Development of fluorogenic probe-based PCR assays for the detection and quantification of bovine piroplasmids.

This paper reports two new quantitative PCR (qPCR) assays, developed in an attempt to improve the detection of bovine piroplasmids. The first of these techniques is a duplex TaqMan assay for the simultaneous diagnosis of Babesia bovis and B. bigemina. This technique is ideal for use in South America where bovids harbour no theilerids. The second technique, which is suitable for the diagnosis of both babesiosis and theileriosis worldwide, involves fluorescence resonance energy transfer (FRET) probes. In FRET assays, Babesia bovis, B. divergens, Babesia sp. (B. major or B. bigemina), Theileria annae and Theileria sp. were all identifiable based on the melting temperatures of their amplified fragments. Both techniques provided linear calibration curves over the 0.1fg/microl to 0.01ng/microl DNA range. The assays showed good sensitivity and specificity. To assess their performance, both procedures were compared in two separate studies: the first was intended to monitor the experimental infection of calves with B. bovis and the second was a survey where 200 bovid/equine DNA samples from different countries were screened for piroplasmids. Comparative studies showed that duplex TaqMan qPCR was more sensitive than FRET qPCR in the detection of babesids.

A new set of molecular markers for the genotyping of Babesia bovis isolates.

Babesia bovis is a tick-borne apicomplexan pathogen that remains an important constraint for the development of cattle industries worldwide. The existence of different strains and subpopulations has long been described in this hemoparasite. However, few molecular markers have been reported for strain genotyping and characterization. Minisatellite sequences show high levels of variation and therefore provide excellent tools for both the genotyping and population genetic analysis. In this work we report a set of five molecular markers containing minisatellites that showed a variable degree of polymorphism in several American strains. We have used a bioinformatics approach to search for marker sequences contained in open reading frames. Five genes were chosen and primers were designed in conserved regions flanking the repeat region. Two of the genes were the previously described Bv80/Bb-1 and TRAP. The other three genes were named p200, Antigen 3 and Desmoyokin. Amplification by PCR, sequencing and comparative analysis of 11 strains from Argentina, Brazil, Uruguay, Mexico and USA determined that the tandem repeats varied in number and sequence among the isolates. Genome analysis of the five markers revealed that they were single copy and distributed across the four B. bovis chromosomes. When the new markers were analyzed in an experimental infection, absolute sequence conservation was found, indicating the stability of these markers during the course of infection. These markers were also stable during three syringe passages through calves. The application of this panel of molecular markers could provide new molecular tools for the genotyping of B. bovis isolates and analysis of changes in parasite populations following vaccination.

Identification of novel vaccine candidates against cryptosporidiosis of neonatal bovines by reverse vaccinology.

The apicomplexan protozoan Cryptosporidium parvum is an important causative agent of diarrhea of neonatal bovines. Vaccination has been proposed as an advantageous strategy against cryptosporidiosis of calves since besides protection against disease it has also the potential to prevent dissemination of infective oocysts into the environment. Antigens anchored to the parasite surface via glycosylphosphatidylinositol (GPI) are implicated in host cell attachment and invasion and represent promising vaccine candidates. A reverse vaccinology approach was employed to (i) identify the GPI-anchored proteome of C. parvum using available web-based bioinformatic tools and (ii) characterize previously unrecognized novel vaccine antigens. Altogether, 14 putative GPI-anchored proteins could be determined of which CpH1 and CpSUB2 as well as GP60 were further characterized. Sequencing and comparison of GP60, CpH1, and CpSUB1 alleles amplified from different geographic isolates showed a high degree of conservation. All three antigens were recombinant expressed and immunoblotted using sera of 12 Cryptosporidium-infected calves sampled at age periods 1-11 and 12-28 days after birth. Specific antibody reactions against the studied antigens were detected in all analyzed calves, demonstrating their immunreactivity and expression, and recognition in vivo at an early stage of host infection. Besides the acknowledged GP60 vaccinogen, the presented reverse vaccinology approach reveals the additional vaccine candidates CpH1 and CpSUB1 for inclusion into a subunit vaccine formulation.

A quantitative PCR assay for the detection and quantification of Babesia bovis and B. bigemina.

