Biochimica et Biophysica Acta, 1051 (1990) 71-77 71 Elsevier

BBAMCR 12598

Stimulus-response coupling in FMLP-stimulated U937 monocytes" effect of differentiation on G i2 expression

Kenneth Pollock 1, Judith Creba 1, Fiona Mitchell 2 and Graeme Milligan 2 J Bioscience II, ICI Pharmaceuticals, Alderley Park and 2 Molecular Pharmacology Group, Biochemistry Department,

Glasgow University, Glasgow (U.K.)

(Received 28 April 1989) (Revised manuscript received 11 August 1989)

Key words: Differentiation; Phosphoinositidase C; G protein; (U937 monocyte); (Human monoblastic cell line)

The effect of differentiation on FMLP-stimulated InsP production and G-protein expression was investigated in U937 monocytes. FMLP (0.01-10 pM) stimulated [3H]InsP production in dimethyl sulphoxide-differentiated, but not in immature, U937 cells, lonomycin (1 and 10 pM) stimulated [3H]InsP production equally well in both cell types. The FMLP response was blocked by pertussis toxin (100 n g / m l for 4 h) which catalysed [32p]ADP ribosylation of a 40 kDa 'Grlike' G-protein a subunit in these cells. This protein was also identified immunologically using anti-peptide antibodies that detect 'Gi-like' a subunits (SG2) or Gi2a specifically (LE2). With LE2 a 5-fold increase in Gi2a levels was seen following differentiation of the cells, suggesting that FMLP receptor expression is accompanied by an increase in the G-protein with which these receptors interact.

Introduction

Activation of plasma membrane phosphoinositidase C (PIC) to generate Ins(1,4,5)P 3 is a ubiquitous trans- duction mechanism utilised by many calcium-mobilising agonists following interaction with their cell surface receptors [1]. In common with other membrane trans- duction mechanisms, including the enzyme adenylate cyclase and certain ion channels, receptor control of PIC requires a guanine nucleotide binding, or 'G'-pro- tein; for review see Refs. 2 and 3. The involvement of G-protein(s) in PIC activation has been implied from studies using non-hydrolysable guanine nucleotide ana- logues and fluroaluminates to activate PIC in plasma membranes or in permeabilised cells [4,5]. These G-pro- teins comprise ct and fl/y subunits with the a subunit having the GTP binding/GTPase site, the receptor recognition site and the effector recognition site [6]. Amino-acid sequences have been predicted from cDNA clones for 8-10 different G-protein a subunits (Ga) of both known and unknown function [7]. However, the true identity of the PIC-coupling G-protein(s) remains

Abbreviations: PIC, phosphoinositidase C; PAF, platelet activating factor; DMSO, dimethyl sulphoxide.

Correspondence: K. Pollock, Bioscience II, ICI Pharmaceuticals, A1- derley Park, Cheshire, SK10 4TG, U.K.

unknown, but on the basis of inhibition of effector function they can be pharmacologically subdivided into those that are substrates for pertussis toxin, and those that are not. In most cell types originating from haemopoetic stem cells, receptor-regulated inositol phosphate production is inhibited by pertussis toxin. This is because pertussis toxin catalyses ADP ribosyla- tion of certain G-proteins, rendering them dysfunc- tional [8].

U937 cells are a human monoblastic cell line [9] which have receptors for platelet activating factor (PAF) and leukotrienes that are coupled to PIC [10,11], but in contrast to mature monocytes U937 cells do not have receptors for the chemotactic peptide formylmethionyl- leucylphenylalanine (FMLP) [12,13]. Differentiation of U937 cells to mature phagocytes can be induced by a variety of agents, including 1,25-dihydroxy vitamin D-3, retinoic acid and (DMSO) [12]. Phenotypic changes observed in differentiated U937 cells include expression of FMLP receptors on the cell surface [12] that are coupled to an elevation in cytosolic calcium, presuma- bly as a consequence of Ins(1,4,5)P 3 formation [14].

In common with neutrophils [15] and HL60 leukaemic cells [16], U937 cells express ' G i type' G-proteins which have been identified immunologically using anti-peptide antibodies, and which are pertussis toxin substrates [17]. Three different G~-like a subunits have been identified from cDNA clones, and it has been suggested that

016%4889/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)

72

Gi2c~ may couple receptors to PIC in these cell types [181.

