Serine-threonine Phosphatase Inhibitor Research Articles

Induction of CD45 Expression and Proliferation in U-266 Myeloma Cell Line by Interleukin-6

Recently, there has been an increasing interest in the expression pattern and biological significance of the CD45 molecule in myeloma cells. In this study, we have further defined the phenotypic pattern of CD45 expression on myeloma cells. Using a panel of myeloma cell lines, we showed that CD45 showed a remarkably heterogeneous pattern of expression. Whereas some cell lines were CD45(+) and others were CD45(-), the U-266 cell line, although predominantly CD45(-), still had a considerable subpopulation of CD45(+) cells. Among the myeloma cell lines examined, there was a direct correlation between interleukin-6 (IL-6) dependency and CD45 positivity. Moreover, we showed that IL-6 stimulation led to the induction of expression of CD45 and cellular proliferation. Using independent experimental approaches, we could show that the IL-6-induced effects were closely linked to CD45 expression. First, sorting out CD45(+) and CD45(-) subsets of U-266 cell line followed by IL-6 stimulation, only the CD45(+) cells showed a proliferative advantage after IL-6 stimulation. Second, IL-6 stimulation of sorted CD45(-) cells was gradually followed by phenotypic conversion to CD45(+) cells that started after 2 days as judged by the detection of CD45 mRNA by reverse transcription polymerase chain reaction (RT-PCR) and immunophenotypic analysis by flow cytometry. Withdrawal of IL-6 from the medium led to gradual loss of CD45 expression in CD45(+) flow-sorted U-266 cells. Third, the use of vanadate, a potent inhibitor of protein tyrosine phosphatase (PTP), abrogated the IL-6-induced proliferation in the CD45(+) myeloma cells. On the other hand, cellular proliferation induced by IL-6 was not affected by the serine-threonine phosphatase inhibitor okadaic acid. Our data show that the expression pattern of CD45 in myeloma cell lines is heterogeneous and show for the first time that CD45 expression can be induced by IL-6 stimulation. Finally, these data shed some light on the biological role of CD45 in myeloma by determining the proliferative population among myeloma cells.

A Protein Phosphatase Is Involved in the Inhibition of Histone Deacetylation by Sodium Butyrate

Treatment of cells with sodium butyrate is known to increase histone acetylation by inhibiting deacetylases. Here we have observed, in cultured hepatoma cells, that the potent serine-threonine phosphatase inhibitors, okadaic acid or calyculin A, inhibited phosphatase activity and concomitantly decreased the histone acetylation classically maintained by sodium butyrate. These results suggest that a protein phosphatase may mediate the sodium butyrate effect on deacetylases. Since we have previously found that such a protein would also mediate the sodium butyrate effect on gene expression, we propose that a phosphatase activity constitutes an early and essential step in the sodium butyrate-triggered signalling pathway.

Stimulation of membrane serine-threonine phosphatase in erythrocytes by hydrogen peroxide and staurosporine.

Indirect evidence has suggested that K-Cl cotransport in human and sheep erythrocytes is activated physiologically by a serine-threonine phosphatase. It is activated experimentally by H2O2 and by staurosporine, a kinase inhibitor. Activation by H2O2 and staurosporine is inhibited by serine-threonine phosphatase inhibitors, suggesting that the activators stimulate the phosphatase. The present study shows that sheep and human erythrocytes contain membrane-associated as well as cytosolic serine-threonine phosphatases, assayed from the dephosphorylation of 32P-labeled glycogen phosphorylase. In cells from both species, the relatively low sensitivity of the membrane enzyme to okadaic acid suggests it is type 1 protein phosphatase. The cytosolic phosphatase was much more sensitive to okadaic acid. Membrane-associated phosphatase was stimulated by both H2O2 and staurosporine. The results support earlier conclusions that the membrane-associated type 1 phosphatase identified here is regulated by phosphorylation and oxidation. The results are consistent with the phosphatase, or a portion of it, being responsible for activating K-Cl cotransport.