The haemoparasites Babesia bovis and Babesia bigemina affect cattle over vast areas of the tropics and temperate parts of the world. Microscopic examination of blood smears allows the detection of clinical cases of babesiosis, but this procedure lacks sensitivity when parasitaemia levels are low. In addition, differentiating between similar haemoparasites can be very difficult. Molecular diagnostic procedures can, however, overcome these problems. This paper reports a quantitative PCR (qPCR) assay involving the use of SYBR Green. Based on the amplification of a small fragment of the cytochrome b gene, this method shows both high sensitivity and specificity, and allows quantification of parasite DNA. In tests, reproducible quantitative results were obtained over the range of 0.1 ng to 0.1 fg of parasite DNA. Melting curve analysis differentiated between B. bovis and B. bigemina. To assess the performance of the new qPCR procedure it was used to screen for babesiosis in 40 cows and 80 horses. B. bigemina was detected in five cows (three of these were also found to be positive by standard PCR techniques targeting the 18S rRNA gene). In addition, B. bovis was detected in one horse and B. bigemina in two horses using the proposed method, while none was found positive by ribosomal standard PCR. The sequences of the B. bigemina cytochrome b and 18S rRNA genes were completely conserved in isolates from Spain and Argentina, while those of B. bovis showed moderate polymorphism.

Taking advantage of the polymorphism of the MSA-2 family for Babesia bovis strain characterization.

The Merozoite Surface Antigen-2 (MSA-2) family of Babesia bovis is a group of variable genes which share conserved 5' and 3' conserved ends. These genes encode membrane anchored glycoproteins, named MSA-2a1, a2, b and c, which are immunodominant antigens located on the surface of sporozoites and merozoites. In this work, we have analyzed the sequences of the msa-2a1, a2 and 2b genes in two geographically distant strains from Mexico and Argentina and detected a certain degree of genotypic diversity that can be exploited for the development of a new molecular tool for the discrimination of B. bovis field samples. Here, we describe a PCR restriction assay based on the msa2-a1, -a2 and -2b genes of B. bovis. When field strains from Argentina, Mexico and USA were analyzed, the results showed a strain-specific band pattern indicating the presence of differentially located BspMI restriction sites. This approach provides a simple method for the genotyping/strain differentiation of B. bovis field samples.

Search for Babesia bovis vaccine candidates.

Babesia bovis is a tick-borne apicomplexan pathogen that remains an important constrain for the development of cattle industries worldwide. Effective control can be achieved by vaccination with live attenuated forms of the parasite, but they have several drawbacks and thus the development of alternative subunit vaccines, either based in recombinant versions of full size proteins or in recombinant or synthetic peptides containing combinations of protective B-cell and T-cell epitopes is needed. Our current strategies for the identification of vaccine candidate antigens include the identification of functionally relevant antigens, bioinformatics, and comparative genomics using the recently sequenced B. bovis genome. These led us to the functional and immunological characterization of members of the VMSA gene family, a group of well conserved putative cysteine and serine proteases, and to the definition of a surface exposed B-cell epitope present in the Merozoite Surface Antigen-2c. Work in progress is focused in defining additional epitopes, and to determine whether they are neutralization-sensitive. These approaches might unravel useful vaccine candidates for B. bovis, and will increase our understanding of the pathogenicity mechanisms of these and related hemoparasites.

Temperature acclimation of Trypanosoma cruzi epimastigote and metacyclic trypomastigote lipids.

This study examines the changes in cellular lipids that take place when Trypanosoma cruzi epimastigotes and metacyclic trypomastigotes are transferred from 28 to 37 degrees C. We found a rise in the sterol to phospholipid ratio, as well as in the triacylglycerol and steryl ester cellular content in T. cruzi epimastigotes. In addition, saturated to unsaturated fatty acid ratios in phospholipids increase. This latter effect appears to be due to two concurrent processes. Firstly, fatty acyl delta9 and, especially, delta12 desaturations are significantly diminished at 37 degrees C. Secondly, triacylglycerols and steryl esters undergo changes in their fatty acyl composition opposite to those simultaneously observed in phospholipids, i.e. the ratio of saturated to unsaturated fatty acids markedly decreases. Similar alterations in each of the lipid classes and in the fatty acid composition of polar and neutral lipids were found in cultured metacyclic trypomastigotes on exposure to the same shift-up. These observations suggest that a global remodeling of cellular lipids that involves extensive fatty acid exchange between neutral and polar lipid pools represents a novel and important mechanism of adaptation of the parasites to the temperature changes they encounter in their life cycle.

