Effect of phorbol esters on cytosolic protein kinase C content and activity in the human monoblastoid U937 cell

12-O-Tetradecanoylphorbol-13-acetate (TPA) stimulates the human monoblastoid U937 cell to differentiate into a mature monocyte/macrophage-like cell. Since TPA may produce cellular responses by activating protein kinase C, the effects of TPA on kinase activity in the U937 cell were investigated. Brief exposures (less than or equal to 60 min) to TPA dramatically diminished protein kinase C-dependent phosphorylation of histone and endogenous substrates. However, using a peptide substrate corresponding to residues 720-737 of protein kinase C-epsilon, Ca2(+)-, phospholipid-, and diacylglycerol-dependent kinase activity was reduced only modestly after exposure to TPA. This phospholipid-dependent kinase activity coeluted on DEAE chromatography with protein kinase C. Examination of cytosolic protein kinase C content by Western blot analysis demonstrated a moderate decline in kinase content after TPA treatment. The decline was due primarily to loss of an 80-kDa species with preservation of a 76-kDa protein. The immunoreactive 76-kDa protein observed after TPA treatment comigrated on DEAE chromatography with the kinase activity phosphorylating the protein kinase C-epsilon peptide and had an elution profile similar to protein kinase C derived from untreated cells. Using antisera recognizing the catalytic and regulatory domains of the kinase, no evidence for proteolytic degradation of protein kinase C was observed. Although incubation of extracts from vehicle and TPA-treated cells inhibited the activity of partially purified protein kinase C, the degree of inhibition was similar in the two extracts. These findings suggest that TPA markedly diminishes protein kinase C-dependent phosphorylation of histone and endogenous substrates in part by altering kinase substrate specificity. These observations provide evidence for a novel post-translational process that can modulate protein kinase C-dependent phosphorylation.     

Prosaposin: a new player in cell death prevention of U937 monocytic cells

We report that prosaposin binds to U937 and is active as a protective factor on tumor necrosis factor alpha (TNFalpha)-induced cell death. The prosaposin-derived saposin C binds to U937 cells in a concentration-dependent manner, suggesting that prosaposin behaves similarly. Prosaposin binding induces U937 cell death prevention, reducing both necrosis and apoptosis. This effect was inhibited by mitogen-activated protein ERK kinase (MEK) and sphingosine kinase (SK) inhibitors, indicating that prosaposin prevents cell apoptosis by activation of extracellular signal-regulated kinases (ERKs) and sphingosine kinase. Prosaposin led to rapid ERK phosphorylation in U937 cells as detected by anti-phospho-p44/42 mitogen-activated protein (MAP) kinase and anti-phosphotyrosine reactivity on ERK immunoprecipitates. It was partially prevented by apo B-100 and pertussis toxin (PT), suggesting that both lipoprotein receptor-related protein (LRP) receptor and Go-coupled receptor may play a role in the prosaposin-triggered pathway. Moreover, sphingosine kinase activity was increased by prosaposin treatment as demonstrated by the enhanced intracellular formation of sphingosine-1-phosphate (S-1-P). The observation that the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin prevented the prosaposin effect on cell apoptosis suggests that sphingosine kinase exerts its anti-apoptotic activity by the PI3K-Akt pathway. Thus, cell apoptosis prevention by prosaposin occurs through ERK phosphorylation and sphingosine kinase. The biological effect triggered by prosaposin might be extended to primary cells because it triggers Erk phosphorylation in peripheral blood mononuclear cells (PBMCs). This is the first evidence of a biological effect consequent to a signal transduction pathway triggered by prosaposin in cells of non-neurological origin.     