Involvement of phosphorylation in doxorubicin-mediated myofibril degeneration. An immunofluorescence microscopy analysis.

Loss of myofilaments has been observed in both adaptive cardiac responses (i.e., hypertrophy) as well as in chemotheraputic use of antineoplastic drugs with cardiotoxic side effects (i.e., doxorubicin). An understanding of the degenerative process is a prerequisite for determining approaches to limit the cardiomyopathic changes associated with chronic heart disease or long-term chemotheraputic treatments. However, little is known about the specific events and molecular changes that initiate the degenerative process. To study this process, neonatal rat cardiomyocytes were treated with doxorubicin, which induced rapid and widespread thin-filament degeneration as observed by fluorescence confocal microscopy. Which demonstrated deterioration of sarcomeric thin-filament structure. Changes in the spontaneous beating of cardiomyocytes corresponding with myofibrillar degeneration were apparent using differential interference contrast video microscopy. After finding induction of kinase activity by doxorubicin in cultured cardiomyocytes, the protective effects of specific inhibitors of kinase activity were assessed for their ability to inhibit doxorubicin-induced myofibrillar break-down. Doxorubicin-induced changes appeared similar to the degeneration observed after treatment with a protein kinase activator (phorbol 12-myristate 13-acetate) or a serine-threonine protein phosphatase inhibitor (okadaic acid). Collectively, these results indicate that activation of protein kinase is an important event in the initiation of myofibrillar degeneration by doxorubicin. Further analyses of myofibrillar proteins with respect to biochemical modifications will be necessary to determine if phosphorylation events transmit signal(s) to initiate degeneration.

Synthesis of functionalised cyclic pentapeptide analogues of the serine-threonine protein phosphatase inhibitor nodularin

A generic synthesis of cyclic peptidic analogues of nodularin incorporating suitable functionality for synthetic elaboration is described, providing access to new protein phosphatase inhibitors.

C-Jun Is a Downstream Target for Ceramide-activated Protein Phosphatase in A431 Cells

Stimulation of [3H]serine-labeled A431 cells with tumor necrosis factor-alpha (TNFalpha) or bacterial sphingomyelinase (SMase) resulted in a rapid decrease (approximately 50% by 15 min) in cellular [3H]sphingomyelin content and generation of the lipid moiety [3H]ceramide, which remained elevated 60 min later. Sphingomyelin hydrolysis in response to TNFalpha or bacterial SMase resulted in a time-dependent decrease in the phosphorylation state of c-Jun protein, an effect that was also observed in cells treated with the membrane-permeable ceramide analogue N-hexanoylsphingosine (C6-ceramide). The rapid dephosphorylation of the c-Jun gene product in response to TNFalpha, SMase, or C6-ceramide was not observed in A431 cells treated with the serine-threonine phosphatase inhibitor okadaic acid. After the initial steps of previously described methods for the purification of a ceramide-activated protein phosphatase termed CAPP (Dobrowsky, R. T., Kamibayashi, C., Mumby, M. C., and Hannun, Y. A. (1993) J. Biol. Chem. 268, 15523-15530), we obtained a cytosolic fraction from A431 cells that specifically dephosphorylated 32Pi-labeled c-Jun protein used as substrate in an immunocomplex phosphatase assay. Phosphatase activity in vitro was apparent only in the presence of ceramide (5 micro) and was specifically abrogated when okadaic acid (1 n) was included in the immunocomplex phosphatase assay. These results provide strong evidence for c-Jun as a downstream target for CAPP activated in response to post-TNF signaling in A431 cells.

The Phosphatase Inhibitor Calyculin Antagonizes the Rapid Initiation of Apoptosis by Photodynamic Therapy

DNA fragmentation and internucleosomal cleavage were rapidly initiated after lysosomal photodamage to murine leukemia cells, with apoptotic chromatin and DNA `ladders' detected within 30 min after irradiation. Apoptosis was inhibited by concurrent exposure of cells to a 10 nM concentration of the serine-threonine phosphatase inhibitor calyculin A and promoted by a serine/threonine kinase inhibitor. These results indicate that a late stage in apoptosis requires serine/threonine dephosphorylation.

