Protein kinase C-beta, fibronectin, alpha(5)beta(1)-integrin, and tumor necrosis factor-alpha are required for phorbol diester-induced apoptosis in human myeloid leukemia cells.

The human myeloid HL-60 cell line and its cell variant HL-525 were used to study signaling events leading to apoptosis induction by phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC) enzymes. Unlike parental cells, HL-525 cells are PKC-beta deficient and resistant to PMA-induced apoptosis. These cells regain susceptibility to apoptosis induction after transfection with a PKC-beta expression vector. By using this vector and specific neutralizing monoclonal antibodies (mAbs), it was established that PMA-induced apoptosis also called for an interaction between cell-surface alpha(5)beta(1)-integrin and its deposited ligand fibronectin (FN), which is downstream of PKC-beta activation. Experiments with mAbs, the PKC-beta vector, and exogenous FN revealed that the next step entailed an interaction between secreted tumor necrosis factor-alpha and its type I receptor. By using a sphingomyelinase inhibitor, it was concluded that the subsequent step involved ceramide production. Moreover, a permeable ceramide was effective in inducing apoptosis in both HL-60 and HL-525 cells, and this induction was caspase-1 and/or -4 dependent because an inhibitor of these caspases abrogated the induced apoptosis. Based on these and related differentiation studies, we conclude that the above signaling events, the early ones in particular, are shared with PMA-induced macrophage differentiation in the HL-60 cells. It is likely that once these cells acquire their macrophage phenotype and perform their tasks, they become superfluous and are eliminated from the body by a self-triggered apoptotic process that involves our proposed signaling scheme.     

JAK-STAT signaling involved in phorbol 12-myristate 13-acetate- and dimethyl sulfoxide-induced 2'-5' oligoadenylate synthetase expression in human HL-60 leukemia cells

The JAK-STAT signal transduction cascade participates in various cellular processes, including immune response, cell replication, differentiation and oncogenesis. Here, we report that this cascade is induced in two human myeloid HL-60 leukemia cell variants by the granulocyte differentiation inducer dimethyl sulfoxide (DMSO) and macrophage differentiation inducer phorbol 12-myristate 13-acetate (PMA). DMSO and PMA also induced the expression and catalytic activity of 2'-5' oligoadenylate synthetase (2-5A synthetase), a known interferon (IFN) inducible enzyme. The HL-60 cell variants included HL-205, which is susceptible to DMSO- and PMA-induced differentiation, and HL-525, which is susceptible to DMSO- but not to PMA-induced differentiation. Treatment of HL-205 and HL-525 cells with DMSO and HL-205 cells with PMA-induced JAK1 phosphorylation, JAK1/STAT1 association, formation of STAT1-STAT2 heterodimers, and the binding of the active IFN stimulating growth factor 3 (ISGF3) to the IFN-stimulated response element (ISRE) fragment isolated from the 2-5A synthetase promoter. These events were either reduced or absent in the resistant HL-525 cells treated with PMA. Taken together, our data implicate the above signaling cascade in DMSO- and PMA-induced 2-5A synthetase expression and catalytic activity in the HL-60 cell system.     

Up regulation of the phorbol ester receptor-protein kinase C in HL-60 variant cells

The human promyelocytic leukemia cell line HL-60 can be induced to differentiate into macrophage-like cells by nanomolar concentrations of phorbol esters. A phorbol ester-resistant variant R1B6 obtained by culturing HL-60 cells with increasing concentrations of 12-O-tetradecanoylphorbol-13-acetate, is reversibly resistant. These cells have been growing continuously in the presence of phorbol esters for more than 1 yr, but when the phorbol ester is removed, the cells gradually regain their sensitivity and express characteristics of macrophage-like cells upon readdition of phorbol ester. The concentration of phorbol ester receptors in R1B6 is about one-third that in the parental HL-60 cells. The reversion of the variants to sensitivity to phorbol esters is associated with the up regulation of the cytosol and membrane phorbol ester receptors. When partially purified, these receptor populations contain protein kinase C activity, in support of the identity of protein kinase C and the receptor. This study demonstrates that a phenotypic change in a clonal cell population correlates with the up regulation of the phorbol ester receptor-calcium-activated phospholipid-dependent protein kinase. This variant cell line is a useful model for analyzing the relationship between phorbol ester binding and protein kinase C during differentiation of HL-60 cells.     