Binding of platelet activating factor by isolated membranes from U937 cells

Platelet activating factor (PAF), 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (1-O-alkyl-2-acetyl-GPC) has been known to have biological effect on cells. The mechanisms of the effect of the potent phospholipid on cells has not been established. We have used 1-O-[3H]alkyl-2-acetyl-GPC [( 3H]PAF) to study the interaction on the isolated membranes of U937 cells. The binding process was time, protein concentration, temperature dependent and reversible. The binding of [3H]PAF to the U937 cell membranes was slightly inhibited by the addition of PAF analogue, 3-O-Hexadecyl-2-acetyl-sn-glycerol-1-phosphorylcholine. U937 cell membranes showed high affinity binding sites for PAF with equilibrium dissociation constant (Kd) of 5 x 10(-9) M. The displacement of bound [3H]PAF with 500-fold excess of nonlabeled PAF was not altered suggesting that the bound [3H]PAF was not degraded during the binding. Binding of [3H]PAF on U937 cell membranes was inhibited by PAF antagonist, 59227RP. The kinetic of the inhibition by PAF antagonist is competitive suggesting that PAF and PAF antagonist bind at the same site.     

CD45 negatively regulates lyn activity by dephosphorylating both positive and negative regulatory tyrosine residues in immature B cells.

Using CD45-deficient clones from the immature B cell line, WEHI-231, we previously demonstrated that CD45 selectively dephosphorylates the Src-family protein tyrosine kinase Lyn and inhibits its kinase activity. To further define the mechanisms of CD45 action on Lyn, we metabolically labeled Lyn from CD45-positive and -negative WEHI-231 cells and analyzed cyanogen bromide fragments by SDS-PAGE analysis. Phosphoamino acid analysis confirmed that Lyn is tyrosine phosphorylated with little serine or threonine phosphorylation. In CD45-negative cells, two bands at 8.2 and 4.1 kDa were phosphorylated in the absence of B cell Ag receptor (BCR) ligation. The 8.2-kDa band corresponded to a fragment containing the positive regulatory site (Tyr397), as assessed by its size and its phosphorylation in an in vitro kinase assay. The 4.1-kDa band was phosphorylated by COOH-terminal Src kinase, suggesting that it contains the COOH-terminal negative regulatory site (Tyr508). CD45 was also shown to dephosphorylate autophosphorylated Lyn in vitro. Thus, CD45 dephosphorylates not only the negative but also the positive regulatory tyrosine residues of Lyn. Furthermore, coimmunoprecipitations using anti-Igalpha Ab demonstrated that Lyn associated with the resting BCR was constitutively phosphorylated and activated in CD45-negative cells. In the parental cells, both regulatory sites were phosphorylated on BCR ligation. Taken collectively, these results suggest that CD45 keeps both BCR-associated and total cytoplasmic pools of Lyn in an inactive state, and a mechanism by which Lyn is activated by relative reduction of CD45 effect may be operative on BCR ligation.     

Activation of tyrosine phosphorylation in human T cells via the CD2 pathway. Regulation by the CD45 tyrosine phosphatase

In this study we compare the effect of CD3 and CD2 ligation on tyrosine kinase activation in human peripheral blood T cells. Using antiphosphotyrosine antibody to detect tyrosine phosphorylation of cellular substrates, we demonstrate that mAb stimulation of either CD3 or CD2 results in tyrosine phosphorylation of the TCR-zeta chain and 135- and 100-kDa proteins. However, differences are observed between CD3 and CD2 ligation; only the former results in rapid tyrosine phosphorylation of 72-, 65-, and 40-kDa substrates. Co-aggregation of CD2 and CD45, a tyrosine phosphatase, results in inhibition of intracellular calcium elevation and T cell proliferation. We demonstrate in this study that this manipulation also inhibits polyphosphoinositide hydrolysis and tyrosine phosphorylation of the 100-kDa substrate. The failure of tyrosine phosphorylation of the 100-kDa substrate is specific in that phosphorylation of the 135-kDa protein is not inhibited. Similar results are observed when CD2 and CD45 are independently cross-linked rather than co-aggregated. The observation that CD45 cross-linking alters tyrosine phosphorylation of T cell substrates and effects polyphosphoinositide hydrolysis is further evidence that tyrosine phosphorylation regulates early events in T cell activation including, perhaps, phospholipase C activity.     

