Lipocortins: an update

Induction of annexin-1 at transcriptional and post-transcriptional level in rat brain by methylprednisolone and the 21-aminosteroid U74389F

Brain tissue of rats pretreated with methylprednisolone or with the 21-aminosteroid U74389F, and that of untreated control rats, was assessed for the expression of annexin-1 (Anx-1) and the transcription of its mRNA. For this purpose Anx-1 cDNA was amplified and simultaneously a T7-RNA-polymerase promoter was incorporated into the cDNA using a polymerase chain reaction (PCR). Then digoxigenin-11-UTP was incorporated into the transcribed cRNA with T7-RNA-polymerase. With this probe in situ hybridization was carried out on sections of the brain. The probe was visualized by an immunoassay using an antidigoxigenin antibody conjugate. Anx-1 protein was assessed by means of immunohistochemistry using a polyclonal antibody. The various brain areas of the control animals showed an appreciable amount of Anx-1 at mRNA or protein level; on the other hand, the animals which had been pretreated with either steroid, showed a more intense Anx-1 mRNA signal than the controls in many areas. In the pretreated animals Anx-1 immunostaining was unchanged in cortex, basal ganglia, amygdala and septum, but more intense in hippocampus, hypothalamus and thalamus. In ependyma, choroid plexus, meninges, and vascular walls there was no Anx-1 mRNA transcription detectable. An opposite profile was shown by the Anx-1 immunoreactivity, the protein was present in control animals as well as the steroid-pretreated animals, suggesting that here the protein was either from systemic origin, or has diffused from adjacent structures. The results indicated that Anx-1 mRNA transcription is upregulated by either steroid, and that in the untreated animals there is a resting level of Anx-1 mRNA transcription, presumably reflecting physiological influences on Anx-1 expression.

Annexin V and phospholipid metabolism

Annexins, protein kinases C and cytosolic phospholipase A2 belong to three families of ubiquitous cytoplasmic proteins involved in signal transduction. All annexins share the property of binding to phospholipids in the presence of calcium. Most annexins are substrates for protein kinases C except annexin V, the most ubiquitous and abundant annexin. Protein kinases C (PKC) belong to three distinct groups of kinases, conventional PKCs (cPKCs) that depend on calcium, diacylglycerol and negatively charged phospholipids for their activity, novel PKCs (nPKCs) and atypical PKCs (aPKCs), that do not require calcium for their activity, although they both require negatively charged phospholipids. Cytosolic phospholipase A2 (cPLA2) depends on calcium for its catalytic activity as well as on serine phosphorylation by MAP kinases. We report that annexin V modulates the activity of cPKCs as well as of cPLA2 by interfering with their ability to bind to negatively charged phospholipids and calcium. We propose that annexin V could interfere with the calcium and phospholipid signalling pathway.

In adrenocortical tissue, annexins II and VI are attached to clathrin coated vesicles in a calcium-independent manner

We have previously characterized three populations of clathrin coated vesicles (CCVs) isolated from bovine adrenocortical tissue and designated them as large, medium and small coated vesicles, i.e., LCV, MCV and SCV, respectively. Here, we show that annexins II and VI, two of the annexins involved in membrane traffic, are present in the three populations of CCVs but with different distributions between coat proteins (CP) and lipidic vesicle membrane. Annexin VI is only associated with the membrane, whatever the CCV population. In contrast, annexin II is differently distributed between coat and membrane, depending on the CCV population. Both annexins are bound to membranes in a calcium-independent manner and solubilization studies in Triton X114 (TX114) suggest that they interact poorly with lipids by hydrophobic interactions. Ligand blotting experiments show that both annexins bind to CCV proteins: annexin II to a 200-kDa component in all CCVs and annexin VI to a 100-kDa component in LCV and SCV identified as dynamin, a GTPase essential for endocytic CCV pinching off. Dynamin is tightly associated to annexin VI only in LCVs, the endocytic [transferrin (Tf) positive] vesicles. Our data suggest that annexins II and VI could define specific protein-lipid interaction microdomains that could play a role in the different functions of the CCVs.