Here we investigate agonist-dependent inositol phos- phate production and G-protein expression in both immature and DMSO-differentiated U937 cells, and provide evidence to show that FMLP receptor expres- sion and coupling to PIC is accompanied by increased Gi2a expression.

Materials and Methods

Materials Tissue culture reagents were obtained from Gibco,

Paisley, U.K. Electrophoresis and immunoblotting re- agents were from Bio-Rad myo[2-3H]inositol (10-20 Ci/mmol), and [adenylate-32p]nicotinamide adenine di- nucleotide (10-50 Ci/mmol) were from New England Nuclear. Pertussis toxin was purchased from Porton Products, Porton Down, U.K., FMLP was from Sigma, Poole, U.K. and ionomycin was from Calbiochem, Cambridge, U.K. All other reagents were of analytical grade.

Cell culture U937 cells were cultured in RPMI 1640 medium

supplemented with 10% (v/v) foetal calf serum, 2 mM L-glutamine, and 3 #g gentamycin/ml at 37°C in an atmosphere of 95% air/5% CO z. Cells were reseeded at less than 105/ml by dilution with fresh medium. For labelling (2/~ Ci/ml [ 3 H]inositol) and/or differentiation (1.25% (v/v) DMSO), cells were spun down and reseeded at 5 x 105/ml in medium 199 supplemented as above but with 1% (v/v) foetal calf serum, and in- cubated for a further 72 h.

lnositol phosphate studies Labelled cells were harvested by centrifugation (500

× g for 10 min 22°C) and resuspended at 5.106 cells/ml in Hepes-buffered Tyrodes (HBT) (145 mM NaC1, 5 mM KC1, 0.5 mM Na2HPO 4, 1 mM MgSO 4, 5 mM D-glucose, 10 mM Hepes (pH 7.35), 1 mg/ml bovine albumin. The cells were spun down and resus- pended as above in HBT containing 10 mM LiC1; the presence of lithium amplifies the agonist-stimulated in- ositol phosphate response [19]. Aliquots (1 ml) of cells were incubated at 37 ° C in the presence of 1 mM CaC12 plus agonists. Reactions were quenched with 6% per- chloric acid and the acid-soluble inositol phosphates were neutralised with 1.2 M KOH/60 mM EDTA/75 mM Hepes. These samples were diluted to 4 ml in water and applied to Dowex formate (AG l-X8 200-400 mesh) columns (1 ml bed volume) for fractionation by anion-exchange chromatography essentially as de- scribed [20]. Samples (5 ml) were mixed with 15 ml of Insta-gel for liquid scintillation counting. For HPLC analysis, acid-soluble inositol phosphates were neu-

tralised by the modified method of Sharpes and McCarl using freon/octylamine (1:1, v/v) [21,22], after which samples (1 ml injection volume) were loaded onto a Partisil Sax column (Jones Chromatography). Water was pumped through for 5 min then [3H]InsPs were eluted using a gradient of 0-100% 1.7 M ammonium formate (pH 3.7) [23] from 5 to 40 min, flow rate 1.25 ml/min. Fractions (0.4 min) were mixed with 1 ml of 50% methanol and 4 ml of Insta-gel for counting. Peaks of radioactivity were identified using authentic stan- dard , [3H]Ins(1,4)P2; [3H]Ins(1,4,5)P3 and [3H]Ins(1,3,4,5)P4 in parallel runs.

G-protein studies Control and DMSO-differentiated cells were

harvested as above except that after the wash in HBT the cell pellets (5 • 107 cells) were resuspended in 0.5 ml of 10 mM Tris/0.1 mM EDTA (pH 7.5) and samples were stored at - 8 0 ° C. Membranes were prepared by disrupting the thawed cells in 8 ml of Tris/EDTA using 20-25 strokes in a Teflon/glass homogeniser. Intact cells and debris were pelleted by centrifugation (1000 × g for 10 min at 4°C); the supernatant was then centri- fuged (100000 × g for 10 min at 4°C) and the resulting pellet was resuspended at 5-10 mg of protein per ml in Tris/EDTA.