Tumor necrosis factor signal transduction. Tissue-specific serine phosphorylation of a 26-kDa cytosolic protein

Binding of tumor necrosis factor-alpha (TNF-alpha) to its receptor on U937 cells results in rapid and TNF dose-dependent phosphorylation of a cytosolic protein with an apparent molecular mass of 26,000 kDa (p26) and an isoelectric point of 5.6. Half-maximal phosphorylation of p26 was achieved at concentrations of 1.8 ng/ml and was detectable within 20 s of TNF-alpha treatment. p26 is phosphorylated exclusively at serine residues. p26 phosphorylation occurs at 37 degrees C as well as at 14 degrees C, indicating that internalization of the TNF receptor is not required for serine kinase activation. Dephosphorylation of p26 starts 10 min after TNF-induced phosphorylation, suggesting a possible regulatory function of this cytosolic protein within the post-TNF receptor signaling system. p26 is also phosphorylated upon treatment with lymphotoxin. In contrast, both interferon-gamma and lipopolysaccharide fail to induce p26 phosphorylation. Whereas phosphorylated p26 was detected in the TNF-sensitive breast cancer cell line CRL1500, other TNF-responsive tumor cell lines investigated lacked enhanced phosphorylation of p26 in response to TNF, indicating that the 26-kDa phosphoprotein (pp26) may be a cell type-specific second messenger molecule involved in TNF signal transduction in some, but not all, target cells. p26 is also phosphorylated in a subclone of U937 (U937.C27) that responds to TNF-alpha with differentiation, yet is resistant to TNF-alpha-mediated growth inhibition. In contrast, p26 is not phosphorylated in another U937 derivative (U937.G3) that is resistant to both TNF-alpha-induced growth arrest and differentiation, suggesting that pp26 may play a role in the TNF signaling pathway linked to differentiation processes rather than to growth control.     

Phospholipase C activity in NCB-20 cells is inhibited by protein kinase A-mediated phosphorylation of low molecular mass GTP-binding proteins.

We have previously shown that bradykinin-induced production of second messengers such as inositol trisphosphate and diacylglycerol in neurotumor cells is inhibited by raising cellular cyclic AMP levels, which in turn inhibit phospholipase C. A monoclonal antibody to phospholipase C-II immunoprecipitated the 140-kDa form of phospholipase C-II from [35S]methionine/[3H]eucine-labeled cells, but not [32P]orthophosphate-labeled phospholipase C-II, following treatment with either forskolin or dibutyryl cyclic AMP. This suggested that phospholipase C is not the target for cyclic AMP-dependent protein kinase-mediated phosphorylation. In vitro studies confirmed that phospholipase C activity was inhibited by raising cellular cAMP levels, and partial sensitivity to Bordetella pertussis toxin suggested the involvement of a GTP-binding protein which could be the target for protein kinase A. The involvement of a GTP-binding protein in coupling the bradykinin receptor to phospholipase C was further suggested by the ability of both guanosine 5'-O-(thio-triphosphate) and fluoride (NaF) to release inositol phosphates from NCB-20 cell membranes previously labeled with [3H]inositol. Both effects were blocked by pretreatment of the cells with protein kinase A activators, further suggesting a GTP-binding protein as the target for protein kinase A-mediated phosphorylation. When whole NCB-20 cell extracts were blotted onto nitrocellulose and incubated with [alpha- 32P]GTP, a major 24-kDa band plus minor bands at 22 and 20 kDa were revealed by autoradiography. A pH 3.0/6.0 soluble (basic protein) NCB-20 cell extract revealed the major 24-kDa band plus the 20-kDa band, and similar basic proteins were shown to be heavily phosphorylated following [32P]orthophosphate labeling and pretreatment with forskolin. The size and ability to bind GTP on Western blots are characteristic of the ras, rho, smg, etc. family of GTP-binding proteins recently suggested to be the much sought after GPLC (Lapetina, E.G., Lacal, J. C., Reep, B. R., and Molina y Vedia, L. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 3131-3134; Wang, P., Nishihata, J., Takabori, E., Yamamoto, K., Toyoshima, S., and Osawa, T. (1989) J. Biochem. (Tokyo) 105, 461-466; Nagata, K.-I., Nagao, S., and Nozawa, Y. (1989) Biochem. Biophys. Res. Commun. 160, 235-242). We propose that GPLC is uniquely sensitive to protein kinase A-mediated phosphorylation and that phosphorylation inhibits stimulus-secretion coupling in these cells.     