Maturational changes in rabbit renal cortical phospholipase A2 activity

Several studies have demonstrated that the neonatal kidney has a markedly attenuated response to parathyroid hormone (PTH); however, the cause for this blunted response is unknown. PTH stimulated cAMP production by 215 +/- 18% in neonatal proximal tubule suspensions compared to a 35 +/- 7% increase in adult proximal tubules. Thus, neonatal proximal tubules have functioning PTH receptors and a greater adenylate cyclase response than the adult segment. In adult proximal tubules, PTH stimulates phospholipase A2 (PLA2) activity and the inhibition of Na,K-ATPase activity by PTH is blocked by inhibitors of PLA2. We examined whether maturational changes in renal cortical activity could play a role in the attenuated response to PTH in the neonatal proximal tubule. Compared to adults, neonates had a lower renal cortical cytosolic PLA2 (cPLA2) activity, assessed as the release of 14C-arachidonic acid (AA) from labeled phosphatidyl choline (0.44 +/- 0.10 vs. 0.74 +/- 0.06% 14C-AA released/min/mg protein, P < 0.05) and microsomal PLA2 activity (0.32 +/- 0.03 vs. 1.20 +/- 0.13% 14C-AA released/min/mg protein, P < 0.001). The protein abundance of cPLA2 was not different between the neonatal and adult renal cortex as assessed by immunoblot assay. Thus, the difference in activities must be due to a difference in regulation of cPLA2. Annexin 1 (lipocortin 1) has been shown to inhibit PLA2 activity by binding to phospholipid substrate. Annexin 1 protein abundance was higher in neonatal than in adult renal cortex (P < 0.001). Thus, the lower activity of PLA2 in the neonatal tubules may be due in part to higher expression of annexin 1. PLA2 activation by PTH, -8-bromo-cAMP and PMA was assessed as 3H-AA release from prelabeled suspensions of neonatal and adult proximal tubules. PTH (10(-7) M), 8-bromo-cAMP (10(-4) M) and PMA (5 x 10(-8) M) significantly increased 3H-AA release from adult tubules (P < 0.05) but had no effect on neonatal tubules (P = NS). Thus, PTH, 8-bromo-cAMP and PMA stimulated PLA2 in adult but not neonatal proximal tubules. In conclusion, the maturational changes in renal cortical PLA2 activity may be a factor in the blunted response of neonatal proximal tubules to PTH.

Inhibition of cytosolic phospholipase A2 by annexin V in differentiated permeabilized HL-60 cells. Evidence of crucial importance of domain I type II Ca2+-binding site in the mechanism of inhibition

Annexin V belongs to a family of proteins that interact with phospholipids in a Ca2+-dependent manner. This protein has been demonstrated to have anti-phospholipase A2 activity. However, this effect has never yet been reported with the 85-kDa cytosolic PLA2 (cPLA2). We studied, in a model of differentiated and streptolysin O-permeabilized HL-60 cells, the effect of annexin V on cPLA2 activity after stimulation by calcium, GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), formyl-Met-Leu-Phe, or phorbol 12-myristate 13-acetate. Both recombinant and human placental purified annexin V inhibit cPLA2 activity whatever the stimulus used. The decrease of arachidonic acid release is of 40 and 50%, respectively, at [Ca2+] of 3 and 10 microM. The mechanism of inhibition was also analyzed. cPLA2 requires calcium and protein kinase C (PKC) or mitogen-activated protein kinase phosphorylation for its activation. As annexin V was shown to be an endogenous inhibitor of PKC, PKC-stimulated cPLA2 activity was analyzed. Using GF109203x, a specific PKC inhibitor, we demonstrated that this pathway is of minor importance in our model. cPLA2 inhibition by annexin V is not linked to PKC inhibition. To test the hypothesis of phospholipid depletion, mutants of annexin V were constructed using mutagenesis directed to Ca2+ site. We demonstrate that the Ca2+ site located in domain I is necessary for the inhibitory effect of annexin V on cPLA2 activity. The site in domain IV is also involved but with less efficiency. In contrast, mutations in site II and III do not modify this effect. Moreover, annexin V mutated on all sites does not inhibit cPLA2. Thus, we propose a predominant role of module (I/IV) in the biological action of annexin V, which, in physiological conditions, may control cPLA2 activity by depletion of the phospholipid substrate.

