The role of the nuclear Akt activation and Akt inhibitors in all-trans-retinoic acid-differentiated HL-60 cells

The pharmacological inhibitors of phosphoinositide 3-kinase (PI3K)/Akt pathway have been proposed in the treatment of leukemia based on their antiproliferative effects. However, several studies demonstrated the activation of PI3K in the nuclei of all-trans-retinoic acid (ATRA) - differentiated HL-60 cells, raising the possibility that PI3K/Akt-inhibitors may block antitumor properties of retinoids. The aim of the present study was to investigate the possible activation of nuclear Akt in ATRA-treated cells and to test the effects of Akt-inhibitors on ATRA-mediated differentiation. The Akt-activity was found to be increased in the nuclei and lysates of ATRA-differentiated HL-60 and NB4 cells. The down-modulation of the expression of Akt protein in HL-60 cells using siRNA reduces the CD11b expression in ATRA-treated cells. The treatment of both cell lines with the commercially available Akt inhibitors inhibited the growth of both control and ATRA-treated cells. Akt-inhibitors had no inhibitory effects on ATRA-mediated growth arrest and the expression of CD11b in HL-60 cells, but increased the percentage of control cells expressing CD11b. In contrast, the presence of Akt inhibitors reduced the expression of CD11b in ATRA-treated NB4 cells.

Stimulation of T-cell proliferation by pancreastatin and its C-terminal fragment (33-49)

We have studied the effect of pancreastatin and its C-terminal fragment (33-49) on mitogen-stimulated T lymphocyte proliferation. In a concentration range from 10(-12) to 10(-8) M they exhibit a dose-dependent stimulatory effect on concanavalin A-induced response with the maximal effect at 10(-8) M concentration. They were inactive in response to a B-cell mitogen, lipopolysaccharide, which points to an involvement of T but not B lymphocytes in their response. Pancreastatin can still produce a stimulatory effect when added 18 h after incubation of cultures with concanavalin A and apparently uses a diacylglycerol independent mechanism. When cells were preincubated for 4, 16 or 24 h with pancreastatin or its fragment and then stimulated with concanavalin A, a ten times lower concentration of peptides was needed (10(-9) M) to obtain the maximal response. This suggests that resting cells are more sensitive to pancreastatin and its fragment. Both peptides exhibit a very similar pharmacological profile, indicating that the C-terminal part of the molecule is responsible for the effect on T-cell proliferation.

Neutrophil signal transduction in Met-enkephalin modulated superoxide anion release

The present study explored the involvement of signal transduction system(s) in Met-enkephalin (MENK) modulated superoxide anion (O2-) release from human neutrophils. This opioid pentapeptide stimulated the O2- release in all samples if present at 10(-8) M concentration while in lower concentrations the stimulatory concentration was donor-dependent. The most abundant product of MENK degradation, Tyr-Gly-Gly (TGG), suppressed O2- release over a wide range of concentrations (10(-12)-10(-8) M). MENK induced O2- release was associated with a dose-dependent increase of diacylglycerol (DAG) concentration and protein-kinase C (PKC) translocation to the neutrophil membranes, with an increase of cytosolic Ca++, and could be abolished by H7, a PKC inhibitor. On the contrary, the suppressive effect of TGG was not associated with alteration of DAG concentration in neutrophil membranes. Superoxide anion release induced by low concentrations of MENK (10(12)-10(-10) M), could be blocked by NDGA, an inhibitor of the lipoxygenase pathway. We concluded that MENK-induced O2- release results mainly due to DAG/PKC pathway activation, although other secondary messengers might be involved.

Acetylcholine and cholecystokinin receptors functionally couple by different G-proteins to phospholipase C in pancreatic acinar cells

We have studied the involvement of GTP-binding proteins in the stimulation of phospholipase C from rat pancreatic acinar cells. Pretreatment of permeabilized cells with activated cholera toxin inhibited both cholecystokinin-octapeptide (CCK-OP) and GTP gamma S but not carbachol (CCh)-induced production of inositol trisphosphate. Pertussis toxin had no effect. Neither vasoactive intestinal polypeptide, a stimulator of adenylyl cyclase, nor the cAMP-analogue, 8-bromo cAMP, mimicked the inhibitory effect of cholera toxin on agonist-induced phospholipase C activation. This indicates that inhibition by cholera toxin could not be attributed to a direct interaction of cholera toxin activated Gs with phospholipase C or to an elevation of cAMP. In isolated rat pancreatic plasma membranes cholera toxin ADP-ribosylated a 40 kDa protein, which was inhibited by CCK-OP but not by CCh. We conclude from these data that both CCK- and muscarinic acetylcholine receptors functionally couple to phospholipase C by two different GTP-binding proteins.