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- Brattsand, G, et al.
(author)
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Quantitative analysis of the expression and regulation of an activation-regulated phosphoprotein (oncoprotein 18) in normal and neoplastic cells.
- 1993
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In: Leukemia. - 0887-6924 .- 1476-5551. ; 7:4, s. 569-79
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Journal article (peer-reviewed)abstract
- Activation of protein kinase C results in phosphorylation of a 19-kDa protein termed 19K. Isolation and sequence analysis of a cDNA encoding the 19K protein revealed that this protein has been studied in other systems under different names. The name oncoprotein 18 (Op18) has been proposed on the basis of a postulated up-regulation in neoplastic cells. In the present report we adopt the designation Op18 for the 19K protein, and quantify this phosphoprotein in a series of leukemia/lymphoma cell lines, a panel of non-transformed cells and some terminally differentiated cell types. For this purpose we have developed reagents allowing quantitative Western-blot analysis, and quantification of Op18 on the single cell level by flow cytometric analysis. The data demonstrates a pronounced up-regulation of the Op18 protein in most leukemia/lymphoma cell lines. The HPB-ALL cell line provided the most extreme case and expressed 7 x 10(6) Op18 molecules/cell, which compares with 0.65 x 10(6) Op18 molecules/cell in non-transformed lymphoblastoid cells. The expression of Op18 appears to be restricted to cell types with proliferative potential, but it is clear from our results that up-regulation of Op18 is uncoupled from cellular proliferation. Moreover, by employing an Epstein-Barr virus based shuttle vector, we expressed Op18 cDNA in lymphoblastoid cells. This resulted in a three to fourfold up-regulation of Op18 that did not have any detectable consequences for cell-surface phenotype or cell size. However, increased expression of Op18 resulted in a partial inhibition of cell proliferation. Taken altogether, the results suggest that up-regulation Op18 levels in leukemia/lymphoma cells are strongly associated with, but not a direct cause of tumour progression.
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| 2. |
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| 3. |
- Marklund, U, et al.
(author)
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Serine 25 of oncoprotein 18 is a major cytosolic target for the mitogen-activated protein kinase.
- 1993
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In: Journal of Biological Chemistry. - 0021-9258 .- 1083-351X. ; 268:20
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Journal article (peer-reviewed)abstract
- Oncoprotein 18 (Op18) is an 18-19-kDa cytoplasmic phosphoprotein, of unknown function, that is frequently up-regulated in transformed cells. Stimulation of various cell-surface receptors results in extensive phosphorylation of Op18 and this protein has, therefore, previously been implicated in intracellular signaling. In the present study, by expression of specific Op18 cDNA mutant constructs and phosphopeptide mapping, we have identified in vivo phosphorylation sites. In conjunction with in vitro phosphorylation experiments, using purified wild-type and mutant Op18 proteins in combination with a series of kinases, these results have identified two distinct proline-directed kinase families that phosphorylate Op18 with overlapping but distinct site preference. These two kinase families, mitogen activated protein (MAP) kinases and cyclin dependent cdc2 kinases, are involved in receptor and cell cycle-regulated phosphorylation events, respectively. Therefore, Op18 may reside at a junction where receptor and cell cycle-regulated kinase families interact with the same substrate. The present study shows that the MAP kinase has a 20-fold preference for Ser25 as opposed to Ser38 of Op18, while cdc2 kinases have a 5-fold preference for the Ser38 residue. Only a minor fraction of the 4.5 x 10(6) Op18 molecules/cell in a leukemic T-cell line are normally in their Ser25 phosphorylated form. However, antigen receptor stimulation of this cell line is shown to result in a rapid conversion of 35-45% of all Op18 molecules to the Ser25 phosphorylated form. These results suggest that Ser25 of Op18 may be a major cytoplasmic target for the MAP kinase in cells with high expression of Op18.
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