Annexins and protein kinases C

Annexins and protein kinases C belong to two distinct families of ubiquitous cytoplasmic proteins involved in signal transduction. All annexins share the property of binding calcium and phospholipids in the presence of calcium. Protein kinases C belong to three distinct groups of kinases: cPKCs (conventional PKCs) depend on calcium, diacylglycerol and negatively charged phospholipids for their activity, nPKCs (novel PKCs) depend on diacylglycerol and negatively charged phospholipids and aPKCs (atypical PKCs) only require negatively charged phospholipids. Almost all annexins are both in vitro and in vivo substrates for PKCs except annexin V. All annexins have a putative binding site for PKCs but only annexin V would possess a potential pseudo-substrate site. We propose that annexin V modulates the activity of some cPKCs on their substrates which may be the other annexins.

In vivo and in vitro phosphorylation of annexin II in T cells: potential regulation by annexin V

In order to understand how signal transduction occurs during T cell activation, it is necessary to identify the key regulatory molecules whose function is influenced by phosphorylation. Annexins II (A-II) and V (A-V) belong to a large family of Ca(2+)-dependent phospholipid-binding proteins. Among many putative functions, annexins may be involved in signal transduction during cellular proliferation and differentiation. In the present study we show that A-II is phosphorylated in vivo in the Jurkat human T cell line. Indeed, A-II is phosphorylated after stimulation by phorbol myristate acetate and on serine residues after T cell antigen receptor (TcR) stimulation. In cytosol from Jurkat cells, A-II is phosphorylated only by Ca2+/phospholipid-stimulated kinases such as Ca(2+)-dependent protein kinases C (cPKCs). A-V inhibits the phosphorylation of A-II and other substrates of cPKCs and has no effect on kinases activated only by phospholipids. In conclusion, A-II is phosphorylated both in vitro and in vivo in Jurkat cells, and may play a role as a substrate during signal transduction in lymphocytes via the TcR through the PKC pathway. On the other hand, A-V could act as a potent modulator of cPKCs in Jurkat cells.

Modulation of expression of genes involved in the inflammatory response by lipopolysaccharide and temperature in cultured human astroglial cells

In bacterial sepsis and meningitis, large concentrations of interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) correlate directly with morbidity and mortality. This laboratory has reported previously that elevated temperature in the physiologic range is associated with down regulation of IL-1 beta and TNF alpha expression in cultured astroglia after lipopolysaccharide (LPS) stimulation. To further investigate the role of elevated temperature in the CNS inflammatory response, the effects of LPS and elevated temperature on the expression of genes that participate in the inflammatory response were determined in cultured transformed human fetal astrocytes and in an astrocytoma cell line. The effect of physiologic temperature elevation on cytokine concentrations in cerebrospinal fluid (CSF) was also investigated in a rabbit meningitis model. The findings indicate that astrocytes express a wide variety of cytokines, growth factors, growth factor receptors, and other genes that could play important roles in CNS inflammation. Furthermore, temperature elevation in the febrile range can lead to alterations in the patterns of expression of many genes involved in the inflammatory response of these cells.