Novobiocin- and phorbol-12-myristate-13-acetate-induced differentiation of human leukemia cells associated with a reduction in topoisomerase II activity

Studies were conducted to determine the possible involvement of DNA topoisomerase II (Topo II) in the induction of differentiation in two human promyelocytic HL-60 leukemia cell variants that are either susceptible or resistant to differentiation induced by phorbol-12-myristate-13-acetate (PMA), a protein kinase C activator. The acquisition of maturation markers and changes in the activity, level, and phosphorylation of Topo II were determined after treatment with either novobiocin, a Topo II inhibitor, or PMA. Novobiocin at 50-150 microM induced differentiation in the HL-205 cells but not in the HL-525 cells, although both cell types were equally susceptible to novobiocin-evoked cytotoxicity. In both cell types, novobiocin induced similar reductions in topoisomerase I activity but different reductions in Topo II activity. Treatment with novobiocin at 150 microM for 6 h or at 2 mM for 30 min resulted in a 4-fold or higher reduction in Topo II activity in the differentiation-susceptible HL-205 cells but not in the differentiation-resistant HL-525 cells. A differential response in Topo II activity was also observed after treatment with PMA. The novobiocin-evoked decrease in Topo II activity seems to be due to an enhanced enzyme proteolysis, whereas the PMA-elicited decrease in Topo II activity is associated with an increase in Topo II phosphorylation. 1-(5-Isoquinolinesulfonyl)-2-methylpiperazine, which is an inhibitor of protein kinases, including protein kinase C, diminished the novobiocin-elicited proteolysis of Topo II and the PMA-induced Topo II phosphorylation, as well as the decrease in Topo II activity and the acquisition of differentiation markers induced by either novobiocin or PMA. These results suggest that induction of differentiation in HL-60 cells by novobiocin or PMA is associated with a reduction in Topo II activity, mediated directly or indirectly by a protein kinase(s), perhaps protein kinase C.     

Inhibition of the chemotactic peptide-induced elevation of intracellular calcium in differentiated human leukemic (HL-60) cells by the phorbol ester phorbol 12-myristate 13-acetate

The chemotactic peptide formylmethionylleucylphenylalanine rapidly elevated the intracellular calcium concentration, in a concentration-dependent manner, of human leukemic (HL-60) cells which had been differentiated to polymorphonuclear leukocyte-like cells by pretreatment with dimethyl sulfoxide (1.3%). Preincubation of the cells with phorbol 12-myristate 13-acetate, a protein kinase C-activating phorbol ester, inhibited the formylmethionylleucylphenylalanine-induced rise in intracellular calcium in a time- and concentration-dependent manner. 4 alpha-Phorbol 12,13-didecanoate, which does not activate protein kinase C, was inactive in this capacity. The use of calcium-free medium suggested that the elevation of intracellular calcium by formylmethionylleucylphenylalanine consisted of rapid intracellular release followed by calcium influx. Phorbol 12-myristate 13-acetate (10(-7) M) inhibited both components of the elevation of intracellular calcium, whereas 10(-8) M phorbol 12-myristate 13-acetate inhibited only the calcium influx. The influx of calcium was not prevented by verapamil, which blocks the voltage-dependent calcium channels. These data suggest that, in differentiated HL-60 cells, 12-O-tetradecanoylphorbol-13-acetate rapidly inhibits both the intracellular release of calcium and calcium influx through non-voltage-dependent calcium channels in response to formylmethionylleucylphenylalanine.     

Decreased expression of type II protein kinase C in HL-60 variant cells resistant to induction of cell differentiation by phorbol diester

To evaluate the molecular basis for susceptibility of the cell differentiation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), we examined biochemical activities and expression of subspecies of protein kinase C from HL-60 cells that are susceptible to differentiation induced by TPA and HL-60R cells, HL-60 variant cells that are resistant to such induction. Analysis of the subcellular distribution of protein kinase C revealed that the activity of this kinase in the cytosol from HL-60R cells was 30% of that from parental HL-60 cells but that the enzyme activities in the membrane showed similar values in these cells. Treatment of HL-60 cells with 100 nM TPA for 30 min resulted in a 75% decrease in protein kinase C activity in the cytosol and a 300% increase in this activity in the membrane. A minor subcellular redistribution of the enzyme activity was found in HL-60R cells after TPA treatment. Based on analysis of protein kinase C isozymes by hydroxyapatite column chromatography, the relative activities of types I, II, and III in the cytosol of HL-60 cells were 11, 80, and 9%, whereas those in HL-60R cells were 27, 36, and 37%, respectively. Type II isozyme was a major protein kinase C in the cytosol of HL-60 cells, but type II was less in the HL-60R cells. Among the three isozymes, type II enzyme was most sensitive to TPA with histone H1 as the substrate, although all three isozymes were activated Ca2+-dependently in the presence of phosphatidylserine. We suggest that the acquired resistance of HL-60R cells toward induction of cell differentiation by TPA may be associated with a decrease in the expression of the type II isozyme of protein kinase C.     