Total Synthesis of the Serine-Threonine Phosphatase Inhibitor Microcystin-LA

Reversible protein phosphorylation, which is mediated by kinases and phosphatases, is a major control element of the cell. There is a diverse group of toxic natural products that inhibit certain phosphatases, thereby disrupting normal biochemical pathways. These toxins can be useful for dissecting the individual biochemical pathways associated with each of these enzymes. This Article describes the first total synthesis of one such toxin, the cyclic heptapeptide microcystin-LA. The synthesis features a convergent route that is amenable to analog preparation in the search for selective phosphatase inhibitors. A new route to the unusual amino acid Adda is described, which incorporates an efficient diastereoselective aspartate alkylation and diene synthesis via a Suzuki coupling reaction. This work also features an efficient preparation of an N-methylalanine containing peptide via a Horner−Emmons condensation and several difficult amino acid coupling reactions that relied heavily on Carpino's remarkable HATU ...

Role of protein phosphorylation in TNF-induced apoptosis: phosphatase inhibitors synergize with TNF to activate DNA fragmentation in normal as well as TNF-resistant U937 variants.

This study examined the role of protein phosphorylation in TNF induction of apoptosis in several tumor cell lines by testing the effects of agents that either stimulate or inhibit protein phosphorylation. The serine-threonine phosphatase inhibitors, okadaic acid (OKA) and calyculin A (CLA), synergistically augmented TNF-induced apoptosis in several TNF-sensitive tumor cell lines including the U937 histiocytic lymphoma, the BT-20 mammary carcinoma, and the LNCap prostatic tumor cell line. Furthermore, the phosphatase inhibitors completely reversed the TNF resistance of a variant (U9-TR) derived from U937. CLA also inhibited phosphatase activity in cell-free extracts from both U937 and U9-TR at the same concentrations (0.4-2.0 nM) that it synergized with TNF. In contrast, TNF treatment of U937 cells did not result in inhibition of phosphatase activity mediated by protein phosphatase 1 (PP1) and PP2A in cell extracts. Since the phosphatase inhibitors are known to increase the overall levels of protein phosphorylation in cells, this suggested that TNF may act by stimulating protein kinase (PK) activity. This hypothesis was supported by the results of testing a panel of relatively specific protein kinase inhibitors. TNF activation of DNA fragmentation was blocked by a potent inhibitor of myosin light chain kinase (MLCK) but was unaffected by inhibitors of cAMP or cGMP-dependent PKs. We postulate that a defect in the activation of MLCK or possibly some other as yet unknown PK may be responsible for the TNF resistance of U9-TR. Furthermore, this resistance may be circumvented by promoting protein phosphorylation with the serine-threonine-dependent phosphatase inhibitors.

K-Cl cotransport in rabbit red cells: further evidence for regulation by protein phosphatase type 1

We have examined inhibition of swelling-induced K-Cl cotransport in rabbit red blood cells by calyculin A, a potent serine-threonine protein phosphatase inhibitor, to determine whether transport is regulated by phosphatase type 1 or type 2A. Calyculin A blocks K(Rb) influx [half-maximal inhibitory concentration (IC50) = 3-6 nM] 10 times more potently than a second phosphatase inhibitor, okadaic acid (IC50 = 40 nM), consistent with earlier pharmacological studies showing that calyculin A inhibits phosphatase type 1 10 times more effectively than does okadaic acid. Calyculin A always inhibits Rb influx when added either before or after cell swelling, indicating that the phosphatase must operate continually to first activate and then maintain high transport rates in swollen cells. Similarly, N-ethylmaleimide (NEM) fails to stimulate K-Cl cotransport only when added to cells pretreated with calyculin A. Therefore, like cell swelling, activation of K-Cl cotransport by NEM involves a phosphatase sensitive to calyculin A. We conclude that cell swelling and NEM activate K-Cl cotransport via a net dephosphorylation that appears to involve protein phosphatase type 1.