Annexin I in female rabbit reproductive organs: varying levels in relation to maturity and pregnancy

The level of annexin I, a 36 kDa calcium-dependent phospholipid-binding protein (36 kDa PLBP) in the reproductive organs of young, mature, and pregnant rabbits was determined immunologically with antibodies raised against purified rabbit lung annexin I. In the cytosolic fractions of the ovary, fallopian tube, uterus, and placenta, annexin I was the only major immunoreactive protein. The reproductive organs appeared to have higher annexin I levels than most nonreproductive organ tissues, except the lung and the spleen which were also rich in annexin I. A small amount of annexin I and a nearly equal amount of its hydrolytic product, a 33 kDa polypeptide, were detected in the amniotic fluid between 21 and 27 days gestation. Structural similarity of annexin I in the reproductive organs and in the lung was suggested by their identical isoelectric point values. Annexin I in the ovary of adult rabbits was 70% higher than that in the respective organ of immature rabbits. The uterus of pregnant rabbits had about 84% higher annexin I contents than that of the nonpregnant rabbits. The placenta had more annexin I per mg cytosolic protein than either the ovary or the uterus during pregnancy. The high concentration of annexin I in the reproductive organs may reflect specific functions of these organs in the reproductive years and during the reproductive cycle.

The annexins: specific markers of midline structures and sensory neurons in the developing murine central nervous system

The annexins are a family of cytoplasmic proteins that have been shown to have numerous actions within a cell. Recent evidence suggests that at least one of these proteins plays a role in the development of the central nervous system (CNS). The present study examines the temporal expression and spatial distribution of annexins I, II, IV, V, and VI during development and at maturity in the murine CNS by immunocytochemical analysis. The results demonstrate that annexins I, II and IV exhibit clear immunolabeling in the murine CNS with distinct patterns of temporal and spatial expression. Annexin IV is the first annexin to be expressed on embryonic day (E) 9.5 while annexin I is the last to be expressed (E11.5). Annexins I, II and IV are found in the floor plate region, but to differing rostrocaudal extents. Annexin I has a very restricted distribution, only present in the midline raphe of the brainstem. Annexin II is present in the spinal cord, brainstem and mesencephalon. Annexin IV has the widest midline distribution, being observed in the floor and roof plates of the developing CNS. Additionally, antibodies against annexin II and IV immunolabel most dorsal root and sensory ganglion cells and their axons. During early postnatal development, immunolabeling with each antibody gradually disappears in many structures, and only first order sensory neurons and their fibers are immunopositive for annexins II and IV at weaning. Three functions of the annexins are suggested by the present findings: (1) to help establish the midline structures of the floor and roof plates, (2) to help direct the decussation of sensory fibers, and (3) to regulate some aspect of sensory neuron processing, such as signal transduction.

Autoantibodies to annexins: a diagnostic marker for cutaneous disorders?

Annexins/lipocortins are a group of structurally related calcium and lipid binding proteins which have been implicated as mediators of the anti-inflammatory action of corticosteroids. Autoantibodies against annexin-1 have been reported in association with autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis and their presence has been hypothesized as the reason for the steroid resistance phenomenon. In this study we investigated IgG- and IgM-autoantibodies against annexin-1,-2,-3,-4,-5 and -6 in sera of 221 patients with skin disorders and 114 healthy blood donors with newly established ELISAs. Patients were clustered into 5 groups according to their diagnosis: autoimmune diseases, psoriasis, leg ulcer, malignant melanoma, and miscellaneous diseases. Autoantibodies directed against each annexin were detectable in all investigated groups, in the control group as well as in the disease groups, without displaying any significant correlation to any of the disease states. The homogenous distribution of annexin-autoantibodies throughout the control group and all the disease groups studied, do not support the implication of annexin-autoantibodies in pathophysiological states and make them an unlikely candidate for use as a diagnostic marker.

