Protein kinase C activity during sphinganine potentiation of retinoic acid-induced differentiation in a human leukemia cell line (HL-60)

Role of G protein and protein kinase signalling in influenza virus budding in MDCK cells

Recently, we have shown that influenza virus budding in MDCK cells is regulated by metabolic inhibitors of ATP and ATP analogues (Hui & Nayak, Virology 290, 329-341, 2001 ). In this report, we demonstrate that G protein signalling stimulators such as sodium fluoride, aluminium fluoride, compound 48/80 and mastoparan stimulated the budding and release of influenza virus. In contrast, G protein signalling blockers such as suramin and NF023 inhibited virus budding. Furthermore, in filter-grown lysophosphatidylcholine-permeabilized virus-infected MDCK cells, membrane-impermeable GTP analogues, such as guanosine 5'-O-(3-thiotriphosphate) or 5'-guanylylimidodiphosphate caused an increase in virus budding, which could be competitively inhibited by adding an excess of GTP. These results suggest that the G protein is involved in the regulation of influenza virus budding. We also determined the role of different protein kinases in influenza virus budding. We observed that specific inhibitors or activators of protein kinase A (H-89 and 8-bromoadenosine 3',5'-cyclic monophosphate) or of protein kinase C (bisindolylmaleimide I and Ro-32-0432) or of phosphatidylinositol 3-kinase (LY294002 and wortmannin) did not affect influenza virus budding. However, the casein kinase 2 (CK2) inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole decreased virus budding. We further observed an increase in the CK2 activity during the replication cycle of influenza virus, although Western blot analysis did not reveal any increase in the amount of CK2 protein in virus-infected cells. Also, in digitonin-permeabilized MDCK cells, the introduction of CK2 substrate peptides caused a down-regulation of virus budding. These results suggest that CK2 activity also regulates influenza virus budding.

Differential Cellular Distribution of Retinoic Acid during Staurosporine Potentiation of Retinoic Acid-Induced Granulocytic Differentiation in Human Leukemia HL-60 Cells

Pretreatment of cells with staurosporine, a protein kinase C (PKC) inhibitor, was found to potentiate the granulocytic differentiation induced by a brief (2 h) retinoic acid treatment. By cell cycle analysis, staurosporine was found to have little effect on the cell cycle. Retinoic acid was distributed equally in the nuclei (40%) and in the plasma membrane (40%) of staurosporine-pretreated cells while less than 20% of retinoic acid was found in the membrane of control non-staurosporine-pretreated cells during the retinoic acid-induced differentiation. These results indicate that the enhancing effect of staurosporine may be somehow associated with the localization of retinoic acid in the plasma membrane of the cell. Copyright 1995 S. Karger AG, Basel

Sphinganine potentiation of cellular differentiation induced by various anti-leukemia drugs in human leukemia cell line HL-60

A slight induction of cellular differentiation (myelocytes and granulocytes) of HL-60 cells occurred after treatment with anti-tumor agents etoposide (VP-16), mitoxantrone (MXT), mitomycin C (MMC), actinomycin D (Act-D) or novobiocin (NOVO). Addition of sphinganine (SP), an inhibitor of protein kinase C (PKC) enhanced (2-3 fold) the VP-16, MXT, MMC or Act-D-induced differentiation but not the NOVO-induced differentiation. No induction of differentiation was observed with 5-fluorouracil (5-FU) in the absence or presence of SP. The addition of SP in the fresh medium after the removal of VP-16, MXT, or MMC (0.5 h treatment) enhanced the induction of differentiation. In contrast, SP post-treatment did not have any effect on enhancing the differentiation which was induced by Act-D short exposure (0.5 h). In an attempt to characterize the biochemical requirements for potentiation of VP-16-induced differentiation, we examined the effects of calcium depletion using calcium chelator ethylene glycol-bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) or calcium channel blocker verapamil. Potentiation of VP-16-induced differentiation by SP was not observed in EGTA- or verapamil-treated cells. Calcium supplementation to the cells during the treatment with EGTA restored the SP-potentiation of VP-16-induced differentiation. Our results also showed that the induction of differentiation was accompanied by a decrease in PKC activity (70% of the control). PKC activity decreased to a greater extent (50% of control) in SP potentiation of differentiation induction. Our results suggested that calcium-dependent biological action of antitumor agents and the inhibition of PKC activity are required for SP-potentiation of differentiation induction.

Cell cycle phase-dependent effect of retinoic acid on the induction of granulocytic differentiation in HL-60 promyelocytic leukemia cells. Evidence for sphinganine potentiation of retinoic acid-induced differentiation

The efficiency of retinoic acid (RA)-induced differentiation was dependent on the position of HL-60 cells in the cell cycle. Our results demonstrated that cells at the G1/S border were more efficiently induced to differentiate by short exposure to RA than cells at other phases of the cell cycle. Synchronization of cells in G1/S phase by aphidicolin (APH) or mimosine (MIMO) increased the sensitivity of cells to RA short exposure treatment. Pretreatment with sphinganine (SP), a protein kinase C (PKC) inhibitor, potentiated RA-induced cell differentiation. By cell cycle analysis, SP was found to block the cell progression through the G1/S phase. Consequently, cells accumulated in the G1/S phase of the cell cycle. The present data therefore suggest a possible mechanism of action of SP to enhance RA-induced differentiation.

Alkaline phosphatase activity during sphinganine potentiation of retinoic acid-induced differentiation of human promyelocytic leukemia cell line, HL-60

Sphinganine (SP) pre-treatment potentiated the retinoic acid (RA)-induced (4-96h exposures) differentiation and increase of alkaline phosphatase (ALP) activity. A higher percentage of SP pre-treated cells in RA exposures resembled mature myelocytes or granulocytes; greater increase in ALP activity was observed. In cells exposed to RA alone for only a period of 24h, the ALP activity could still increase and reach a similar maximum ALP activity (8.5-10.0 units/mg protein) at 48h as it was under continuous RA treatment. In all cells with longer exposures (24-96h) to RA, SP pre-treatment increased ALP activity to more or less the same higher maximum (14.0-15.5 units/mg protein). SP, added 24h before or concomitantly, but not 24 nor 48h after the addition of RA, could potentiate the RA-induced differentiation and increase of ALP activity.