Pertussis toxin-catalysed ADP ribosylation of U937 membrane proteins was carried out exactly as previ- ously described [24] using [32p]NAD+ and thiol-pre- activated toxin (10 /~g/ml). After SDS-polyacrylamide gel electrophoresis (SDS-PAGE), [32p]ribosylated pro- teins were visualised by autoradiography using Kodak X-Omat AR film.

For Western blotting, proteins were separated by SDS-PAGE (12.5% acrylamide) then electroblotted (Biometra Fast-Blot) onto nitrocellulose membranes. After blocking in 3% gelatin for 3 h, blots were in- cubated overnight with antiserum SG2 which was raised against the carboxy-terminal decapeptide of retinal-rod transducin and which recognises both G~la and Gi2a [17], or with antiserum LE2 which was raised against a synthetic peptide corresponding to amino acids 160-169 of Gi2a and is selective for Gi2a [15]. Immunoreactiv- ity was detected using horseradish peroxidase-linked Goat anti-rabbit IgG and Bio-Rad horseradish per- oxidase developing reagent containing 4-chloro-l-naph- thol. Immunoreactive bands were quantified by scan- ning densitometry using a Chromoscan 3 (Joyce Loeble) densitometer, on reflection mode. Raster scans 6 by 5 mm were carried out to obtain integral values of the band intensity.

Results

The concentration-response relationships for FMLP-induced inositol phosphate production was in-

73

3 . 5 0 3 b 6 c

I P 1 ( 5.5 4

3 2 . 5 5

e 2.5 4.5 CU "0 2

4

• >- 3.5

1.5 o

~ 2,5 l m I I

× I - 9 ~ . . . . . &

,, . a . . k ' " ~ . . . . . 2 'O "'"~'

0 . 5 I i I i I , ' , i 0 . 5 , , , , , , , , i 1 . 5 i , i , , , t , i z e r o - 8 - 7 - 6 - 5 z e r o - 8 - 7 - 6 - 5 z e r o - 8 - 7 - 6 - 5

L o g F W l L P ( M )

Fig. 1. Concentrat ion response curves for FMLP-st imulated inositol phosphates in control and DMSO-differentiated U937 cells. U937 cells were exposed to FMLP at the concentrations indicated for 5 rain in the presence of 1 m M Ca 2+. Data shown are from control (zx) or DMSO-differenti- ated ( o ) cells. Inositol phosphates were fractionated by Dowex anion-exchange chromatography, and the results shown are mean values + S.E. of

triplicate determinations. Similar results were observed in another identical experiment.

vestigated in both control and DMSO-differentiated U937 cells. After 5 min agonist addition there is no measurable stimulation of any [3H]InsP fraction in the control (undifferentiated) cells at up to 10 #M agonist as seen in Fig. 1. In differentiated cells there is a concentration-dependent increase in the [3H]InsP1, [3H]InsP2 and [3H]InsP3_4 fractions in response to FMLP. Half-maximum stimulation of [3H]InsP~ and [3H]InsP2 stimulation (Fig. la and b) occurred between 20 and 30 nM, with maximum stimulation (4- and 3.5-fold, respectively), seen at 1 /,M agonist. In the differentiated cells, FMLP caused a significant increase in the [3H]InsP3_4 fraction (Fig. lc). The maximum stimulation, however, was only 1.5-fold over control levels. The high background seen in the [3H]InsP3_4 fraction is presumably due to higher and/or unidenti- fied [3H]InsPs that have accumulated during the 72 h labelling period and have eluted in this fraction. Since any [3H]Ins(1,4,5)P3 formed during receptor occupation will be metabolised to an InsP2, either directly by a 5-phosphatase [25] or indirectly through the Ins(I,3,4, 5)Pa/Ins (1,3,4)P 3 route of metabolism [26], this also contributes to the apparent low level of stimulation in this fraction.