CD45 negatively regulates monocytic cell differentiation by inhibiting phorbol 12-myristate 13-acetate-dependent activation and tyrosine phosphorylation of protein kinase Cdelta

The protein-tyrosine phosphatase CD45 is expressed on all monocytic cells, but its function in these cells is not well defined. Here we report that CD45 negatively regulates monocyte differentiation by inhibiting phorbol 12-myristate 13-acetate (PMA)-dependent activation of protein kinase C (PKC) delta. We found that antisense reduction of CD45 in U937 monocytic cells (CD45as cells) increased by 100% the ability of PMA to enlarge cell size, increase cell cytoplasmic process width and length, and induce surface expression of CD11b. In addition, reduction in CD45 expression caused the duration of peak PMA-induced MEK and extracellular signal-regulated kinase (ERK) 1/2 activity to increase from 5 min to 30 min while leading to a 4-fold increase in PMA-dependent PKCdelta activation. Importantly, PMA-dependent tyrosine phosphorylation of PKCdelta was also increased 4-fold in CD45as cells. Finally, inhibitors of MEK (PD98059) and PKCdelta (rottlerin) completely blocked PMA-induced monocytic cell differentiation. Taken together, these data indicate that CD45 inhibits PMA-dependent PKCdelta activation by impeding PMA-dependent PKCdelta tyrosine phosphorylation. Furthermore, this blunting of PKCdelta activation leads to an inhibition of PKCdelta-dependent activation of ERK1/2 and ERK1/2-dependent monocyte differentiation. These findings suggest that CD45 is a critical regulator of monocytic cell development.     

Affinity-purified c-Jun amino-terminal protein kinase requires serine/threonine phosphorylation for activity.

The addition of phorbol esters to U937 leukemic cells stimulates the phosphorylation of c-Jun on serines 63 and 73. To isolate the protein kinase which stimulates this phosphorylation, we have used heparin-Sepharose chromatography followed by affinity chromatography over glutathione-Sepharose beads bound with a fusion protein of glutathione S-transferase and amino acids 5-89 of c-Jun (GST-c-Jun). Using this procedure we purify a 67-kDa protein which is capable of phosphorylating GST-c-Jun as well as the complete c-Jun protein. By making mutations in serines 63 and 73 and then creating a fusion protein with GST (GST-c-Jun mut), we demonstrate that this protein kinase specifically phosphorylates these sites in the c-Jun amino terminus. Treatment of purified c-Jun amino-terminal protein kinase (cJAT-PK) with phosphatase 2A inhibits its ability to phosphorylate GST-c-Jun. This inactivated enzyme can be reactivated by phosphorylation with protein kinase C (PKC), although PKC is not capable of phosphorylating the GST-c-Jun substrate. Because v-Jun cannot be phosphorylated in vivo, we compared the ability of cJAT-PK to bind to GST-v-Jun or GST-c-Jun mut. The cJAT-PK bound 50-fold better to GST-c-Jun mut than GST-v-Jun suggesting that the delta domain which is missing in v-Jun plays a role in binding the cJAT-PK. These results suggest that there is a protein kinase cascade mediated by protein phosphatases and PKC which regulates c-Jun phosphorylation.     

A very late activating antigen-alpha4 (CD49d) monoclonal antibody,BU49 induces phosphorylation of a cAMP response element-binding protein (CREB), resulting in induction of homotypic cell aggregation and enhancement of interleukin-8 (IL-8) production