Rapid increase in the activity of DNA topoisomerase I, but not topoisomerase II, in HL-60 promyelocytic leukemia cells treated with a phorbol diester

There is significant evidence to suggest that protein kinase C and DNA topoisomerases are functionally linked in signal transduction pathways. Much of this is based on the observation that phosphorylation of topoisomerase II by protein kinase C may lead to its activation in vitro and that inhibitors of topoisomerase II block phorbol diester-induced differentiation in HL-60 cells. In the present study, the activities of the DNA topoisomerases I and II have been quantitated to examine their regulation in phorbol diester-treated HL-60 cells undergoing differentiation. The activity of topoisomerase I increased rapidly after treatment with phorbol myristate acetate (PMA); it increased maximally (150% of control activity) at 3 hr post-treatment and remained elevated for at least 24 hr. Conversely, from the onset of exposure to PMA through 12 hr, there was no measurable alteration in topoisomerase II activity in PMA-treated cells. Moreover, there was a measurable decrease in topoisomerase II activity at the later time points, a result that occurred concomitantly with the loss of proliferative potential in differentiating HL-60 cells. Similar results were obtained when the activities of both enzymes were measured in nuclear extracts. The apparent increase in topoisomerase I activity was not due to an increase in the mass of the enzyme after PMA treatment, as measured by both western blotting and by the formation of camptothecin-dependent, topoisomerase I-DNA complexes. Taken together, these data suggest that the activities of the topoisomerases I and II may have been regulated independently in PMA-treated HL-60 cells, that the activity of topoisomerase II was not increased under conditions in which protein kinase C was activated in vivo, and that an increase in the activity of topoisomerase I may have had a role in the mechanism through which HL-60 cells underwent monocytic maturation in response to phorbol diesters.     

A variant of the HL-60 cell line expressing a high level of PTP1C exhibits increased susceptibility to differentiation by phorbol 12-myristate 13-acetate

A variant of the HL-60 cell line, HL-60/MCSFR4D2, has been found to express twice the amount of PTP1C as compared to the parental HL-60 cell line by immunoblotting and immunoprecipitation. Differentiation of the variant cells after phorbol 12-myristate 13-acetate (PMA) treatment was examined by the appearance of adherence. In 1% fetal calf serum (FCS), 20% of HL-60/MCSFR4D2 cells exhibited adherence after treatment with 0.5 ng/ml PMA for 48 h, 60% exhibited adherence after treatment with 1.0 ng/ml PMA and 80% exhibited adherence after treatment with 5.0 ng/ml PMA, while HL-60 cells exhibited only a slight response. Furthermore, antisense PTP1C oligonucleotides decreased the PMA-induced adherence of HL-60/MCSFR4D2 cells. These results suggest that the high-expression of PTP1C in HL-60 cells may be involved in the enhancement of susceptibility to macrophage-like differentiation by PMA.     

Synergistic stimulatory effect of 12-O-tetradecanoylphorbol-13-acetate and capsaicin on macrophage differentiation in HL-60 and HL-525 human myeloid leukemia cells

Our previous studies demonstrated that 12-O-tetradecanoylphorbol-13-acetate (TPA) had pharmacological activity for the treatment of myeloid leukemia patients. In the present study, we investigated the effects of TPA alone or in combination with capsaicin (8-methyl-N-vanillyl-6-nonenamide) on growth and differentiation in myeloid leukemia HL-60 cells and in a TPA-resistant HL-60 variant cell line termed HL-525. Treatment of HL-60 cells with TPA (0.16-1.6 nM) for 48 h resulted in concentration-dependent growth inhibition and cell differentiation (via the macrophage pathway). Capsaicin (5-50 microM) inhibited the growth of HL-60 cells in a concentration-dependent manner. Treatment of HL-60 cells with capsaicin alone only resulted in a small increase in the number of differentiated cells but treatment of the cells with TPA in combination with capsaicin synergistically increased differentiation. Moreover, inhibitors of protein kinase C (PKC), 7-hydroxystaurosporin (UCN-01; 100 nM) and chelerythrine (0.5 microM), significantly decreased HL-60 cell differentiation induced by the combination of TPA and capsaicin. These results suggest that PKC may be involved in HL-60 cell differentiation induced by TPA in combination with capsaicin. Capsaicin alone caused a very small increase in differentiation in the TPA-resistant HL-525 cells. However, treatment of HL-525 cells with combinations of TPA (0.16 nM) and capsaicin (10-50 microM) caused a strong synergistic increase in differentiation. Results from the present study suggest that a combination of TPA and capsaicin may improve the therapeutic efficacy of TPA and overcome resistance to TPA in some myeloid leukemia patients.     