The calcium ionophore ionomycin stimulated [3H]InsP production in both control and differentiated U937 cells (Table I) consistent with the idea that calcium per se can directly stimulate PIC [28] and providing a positive indication that PIC is functional in the undif- ferentiated cells. Ionomycin (1 #M) caused a 2-fold

increase in [3H]InsP1 and [3H]InsP2 in both cell types. With 10 /~M ionomycin, a concentration that would maximally raise cytosolic calcium in intact cells, there is a 3-4-fold increase in the [3H]InsP1 in both cell types. However, 10 #M ionomycin stimulated the [3H]InsP2 6-7-fold in differentiated cells, which is significantly greater than the 4-5-fold stimulation seen in the undif- ferentiated cells. At both ionophore concentrations in both cell types no significant increase in [3H]InsP3_4 was detected after 15 min, presumably due to catabo- lism by this time of any InsP 3 formed (data not shown). These data suggest that there is no profound change in PIC activity when U937 cells are differentiated with DMSO, although subtle differences may occur in the subsequent metabolism of Ins(1,4,5)P 3, especially dur-

TABLE I

Ionomycin-stimulated inositol phosphate production in control and DMSO-differentiated U937 cells

Experimental condit ions as described in the legend to Fig. 1, except that ionomycin was added for 15 rain. Statistical analysis by Student 's t-test, *, P < 0.05.

[ 3 H]Ins P1 [ 3 H]Ins P2

undiff diff undiff diff

Saline control 492+ 84 551+17 592+ 51 581+ 40 Ionomycin 1 # M 978+288 923+37 1678+499 1244+ 42 Ionomycin 10/~M 1 9 8 4 + 2 6 4 1742+37 2537+320 3728+230 *

74

ing long periods of stimulation (15 min) as were used with ionomycin.

The role of G-proteins in the FMLP regulation of PIC was investigated pharmacologically using DMSO- differentiated cells preincubated for 4 h in the absence or presence of 100 ng pertussis toxin/ml of medium. A time-course of 300 nM FMLP-stimulated [3H]InsP3 and [3H]InsP4 production in control and pertussis toxin- treated cells is summarised in Table II. These data were obtained by HPLC analysis, which separated the InsP2-InsP 4 region more satisfactorily than the protocol employing Dowex. In the absence of pertussis toxin treatment there was a rapid production of [3H]InsP3, which peaked at 6-fold over the zero-time control after 30 s and thereafter returned to 1.5-fold after 5 min. The [3H]InsP3 fraction eluted as a double peak with a retention time close to that of authentic [3 H]Ins(1,4,5)P3- By analogy with other workers using the same HPLC system [23], we presume that the first peak is [3H]Ins(1,3,4)P3 and the second is [3H]Ins(1,4,5)P3. It is worth noting that at time zero the InsP 3 was almost exclusively [3I-I]Ins(1,4,5)P3, whilst within 15 s 50-60% of the radioactivity was due to [3H]Ins(1,3,4)P3. There was a similar rapid increase in the [3H]InsP4 fraction which in contrast to the [3H]InsP3 fraction continued to accumulate up to 5 min after FMLP addition. In the pertussis toxin-treated cell the FMLP response was virtually abolished, presumably due to toxin-catalysed ADP ribosylation of the G-protein linking receptor to effector. The extent of pertussis toxin-catalysed ADP ribosylation in plasma membranes prepared from con- trol and pertussis toxin-treated cells was assayed in vitro using thiol-activated pertussis toxin and [32p]_ NAD ÷. An autoradiograph of the proteins labelled in vitro is shown in Fig. 2. In membranes from non-toxin- treated cells there was pertussis toxin-dependent [32P]ADP ribosylation of a 40 kDa protein (lane D). This [32p]ADP ribosylation was absent in membranes

TABLE II

Effect of pertussis toxin on FMLP-stirnulated inositol phosphate produc- tion

Control or pertussis toxin (100 ng/ml for 4 h)-treated, differentiated U937 cells were incubated with 300 nM FMLP for the times indi- cated. Acid-soluble inositol phosphates from triplicate samples were combined and analysed by HPLC as described in the methods. [3H]lnsP3 is the total dpm eluted from the identified Ins(1,3,4)P 3 plus Ins(1,4,5)P 3 peaks. [3H]InsP4 corresponds to the Ins(1,3,4,5)P a peak.

