs 1 and 2 without affecting the binding to p67, thus indicating the importance of this residue in the regulation of the phosphorylation of these ERKs. These results suggest that fumagillin increases the stability of p67 and its affinity to ERKs 1 and 2 and causes the inhibition of the phosphorylation of ERKs 1 and 2.

Eukaryotic initiation factor 2-associated glycoprotein, p67, shows differential effects on the activity of certain kinases during serum-starved conditions

Phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 is the major regulatory step in the initiation of protein synthesis in mammals. P67, a cellular glycoprotein, protects phosphorylation of eIF2alpha from kinases. P67 has five conserved amino acid residues at the D251, D262, H331, E364, and E459 positions. To determine the roles of these conserved amino acid residues in eIF2alpha phosphorylation during serum-starved conditions, we constitutively expressed D251A, D262A, H331A, E364A, and E459A mutants in rat tumor hepatoma cells. We find that the point mutants D251A, H331A, and E364A lower the levels of eIF2alpha phosphorylation. These low levels of phosphorylation decrease when serum-starved cells are grown in medium containing serum. To understand the mechanism of action of the p67 mutants in eIF2alpha phosphorylation during serum-starvation, we performed detailed biochemical analyses with the D251A mutant. We find that neither the O-GlcNAc modification on the D251A mutant nor the binding of D251A mutant with eIF2gamma has significant effects on eIF2alpha phosphorylation during serum-starved conditions. However, the D251A mutant inhibits p67's activity to suppress the activity of ERK1/2. Our data suggest that both p67 and the D251A mutant bind to ERK1, thus strengthening the idea that p67 regulates the activity of ERK1. During serum-starvation conditions, both PKR and PERK are phosphorylated and the D251A mutant shows increased stability of PERK as well as a slight decrease in its activity. Altogether, our data provide evidence to suggest that p67 modulates the expression and activity of certain eIF2alpha-specific kinases.