Prostratin, a nonpromoting phorbol ester, inhibits induction by phorbol 12-myristate 13-acetate of ornithine decarboxylase, edema, and hyperplasia in CD-1 mouse skin

Pretreatment of CD-1 mouse skin with prostratin (12-deoxyphorbol 13-acetate) inhibited biological response to phorbol 12-myristate 13-acetate. The three responses examined were hyperplasia, induction of ornithine decarboxylase, and edema; the characteristics of inhibition depended on the specific response. Hyperplasia is the best short-term correlate of tumor promotion. Two or more pretreatments with 2.56 mumol (1 mg) prostratin, administered at intervals of 1-4 days, almost completely blocked the hyperplasia induced by phorbol 12-myristate 13-acetate applied 15 min to 6 h after the last pretreatment. Inducibility of hyperplasia was partially restored at 2 days and recovered by 4 days. Prostratin was more potent for inhibition of ornithine decarboxylase induction (50% inhibitory dose = 25.6 nmol) than it was for hyperplasia: the inhibition was largely attained by the first application, and the recovery from inhibition was slower (8 days). Edema was partially inhibited by prostratin (dose giving 50% of maximal inhibition = 512 nmol). We have previously demonstrated that prostratin is a protein kinase C activator. Our present results show that prostratin is a functional antagonist for a class of protein kinase C mediated responses. The findings emphasize the diversity of biological outcome for protein kinase C activators, presumably driven by the extensive heterogeneity in the protein kinase C pathway.     

Cyclic AMP induces activation of extracellular signal-regulated kinases in HL-60 cells: role in cAMP-induced differentiation

It is well known that elevated intracellular cAMP induces growth arrest and the differentiation of HL-60 cells to neutrophil-like cells. The present study was designed to assess the regulation of the extracellular signal-regulated kinase (ERK) pathway by cAMP and its association with differentiation in HL-60 cells. We found that 8-bromoadenosine-3',5'-cyclic-monophosphate (8Br-cAMP)-induced the activation of ERK and mitogen-activated protein kinase (MEK), but inhibited B-Raf kinase via a protein kinase A (PKA)-mediated mechanism. Prolonged exposure to 8Br-cAMP increased the phorbol 12-myristate 13-acetate (TPA)-stimulated superoxide generation and CD14 expression that characterize the differentiation phenotype, which was blocked by MEK-1 inhibitor. These data suggest that cAMP-induced ERK activation is essential for the differentiation of HL-60 cells, independently of B-Raf.     

Inhibition of phorbol ester-induced phenotypic differentiation of HL-60 cells by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, a protein kinase inhibitor

To identify the possible role of calcium ions in cell differentiation, we studied the extracellular Ca2+ requirement and the effect of Ca2+/phospholipid-dependent protein kinase (protein kinase C) inhibitor on proliferation and differentiation of human promyelocytic leukemic HL-60 cells. HL-60 cells grew equally well in 0.1 and 1.0 mM Ca2+ media. The addition of 12-O-tetradecanoyl-phorbol-13-acetate (TPA), 1,25-dihydroxyvitamin D3, and all-trans-beta-retinoic acid inhibited the cell growth and induced mature macrophage and granulocyte phenotypes in 1.0 mM Ca2+ medium. 1,25-Dihydroxyvitamin D3 and all-trans-beta-retinoic acid induced HL-60 differentiation to the same degree in 0.1 mM Ca2+ and 1.0 mM Ca2+ media. However, TPA failed to induce HL-60 differentiation or to inhibit proliferation in a 0.1 mM Ca2+ medium. The decrease of extracellular Ca2+ from 1.0 to 0.1 mM caused a significant drop in the intracellular Ca2+ level in undifferentiated and TPA-treated HL-60 cells, although no rapid change in cytosolic Ca2+ was detected in response to TPA addition. 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine (H-7), a protein kinase C inhibitor, inhibited proliferation of HL-60 cells in a dose-dependent manner. In contrast, H-7 selectively restored the proliferation of TPA-treated HL-60 cells and inhibited TPA-induced phenotypic differentiation. However, the same concentrations of 1-(5-isoquinolinylsulfonyl)-2,3-dimethylpiperazin and N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide, analogues of H-7 that inhibit protein kinase C more weakly, had no effect on the proliferation or differentiation induction. H-7 also suppressed 1,25-dihydroxyvitamin D3- and all-trans-beta-retinoic acid-induced phenotypic changes of HL-60 cells but did not eliminate the growth inhibition by these inducers. These results demonstrate the Ca2+ requirement and the protein kinase C involvement in phorbol ester-induced phenotypic differentiation of HL-60 cells.     