A very late activating antigen-alpha4 (CD49d) monoclonal antibody (mAb), BU49 was found to induce phosphorylation of a cAMP response element-binding protein (CREB) in the human monocyte-like cell line, U937. This phosphorylation of CREB was completely inhibited by a protein kinase A (PKA) inhibitor H-89 with the optimum concentration (completely inhibits PKA). Furthermore, BU49 strongly and rapidly (within 5 hr) induced homotypic cell aggregation in the U937 cells accompanied by CREB phosphorylation. This cell aggregation was also completely inhibited by the addition of H-89. Interestingly, both of two mAbs (mAb13 and 4B4) recognizing different epitopes on the CD29 (beta1 integrin) completely inhibited this aggregation at the late phase (18 to 24 hr) but not at the early phase (5 hr) after cultured with BU49. On the other hand, BU49 significantly enhanced interleukin-8 (IL-8) production from the U937 cells into the culture supernatant. In addition, this IL-8 production was significantly blocked in the presence of H-89 with the optimum concentration. However, a CD29 mAb which inhibits homotypic cell aggregation could not block this IL-8 production. Taken together, these findings indicate that BU49 induces CREB phosphorylation mainly mediated by PKA, which finally results in the induction of homotypic cell aggregation and the enhancement of IL-8 production. Furthermore, these findings also indicate that the enhancement of IL-8 production from the U937 cells induced by BU49 partially depends on CREB phosphorylation mainly mediated by PKA.     

Platelet-activating factor enhances complement-dependent phagocytosis of diamide-treated erythrocytes by human monocytes through activation of protein kinase C and phosphorylation of complement receptor type one (CR1)

Oligomerization of band 3 protein has been recently indicated as an early event in senescent or damaged red cell membrane followed by specific deposition of anti-band 3 antibodies and binding of complement C3 fragments. The band 3-anti-band 3-C3b complex is recognized by homologous monocytes, and phagocytosis ensues. This study shows that recognition of the anti-band 3-C3b complex by the monocyte C3b receptor type one (CR1) plays a crucial role in the process of removal of damaged red cells. Indeed, blocking of monocyte CR1 with an anti-CR1 monoclonal antibody abrogated phagocytosis of diamide-treated red cells. Platelet-activating factor (PAF) is a phospholipid mediator involved in inflammatory processes. Nanomolar (R)-PAF enhanced the CR1-dependent phagocytosis of diamide-treated human red cell and of sheep red cells coated with C3b, induced the fast translocation of protein kinase C to monocyte membrane compartment, and stimulated the phosphorylation of monocyte CR1. The biologically inert lyso-PAF and the enantiomer (S)-PAF were inactive. PAF receptor antagonists and inhibitors of protein kinase C blocked the enhancement of phagocytosis induced by PAF. Protein kinase C translocation, phosphorylation of CR1, and stimulation of this receptor to an active state capable of mediating phagocytosis represent a novel pathway by which PAF interferes with red cell homeostasis and possibly modulates inflammatory reactions and host mechanisms against infections.     

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.     

Phorbol myristate acetate inhibits okadaic acid-induced apoptosis and downregulation of X-linked inhibitor of apoptosis in U937 cells

Okadaic acid is a specific inhibitor of serine/threonine protein phosphatase 1 (PP-1) and 2A (PP-2A). The phosphorylation and dephosphorylation at the serine/threonine residues on proteins play important roles in regulating gene expression, cell cycle progression, and apoptosis. In this study, phosphatase inhibitor okadaic acid induces apoptosis in U937 cells via a mechanism that appears to involve caspase 3 activation, but not modulation of Bcl-2, Bax, and Bcl-X(L) expression levels. Treatment with 20 or 40 nM okadaic acid for 24 h produced DNA fragmentation in U937 cells. This was associated with caspase 3 activation and PLC-gamma1 degradation. Okadaic acid-induced caspase 3 activation and PLC-gamma1 degradation and apoptosis were dose-dependent with a maximal effect at a concentration of 40 nM. Moreover, PMA (phorbol myristate acetate), PKC (protein kinase C) activator, protected U937 cells from okadaic acid-induced apoptosis, abrogated okadaic acid-induced caspase 3 activation, and specifically inhibited downregulation of XIAP (X-linked inhibitor of apoptosis) by okadaic acid. PMA cotreated U937 cells exhibited less cytochrome c release and sustained expression levels of the IAP (inhibitor of apoptosis) proteins during okadaic acid-induced apoptosis. In addition, these findings indicate that PMA inhibits okadaic acid-induced apoptosis by a mechanism that interferes with cytochrome c release and activity of caspase 3 that is involved in the execution of apoptosis.     