FMLP incubation [ 3H]InsP3 [ 3 H]InsP4

time (s) (min) control pertussis control pertussis

0 924 744 219 280 15 3268 807 1787 213 30 5674 580 2149 471

300 1570 860 4143 80

M r . x l O -3

2 0 0 -

9 7 -

6 9 -

4 3 -

2 5 -

1 8 -

A B C D

Fig. 2. Pertussis toxin-catalysed ADP ribosylation of U937 membrane proteins from control or pertussis toxin-treated cells. Membranes (50 /~g of protein) prepared from differentiated cells, incubated in the presence (lanes A and B) or absence (lanes C and D) of pertussis toxin (100 ng/ml for 4 h), were incubated in vitro in the presence (lanes B and D) or absence (lanes A and C) of thiol-activated pertussis toxin (10/~g/ml) and [32p]NAD + as described in Materials and Methods. Ribosylated proteins were separated by PAGE (t0%

acrylamide) and visualised by autoradiography.

from pertussis toxin-treated cells (lane B). Also, no [32p]ADP ribosylation was seen in the absence of toxin in vitro (lanes A and C). This suggests that most, if not all, of the G~ in the pertussis toxin-treated cells was already ribosylated, accounting for the inhibition of PIC observed.

Having established that in differentiated U937 ceils newly coupled FMLP receptors appear to regulate PIC through a pertussis toxin-sensitive G-protein, such G- protein expression in undifferentiated and differenti- ated U937 cells was investigated. A direct comparison of pertussis toxin-catalysed ADP ribosylation of mem- branes from undifferentiated and differentiated cells is shown in the autoradiograph in Fig. 3. There is clearly more [32 P]NAD + incorporated into the 40 kDa proteins of the differentiated cell membranes (lane D) than into those of undifferentiated cells. To further characterise this apparent increase in pertussis toxin substrate G- proteins in differentiated U937 cells, these proteins were identified immunologically using anti-peptide anti- bodies. Western blots of G-protein levels in differenti- ated cells are shown in Fig. 4. In the upper panel antibody SG2 was used to identify all 'Gi-like' a sub- units, although not Gi3a, whilst in the lower panel antibody LE2 was used to identify Gi2a selectively. From a 50 /tg protein sample (lanes A-D) antibody SG2 detected a single protein band at 40 kDa. When

A B C D M r × 10 -3

- 2 0 0

- 9 7

- 69

- 4 3

- 2 5

- 1 8

Fig. 3. Effect of DMSO differentiation on pertussis toxin-catalysed ADP ribosylation of U937 membrane. As for Fig. 2 except that in lanes A and B [32p]ADP ribosylation of proteins from undifferenti- ated cells is compared with [nP]ADP ribosylation of proteins from

differentiated cells in lanes C and D.

75

quantified by scanning densitometry, a 2.3-fold increase in immunoreactive Gia was seen in the differentiated cells (C and D) compared with undifferentiated cells (A and B). A similar increase in Gia (1.8-fold) is seen in lanes G and H compared with lanes E and F, where 100 #g of protein was analysed, giving correspondingly stronger immunoreactive signals. However, as antibody SG2 will recognise both Gila and Gi2a it was not clear whether all Gia subunits were induced or whether one in particular was induced. With antibody LE2, a slightly different pattern emerged. 50 #g of protein was insuffi- cient for detection of Gi2a in undifferentiated cells (lanes A and B), whereas in differentiated cells (lanes C and D) there was a faint immunoreactive band at 40 kDa suggesting some increase in Gi2a levels. In lanes E-H with 100 #g of protein separated, immunoreactive Gi2a is clearly visible in all samples. Quantification of these bands by scanning densitometry indicated a 5-fold increase in expression of Gi2a in DMSO-differentiated cells compared with controls.

Discussion

The coupling of FMLP receptors to the inositol phosphate second messenger system in mature leuko-

M r x 10" 3

6 9 -

A B C D E F G H

4 6 -

3 0 -

2 1 -

A n t i b o d y SG 2

6 9 -

A B C D E F G H

4 6 -

3 0 -

[

2 1 - i

Antibody LE 2

Fig. 4. Expression of G-protein a subunits in control and DMSO-differentiated U937 cells. Membrane proteins from undifferentiated (lanes A,B,E and F) and differentiated (lanes C,D,G and H) U937 cells were separated by SDS-PAGE (12.5% acrylamide) for Western blotting. Lanes A to D

represent 50 pg of protein separated, lanes E to H represent 100 pg of protein separated.