Smad6 is a protein kinase X phosphorylation substrate and is required for HL-60 cell differentiation

To gain insight into the function of human protein kinase X (PrKX), a signal-transduction protein required for macrophage differentiation, we identified regulatory subunit I alpha of protein kinase A, T54 and Smad6 as partners for this protein using a yeast two-hybrid interaction screen. Interactions between PrKX and these proteins were substantiated by co-immunoprecipitation. Interaction between Smad6 and PrKX was also confirmed in human myeloid HL-60 cells following their phorbol 12-myristate 13-acetate (PMA)-induced differentiation into macrophages. In vitro phosphorylation assays demonstrated that PrKX phosphorylates Smad6 at a serine residue. Mutagenesis of this site resulted in abrogation of PrKX phosphorylation. Both PrKX and Smad6 were shown to be co-localized to the nuclear compartment of HL-60 cells during their macrophage differentiation where PrKX levels are induced and Smad6 protein levels remain relatively constant while levels of serine phosphorylation of Smad6 increase. By using in vitro electrophoretic mobility shift assays and in vivo chromatin immunoprecipitation, we also demonstrate that during macrophage differentiation Smad6 displays an increased binding to the human osteopontin, Id2, and Hex gene promoters, which correlates to an observed increased expression of these genes. Finally, vector-based RNA interference experiments established that both Smad6 and PrKX proteins are required for PMA-induced cell attachment and spreading.     

Tumor promoters induce basic fibroblast growth factor gene expression in human dermal fibroblasts.

Tumor-promoting phorbol esters have been shown previously to either induce or repress the expression of numerous cellular genes, and this property is likely to be important for the in vitro and in vivo biological effects of these compounds. In this report, we demonstrate that phorbol 12-myristate 13-acetate induces the accumulation of basic fibroblast growth factor mRNA and protein in human dermal fibroblasts. In contrast, acidic fibroblast growth factor expression was unaffected by this compound. The enhancement of basic fibroblast growth factor gene expression by phorbol 12-myristate 13-acetate was blocked by the isoquinolinesulfonamide derivative H7, a potent inhibitor of protein kinase C. Two additional tumor promoters that bind to and activate protein kinase C, phorbol 12,13-didecanoate and mezerein, also increased basic fibroblast growth factor mRNA levels. Basic fibroblast growth factor is a mitogen for many cell types and can stimulate angiogenesis; thus, some tumor promoter-induced cellular responses may be mediated by this polypeptide.     

Glycosaminoglycan metabolism of HL-60 cells during differentiation induction by tetradecanoylphorbol-13-acetate

Many biochemical responses to phorbol ester differentiation inducers have been reported, including alterations in synthesis of specific gene products such as glycoproteins. Stage-specific glycosaminoglycan changes have previously been associated with the differentiation process, including a dramatic reduction in cellular chondroitin 4-sulfate during human myeloid leukemia cell maturation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA). We have demonstrated that treatment of HL-60 human promyelocytic leukemia cells with 4-methyl-umbelliferyl-beta-D-xyloside increases precursor incorporation into glycosaminoglycans linked to beta-D-xyloside, rather than core protein, eliminating the need for core protein and xylosyltransferase. Therefore, these beta-D-xyloside-treated cells were used to study the decreased glycosaminoglycan production during TPA-induced HL-60 differentiation. Exposure of these pretreated HL-60 cells to TPA, which induces macrophage-like maturation, resulted in a 70% reduction of incorporation of [35S]sulfate into cell-associated glycosaminoglycans. Thus, even in HL-60 cells in which glycosaminoglycan production is maximally stimulated by beta-D-xyloside, TPA is a strong inhibitor of free glycosaminoglycan chain production, and this biochemical effect is associated with other features of leukocyte maturation.     