Immunocharacterization and developmental regulation of rabbit lung calcium-dependent phospholipid-binding proteins

The purpose of this work was to use the immunoblotting methods to study the 36 kDa calcium-dependent phospholipid-binding protein (PLBP) in the adult and fetal rabbit lungs to gain insight into the significance of this protein in lung development. The identity of the 36 kDa PLBP and the antigen specificity of the antiserum raised against this protein in the guinea pig were tested against known annexins and antibodies to the annexins. Our results showed that the rabbit lung 36 kDa PLBP contained only one protein which cross-reacted with antibodies against annexin 1. However, the 36 kDa PLBP was slightly smaller (36 vs. 37 kDa) and more acidic (pI 6.0 vs. 6.9) than the recombinant human annexin 1. The guinea pig antiserum only reacted with annexin 1, not with any of the other annexins tested. In the cytosolic fractions of the lung and the alveolar epithelial type II cells, and in the lung lavage fluid, the 36 kDa PLBP was by far the most prominent protein with minor presence of a 33 kDa protein recognized by the guinea pig antiserum. The amount of the 36 kDa PLBP of type II cells was 55% higher than that in the lung tissue and 2.6-times higher than that in the lung lavage (9.3 +/- 0.62, 6.0 +/- 0.31 and 3.6 +/- 0.04 micrograms/mg protein, respectively). The 36 kDa PLBP appeared in the fetal rabbit lungs as early as at 21 days gestation and increased 2-fold to reach the adult level at 27 days gestation (term 31 days). The high content of PLBP in type II cells and the rapid increase in this protein in the fetal lungs at late gestations suggest an important role of the 36 kDa PLBP in lung development and surfactant biogenesis.

Identification of an annexin-like protein and its possible role in the Aplysia eye circadian system

Light and serotonin regulate the phase of the circadian rhythm of the isolated eye of Aplysia. To screen for possible protein components of the eye circadian oscillator, we identified a number of proteins whose synthesis was altered in opposite ways by light and serotonin. The cellular function of one of these proteins was investigated by obtaining a partial amino acid sequence of it and by examining its immunoreactivity. A 38-amino acid sequence was obtained from a 40-kDa (isoelectric point 5.6) protein. A greater than 60% amino acid identity existed between this sequence and sequences of a family of calcium/phospholipid-binding proteins called annexins. Furthermore, the 40-kDa protein reacted with antibodies generated against a conserved amino acid sequence of annexins and with antibodies raised against human annexin I. The identification of the 40-kDa, light- and serotonin-regulated protein as an annexin led us to hypothesize that arachidonic acid metabolism plays a role in the Aplysia eye circadian system. To test this hypothesis, we examined the ability of an inhibitor of the arachidonic acid metabolic pathway to perturb the eye rhythm. Pulse treatments of isolated eyes with a lipoxygenase inhibitor, nordihydroguaiaretic acid, phase shifted the rhythm. The phase-shifting ability of nordihydroguaiaretic acid suggests that arachidonic acid and some of its metabolites may play a role in the eye circadian system. The results of our studies raise the possibility that links may exist between the 40-kDa annexin-like protein, arachidonic acid metabolism, and the circadian oscillator.

Expression of annexin I, II, V, and VI by rat osteoblasts in primary culture: stimulation of annexin I expression by dexamethasone

To determine whether rat osteoblasts synthesize proteins of the annexin family and to evaluate the extent to which glucocorticoids modulate the expression of annexins by these cells, osteoblasts were grown in primary cultures in the absence or presence of dexamethasone, and the expression of annexins was evaluated by immunoblotting using polyclonal antibodies against human annexins. Four different annexins (I, II, V, and VI) were found to be expressed by rat osteoblasts. The expression of annexin I, but not the other annexins studied, was increased in osteoblasts cultured in the presence of dexamethasone (173 +/- 33% increase comparing untreated cells and cells treated for 10 days with 5 x 10(-7) M dexamethasone). Increased expression of annexin I was observed after the third day of exposure to dexamethasone and rose thereafter until day 10; annexin I expression increased with dexamethasone concentrations above 10(-10) M throughout the range of concentrations studied. The increase in annexin I protein was associated with an increase in annexin I mRNA and was completely blocked by the concomitant addition of the glucocorticoid receptor antagonist RU 38486. The increase in annexin I content following dexamethasone treatment was associated with an increase in alkaline phosphatase activity and PTH-induced cAMP stimulation, whereas phospholipase A2 activity in the culture medium was reduced to undetectable levels. The finding that four annexins are expressed in rat osteoblasts in primary culture raises the possibility that these proteins could play an important role in bone formation by virtue of their ability to bind calcium and phospholipids, serve as Ca2+ channels, interact with cytoskeletal elements, and/or regulate phospholipase A2 activity. In addition, the dexamethasone-induced increase in annexin I may represent a mechanism by which glucocorticoids modify osteoblast function.