蛋白激酶C抑制剂对U937细胞清道夫受体功能的影响

为了解细胞内蛋白质磷酸化水平对清道夫受体功能的影响,用蛋白激酶Ｃ抑掉剂形孢菌素（ｓｔａｕｒｏｓｐｏｒｉｎｅ,ＳＴＡ）处理人Ｕ９３７细胞,分别测定对照组和处理组细胞对碘标记的氧化低密度脂蛋白（＾１２５Ｉ）ｏｘ－ＬＤＬ的降解,结合,细胞表面受体复合物的内移以及细胞内脂质蓄积的程度,并利用放射自显影方法观察药物对细胞表面受体表达的影响,结果发现ＳＴＡ可以促进细胞结合（＾１２５Ｉ）ｏｘ－ＬＤＬ增加细胞表面     

Phospholipid-stimulated protein kinase in plants

In membrane fractions from zucchini (Cucurbita pepo L.) hypocotyls, catalytic properties of a platelet-activating factor (PAF)-activated protein kinase were investigated. In the presence of [ethylenebis(oxyethylenenitrilo)]tetraacetic acid, phosphorylation of a 55-kDa membrane polypeptide and, to a lesser extent, several others, including a 120-kDa polypeptide, was stimulated by PAF. The phosphorylation of the 55-kDa polypeptide was used for quantification of the PAF-stimulated protein kinase. Stimulation of protein phosphorylation by PAF increased in a concentration range from 10-200 micrograms/ml (= 19-380 microM) PAF up to 10-fold above the control. Addition of Ca2+ ions in the micromolar range in the presence and in the absence of PAF increased the phosphorylation of the 55- and the 120-kDa polypeptide. Other phospholipids and lipids tested including phorbol ester, diglyceride, mono- and triglyceride, and oleic acid were ineffective. The same lipid specificity was previously observed for the activation of ATP-dependent H+ transport in microsomes (Scherer, G.F.E., Martiny-Baron, G., and Stoffel, B. (1988) Planta 175, 241-253). Lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) were able to stimulate the phosphorylation of the same polypeptides as PAF and H+ transport but both to a lesser extent (PAF greater than LPC greater than LPE). In the presence of EGTA, PAF-stimulated phosphorylation of a 55- and a 57-kDa polypeptide was predominantly associated with vacuolar membranes and those of 42, 61, 63, and 120 kDa were predominantly associated with plasma membranes. Stimulation of ATP-dependent H+ transport by PAF was found in tonoplast vesicles whereas plasma membrane vesicles had only little transport activity and, therefore, an effect of PAF on plasma membrane H+ transport could not be measured. Stimulation of ATP hydrolysis by PAF was observed both in tonoplast- and plasma membrane-containing fractions.     

Nerve growth factor treatment or cAMP elevation reduces Ca2+/calmodulin-dependent protein kinase III activity in PC12 cells

Ca2+/calmodulin-dependent protein kinase III (Ca2+/CaM kinase III) phosphorylates a protein of Mr = 100,000 (the 100-kDa protein), a major substrate for Ca2+/CaM-dependent protein phosphorylation found in many mammalian tissues and cell lines (Nairn, A.C., Baghat, B., and Palfrey, H.C. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 7939-7943). Treatment of PC12 cells with nerve growth factor (NGF) or forskolin resulted in a decrease in the depolarization-dependent phosphorylation of the 100-kDa protein in intact cells and in a decrease in the Ca2+/CaM-dependent phosphorylation of the 100-kDa protein in cytosolic extracts. In experiments using cytosolic extracts, the initial effect of NGF on the phosphorylation of the 100-kDa protein was observed in less than 1 h, was maximal (70% decrease) after 12 h, and began to recover after 24 h. The effect of forskolin was more rapid and the maximal effect was greater (90-95% decrease). Decreased Ca2+/CaM kinase III activity was also found in PC12 cells treated with epidermal growth factor, 2-chloroadenosine plus isobutylmethylxanthine, or dibutyryl cAMP. The effect of forskolin did not reverse unless it was removed. Cycloheximide blocked the recovery of Ca2+/CaM kinase III activity observed following the removal of forskolin but did not affect the ability of forskolin to reduce kinase activity. Short-term treatment with phorbol ester had little effect on Ca2+/CaM kinase III activity; long-term treatment with phorbol ester, which results in the disappearance of enzymatically detectable protein kinase C, had no effect on the ability of NGF or 2-chloroadenosine to reduce Ca2+/CaM kinase III activity. The level of the 100-kDa protein as determined by immunological techniques was not changed by any treatment. These results suggested that the effect of treatment of PC12 cells with NGF or forskolin was to reduce the level of Ca2+/CaM kinase III per se.     