The activation of PI 3-K and PKC zeta in PMA-induced differentiation of HL-60 cells

The human myelocytic leukemia cell line HL-60 is a useful model for the study of cellular differentiation. Phorbol 12-myristate 13-acetate (PMA) induces the monocyte/macrophage-like differentiation of HL-60 cells and results in growth arrest, increasing adherence. In PMA-induced differentiation of HL-60 cells, phosphoinositide 3-kinase (PI 3-K) activity was measured as phosphatidylinositol3P recovery from phosphatidylinositol by in vitro kinase assay. PI 3-K activity was increased in HL-60 cells that were stimulated by 20 nM PMA and the activity was inhibited by pretreatment with 20 microM LY294002, a specific inhibitor of PI 3-K. Members of the protein kinase C (PKC) family have been suggested to be one of the downstream targets of PI 3-K. PKC zeta is one of the atypical PKCs, non-diacylglycerol-responsive PKCs, and the activity was measured by the ability of phosphorylation onto myelin basic protein. PMA also induced the activation of PKC zeta during monocytic differentiation of HL-60 cells, and LY294002-pretreated cells failed to induce PKC zeta activation. The activity of PI 3-K is essential for PKC zeta activation, and LY294002 blocks both monocytic differentiation of HL-60 cells and activation of PKC zeta during PMA-induced cell differentiation. This implies that activated PI 3-K subsequently stimulates the PKC zeta in the process of PMA-induced monocytic differentiation.     

A new derivative of camptothecin, irinotecan hydrochloride (cpt-11) induces programmed cell-death in leukemia-lymphoma cell-lines

Irinotecan Hydrochloride (CPT-11) and 7-ethyl-10-hydroxycamptothecin (SN-38), which are both topoisomerase I inhibitors with potent antitumor effects in vivo and in vitro, were tested for the induction of programmed cell death (PCD) in leukemia/lymphoma cell lines. When the human T-cell leukemia cell line HUT-102 and the human promyelocytic leukemia cell line HL-60 cells were exposed to CPT-11, PCD characterized by a DNA fragmentation ladder of 180-200 bp in agarose gel electrophoresis and loss of cell viability was induced. The PCD inducing activity of SN-38, an active metabolite of CPT-11, was much more powerful than that of CPT-11. Besides inducing PCD in HUT-102 and HL-60 cells, SN-38 also induced PCD in the human erythroblast leukemia cell line K-562, which was resistant to CPT-11. Induction of PCD by SN-38 and CPT-11 was dose- and time-dependent. PCD in HUT-102 cells induced by SN-38 was prevented neither by aurintricarboxylic acid (ATA), an endonuclease inhibitor, as determine by DNA electrophoretic profiles and the number of viable cells, nor by the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA). The present data suggest that the topoisomerase I inhibitors, SN-38 and CPT-11 exert antitumor activity through induction of PCD in involved cells, at least in part. The PCD-inducing activity of the topoisomerase II inhibitor VP-16 was also tested in the above three cell lines and compared with CPT-11 and SN-38.     

Inhibition of chemotactic peptide-induced phosphoinositide hydrolysis by phorbol esters through the activation of protein kinase C in differentiated human leukemia (HL-60) cells

Phorbol-12,13-dibutyrate (PDBU), a tumor-promoting and protein kinase C-activating phorbol ester, inhibited formylmethionylleucyl-phenylalanine-induced generation of inositol mono-, bis-, and tris-phosphates from the hydrolysis of phosphoinositides in human leukemia (HL-60) cells, which had been differentiated to polymorphonuclear leukocyte-like cells by pretreatment with dibutyryl cyclic adenosine 3':5'-monophosphate. PDBU did not alter the binding of formylmethionylleucyl-phenylalanine to the cells. Other protein kinase C-activating substances such as 12-O-tetradecanoylphorbol-13-acetate and 1-oleoyl-2-acetyl-glycerol could substitute for PDBU, but 4 alpha-phorbol-12,13-didecanoate, which is inactive for both tumor promotion and protein kinase C activation, was ineffective in this capacity. Prolonged treatment of the cells with PDBU resulted in the down-regulation and decrease of protein kinase C activity to the level of 30-40% of that in the control cells. In the down-regulated cells, formylmethionylleucylphenylalanine still induced generation of the phosphorylated inositols to the same extent as that in the control cells, but the inhibition of this reaction by PDBU was reduced to 30-50% as compared with that in the control cells. These results strongly suggest that tumor-promoting phorbol esters inhibit the agonist-induced phosphoinositide hydrolysis through the activation of protein kinase C in the differentiated HL-60 cells.