76

cytes, such as neutrophils, is well documented [28]. By contrast, the leukaemic cell line U937 has few, if any, FMLP receptors, but when differentiated these cells express FMLP receptors that are coupled to the stimu- lation of inositol phosphate production (Fig. 1) leading to an elevation in cytosolic calcium [14]. Our results indicate that FMLP receptors on U937 cells are coupled to PIC by interacting with a pertussis toxin-sensitive G-protein as is the case in neutrophils [28]. The 40 kDa protein, which is ADP-ribosylated by pertussis toxin in U937 cells (Fig. 2), corresponds to Gi2c~, which we show here is increased 5-fold during DMSO differenti- ation of the cells. From a technical point of view, when looking for changes in levels of expression, this re- iterates the necessity of using selective antibodies, i.e., using LE2 rather than SG2, which detects only a 2-fold increase in expression, to discriminate subsets of G-pro- teins which may all be pertussis toxin substrates [29] and which may be differentially regulated. Levels of Gi3a in U937 cells do not increase with differentiation, and we have failed to selectively detect G i l a in these cells (Milligan and Mitchell, unpublished data) lending credence to the idea that the increase in 'G~' observed with SG2 can be attributed to an increase in G i 2 a alone. Our results are consistent with, and follow on from similar studies in HL60 and U937 cells, where a 2-fold increase in G i has been shown following differ- entiation of these cells by dibutyryl cyclic AMP and DMSO, respectively [17]. Enhanced G i 2 a expression has been demonstrated in differentiated HL60 cells using both m R N A probes and the selective antibody LE3 [16]. What remains to be demonstrated, however, is direct evidence for FMLP receptor interaction with G~2a. This criterion may possibly be satisfied using cholera toxin-catalysed ADP ribosylation of the 40 kDa pertussis toxin substrate. In HL60 cell membranes such ribosylation is stimulated by FMLP but only in the absence of GTP [30].

Whether the increase in G i 2 a we observed is exclu- sive to the FMLP receptors expressed at the same time, or whether this increase in G i 2 a is available to other receptor types is not clear. Studies with more selective U937 cell differentiating agents such as y-interferon which causes other characteristic phenotype changes including increased Fc but not F M L P receptor expres- sion [31], may help clarify this possibility. Although we would advocate G i 2 a as being the PIC-coupling G protein in U937 cells, it is worth noting that the PAF and LTB 4 receptors activate PIC [10,11] in undifferenti- ated U937 cells. As these agonists function with only 20% of the G~2c~ that is available to the F M L P recep- tors, this suggests that there is a vast excess of G-pro- tein in the differentiated U937 cells. The functional significance of this excess G i 2 a is unknown, but one might expect that activation of PIC by PAF or LTB 4 may become less sensitive to inhibition by pertussis

toxin in the differentiated cells. Preliminary experi- ments by us with PAF have shown this not to be the case. Pertussis toxin (10-100 ng /ml ) inhibited PAF (300 nM) stimulation of inositol phosphate production equally well in both undifferentiated and differentiated cells: around 60% inhibition. However, further experi- ments with lower toxin concentrations are required to fully resolve this matter.

Alternatively, different receptors may utilise the available G-protein differently or each receptor may have a finite ' poo l ' of G-protein. As alluded to above, PAF stimulation of inositol phosphate production is only inhibited by up to 60%, in contrast to FMLP, and both PAF and LTB 4 stimulation of calcium mobilisa- tion in immature U937 cells is only partly inhibited by pertussis toxin [32]. A similar discrepancy occurs in HL60 cells, where ATP stimulation of inositol phos- phate production also has a pertussis toxin-insensitive component, whereas the FMLP response is completely blocked [33]. Activation of protein kinase C can over- come the pertussis toxin-insensitive ATP response, sug- gesting a role for phosphorylation in regulation agonis t -G-prote in interactions for some receptors. In support of this notion it has recently been demonstrated that activation of protein kinase C in hepatocytes, using either phorbol ester or receptor occupancy, leads to the functional inactivation of G i [34].

In conclusion we have shown that increased FMLP receptor expression in U937 cells is accompanied by increased levels of the G-protein which is believed to couple these receptors to PIC, namely G~ 2a. Interaction of other receptor types with this G-protein in mature U937 cells and modification of this process by intra- cellular mediators remains to be investigated.

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