The surface properties of lung 36 kDa Ca(2+)-dependent phospholipid-binding protein

The intrinsic surface activity of a 36 kDa rabbit lung calcium-dependent phospholipid-binding protein (PLBP), a member of the annexin family of such proteins, at the air/water interface has been determined from measurements of surface tension of aqueous solutions, and surface concentration of 14C-labeled PLBP adsorbed from aqueous solution in the absence and presence of Ca2+. It was also possible to spread insoluble monolayers of PLBP to determine surface pressure vs. surface concentration isotherms, as well as surface elasticity and surface viscosity as a function of frequency from electrocapillary wave diffraction measurements. PLBP has been shown to exhibit significant intrinsic surface activity at the air/water interface, comparable to a variety of other hydrophobic proteins known to be quite surface active. In all cases, surface properties were enhanced by the presence of Ca2+, particularly the degree of surface viscoelasticity at close-packing in the monolayer. This is believed to reflect changes in protein conformation at the surface.

Tumor necrosis factors alpha and beta can stimulate bone resorption in cultured mouse calvariae by a prostaglandin-independent mechanism

Human recombinant tumor necrosis factors alpha and beta (TNF-alpha and TNF-beta), at and above 1 ng/ml (approximately equal to 70 pM), caused a dose- and time-dependent enhancement of 45Ca release from neonatal mouse calvarial bones in vitro. In addition, TNF-alpha and TNF-beta (3-100 ng/ml) caused a dose-dependent stimulation of prostaglandin E2 (PGE2) formation in the calvarial bones. TNF-alpha also enhanced the biosynthesis of PGI2, as assessed by analysis of the stable breakdown product 6-keto-PGF1 alpha. The stimulatory actions of TNF-alpha and TNF-beta on PGE2 formation was maximal at 12 h. Indomethacin, flurbiprofen, and meclofenamic acid, three structurally unrelated nonsteroidal antiinflammatory drugs, abolished PGE2 biosynthesis induced by TNF-alpha and TNF-beta (100 ng/ml). The 45Ca release stimulated by TNF-alpha and TNF-beta (100 ng/ml), however, was only slightly reduced by indomethacin, flurbiprofen, and meclofenamic acid. The partial inhibitory effect of indomethacin on 45Ca release was seen over a wide range of TNF-alpha concentrations, without affecting the concentration producing half-maximal stimulatory response. TNF-alpha and TNF-beta (100 ng/ml) stimulated bone matrix breakdown, as assessed by analysis of the release of 3H from bone prelabeled with [3H]proline. Also, the stimulatory effect of TNF-alpha and TNF-beta on bone matrix degradation was partially reduced by indomethacin. Hydrocortisone (1 microM) and dexamethasone (0.1 microM) abolished TNF-alpha- and TNF-beta-induced production of PGE2. In contrast to the cyclooxygenase inhibitors, the corticosteroids did not affect the stimulatory action by the cytokines on 45Ca release.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the presence and location of annexins in human platelets

In this study the identity of annexins in human platelets has been determined together with their ability to be released by agents which induce platelet degranulation. The presence of proteins cross-reacting to antibodies against annexins I and V was detected in human platelets. However, the study provided evidence that these annexins are not located on the surface of the plasma membrane in a Ca++ dependent manner. Moreover, activation of platelets with several agents which induced platelet degranulation did not cause release of annexins I or V as determined by both immunoblotting and ELISA.