Protein kinase C and its substrates in tumor promoter-sensitive and -resistant cells

Calcium- and phospholipid-dependent protein kinase C activity and substrates were characterized in cell lysates of preneoplastic JB6 cells, a model system of genetic variants for sensitivity to tumor promoter-induced neoplastic transformation. Protein kinase C activity was similar for sensitive and resistant variants, as measured by calcium- and phospholipid-dependent phosphorylation of an exogenous substrate (histone HIII). Of 13 endogenous protein kinase C substrates, identified by labeling proteins with [gamma-32P] ATP, at least two (80 and 23 kDa) are potential candidates for mediating events on the pathway for promotion of transformation. 32P incorporation into the 80-kDa protein kinase C substrate was stimulated by tetradecanoylphorbol acetate and correlated with phenotype: the highest incorporation was found in promotion-insensitive cells, an intermediate level in promotion-sensitive cells and the lowest in the transformed cells. The phosphorylation of an 80-kDa protein, found by labeling intact cells in monolayer growth with [32P]orthophosphate, was also stimulated by tetradecanoylphorbol acetate and correlated inversely with phenotype. The 80 kDa protein kinase C substrate from cell lysates and the 80-kDa phosphoprotein from intact cells appear to be identical, as indicated by peptide mapping with protease V8 from Staphylococcus aureus. This finding suggests that the 80-kDa substrate is relevant to promoter-induced signal transduction in the intact cell. The 23-kDa protein kinase C substrate exhibited a band shift in sodium dodecyl sulfate gels in response to another transformation promoter in JB6 cells, the calcium analog, lanthanum (Smith, B. M., Gindhart, T. D., and Colburn, N. H. (1986) Carcinogenesis 7, 1949-1956). In summary, there are no unique substrates that distinguish the variants. Quantitative differences in certain substrates or their phosphorylation may, however, account for the difference in promotion sensitivity among the variants.     

Diacylglycerols modulate phosphorylation of the insulin receptor from human mononuclear cells

It has been found that 1,2- but not 1,3-diacylglycerols stimulated phosphorylation of the insulin receptor of cultured human monocyte-like (U-937) and lymphoblastoid (IM-9) cells both in the intact- and broken-cell systems. The stimulation of the receptor's beta-subunit phosphorylation was dose-dependent, with optimal effect at 100 micrograms/ml of diacylglycerol. The effects of insulin and 1,2-diacylglycerols on the phosphorylation of partially purified insulin receptors were additive. Phosphoamino acid analysis showed a major effect of diacylglycerols on phosphorylation of tyrosine residues. The diacylglycerols also stimulated tyrosine kinase activity of the partially purified U-937 and IM-9 insulin receptors 2.5-3.5-fold when measured by phosphorylation of an exogenous substrate, poly(Glu80Tyr20) in the absence of any added insulin, calcium or phospholipid. Since this diacylglycerol effect could not be reproduced under conditions optimal for protein kinase C activation and the purified protein kinase C did not stimulate phosphorylation of the beta-subunit of the insulin receptor in this system, it is unlikely that the diacylglycerol effect was mediated by protein kinase C. Since these exogenous 1,2-diacylglycerols at the same high concentration also inhibited 125I-insulin binding to the insulin receptor of the intact U-937 and IM-9 cells, diacylglycerols could modulate the function of the insulin receptor and insulin action in human mononuclear cells.