Detection of VAC-beta (annexin-8) in human placenta

A 36 kDa calcium/phospholipid binding protein in human placenta was identified as VAC-beta (annexin-8) by a combination of immunological and peptide mapping analyses. The protein is a minor product in placenta, accounting for less than 1% of extracted annexins. From 150 g of tissue, only 100 micrograms of the protein was isolated. By anion-exchange chromatography on diethylaminoethyl-cellulose annexin-8 coeluted with annexin-3. By gel filtration, the protein chromatographed as a broad peak, where half the product eluted as a monomer and half eluted as a heterodimer that was associated with a 10 kDa subunit. The combination of annexin-8 being a minor component in standard annexin preparations and it co-eluting with annexin-3 by ion exchange chromatography are likely to account for the failure of other labs to characterize the product.

Intracellular Ca2+ mobilization and not calcium influx promotes phorbol ester-stimulated thromboxane A2 synthesis in human platelets

Phorbol esters, potent activators of protein kinase C (PKC), greatly enhance the release of arachidonic acid and its metabolites (TXA2, HETES, HHT) by Ca2+ ionophores in human platelets. In this paper, we report the relationship between intracellular Ca2+ mobilization and external calcium influx into platelets and the ability of PMA plus A23187 to promote thromboxane A2 (TXA2) synthesis. The enhanced levels of TXA2 due to the synergistic stimulation of the platelets with A23187 and phorbol esters are not affected significantly by the presence of external Ca2+ or the calcium-chelator EGTA. PKC inhibitors, staurosporine and sphingosine, abolished phorbol myristate acetate (PMA) potentiation of TXA2 production which strongly supports the role of PKC in the synergism. Platelet aggregation is more sensitive to PMA and external calcium than TXA2 formation. PMA increased TXA2 production as much as 4-fold at low ionophore concentrations. The A23187-induced rise in [Ca2+]i was reduced by pretreatment of human platelets with phorbol esters, both in the presence and absence of EGTA, and staurosporine reversed this inhibitory effect. These results indicate that the synergistic stimulation of TXA2 production by A23187 and phorbol esters is promoted by intracellular Ca2+ mobilization and not by external calcium influx. Our data also suggest that PKC is involved in the regulation of Ca2+ mobilization from some specific intracellular stores and that PKC may also stimulate the Ca(2+)-dependent phospholipase A2 at suboptimal Ca2+i concentrations.

Elevated expression of annexin II (lipocortin II, p36) in a multidrug resistant small cell lung cancer cell line

The doxorubicin-selected multidrug resistant small cell lung cancer cell line, H69AR, is cross-resistant to the Vinca alkaloids and epipodophyllotoxins, but does not overexpress P-glycoprotein, a 170 kDa plasma membrane efflux pump usually associated with this type of resistance. Monoclonal antibodies were raised against the H69AR cell line and one of these, MAb 3.186, recognises a peptide epitope on a 36 kDa phosphorylated protein that is membrane associated, but not presented on the external surface of H69AR cells (Mirski & Cole, 1991). In the present study, in vitro translation and molecular cloning techniques were used to determine the relative levels of mRNA corresponding to the 3.186 antigen. In addition, a cDNA clone containing an insert of approximately 1.4 kb was obtained by screening an H69AR cDNA library with 125I-MAb 3.186. Fragments of this cloned DNA hybridised to a single mRNA species of approximately 1.6 kb that was 5 to 6-fold elevated in H69AR cells. Partial DNA sequencing and restriction endonuclease mapping revealed identity of the cloned DNA with p36, a member of the annexin/lipocortin family of Ca2+ and phospholipid binding proteins.