Babesia bovis merozoite surface protein-2c (MSA-2c) contains highly immunogenic, conserved B-cell epitopes that elicit neutralization-sensitive antibodies in cattle.

The search for vaccine candidates against bovine babesiosis caused by Babesia bovis is greatly focused on the identification of merozoite surface-exposed antigens that are widely conserved, functionally relevant and immunodominant in cattle protected against B. bovis infections. We have recently identified msa-2c, a member of the B. bovis variable merozoite surface antigen (VMSA) gene family, which in contrast to other members, appears to be highly conserved among geographically distant B. bovis strains. In this study, we further investigated the potential of the msa-2c gene product as diagnostic and vaccine candidate for bovine babesiosis. RT-PCR studies demonstrated that MSA-2c is transcribed in merozoites of the Argentine R1A strain. In addition, antibodies against R1A recombinant MSA-2c reacted in immunoblots with a single protein of approximately 30kDa in B. bovis merozoite extracts from both R1A and Australian "S" strains, demonstrating translation of this protein in these two strains and conservation of B-cell epitopes between them. These antibodies reacted with the cell surface of R1A merozoites in fixed immunofluorescence assays, indicating the surface localization of MSA-2c. This localization was confirmed by live immunofluorescence studies in two different strains, R1A and S2P. These results also demonstrate the conservation of MSA-2c surface-exposed B-cell epitopes between these two strains. Sera from cattle either naturally or experimentally infected with Argentine strains of B. bovis specifically recognized rMSA-2c in immunoblots, reinforcing the idea that B-cell epitopes in rMSA-2c are widely conserved among field strains of B. bovis. Furthermore, our results show that these B-cell epitopes are highly immunogenic, suggesting that MSA-2c may be a useful diagnostic tool for the detection of bovine babesiosis by B. bovis. Experimental vaccination of five bovines with rMSA-2c resulted in elicitation of high specific anti-rMSA-2c IgG titers, with similar amounts of IgG(1) and IgG(2) produced. Importantly, bovine anti-rMSA-2c antibodies were able to neutralize in vitro bovine erythrocyte invasion by R1A merozoites suggesting a significant functional role for MSA-2c. Taken together these results postulate MSA-2c as a candidate for the development of novel tools for improved control of bovine babesiosis.

Phosphatidylcholine formation is the predominant lipid biosynthetic event in the hemoparasite Babesia bovis.

This work examines the lipid composition and metabolism of bovine red blood cells infected by apicomplexan Babesia parasites, organisms closely related to Plasmodium sp. We found that erythrocytes infected with Babesia bovis (i-RBC) accumulate lipids and show striking increases in phosphatidylcholine, phosphatidic acid, diacylglycerol and cholesteryl esters as compared to uninfected erythrocytes cultured under the same conditions (n-RBC). A similar pattern was observed in cultures of erythrocytes infected with Babesia bigemina. The lipid profile of purified B. bovis merozoites showed that phosphatidylcholine is the most abundant phospholipid in this parasite (31.8% +/- 2.8 of total phospholipid), markedly differing from bovine n-RBC, in which it is only a minor component (4.8% +/- 0.6). B. bovis cultures incorporate radiolabeled choline into complex lipids, especially phosphatidylcholine, with minor amounts recovered in sphingomyelin and lysophosphatidylcholine. When [14C] stearate was used as precursor, the labeling pattern again gave the highest incorporation into phosphatidylcholine, with lesser incorporation in sphingomyelin, phosphatidylinositol, phosphatidylethanolamine and phosphatidic acid. Diacylglycerol and small amounts of cholesteryl esters were the only labeled neutral lipids found. B. bovis also incorporates [3H] myo-inositol into phosphatidylinositol. Parallel incubations with n-RBC as a control yielded no incorporation into either polar or neutral lipids with any precursor. These results indicate that the lipid changes observed in i-RBC can be explained on the basis of the lipid biosynthetic activities of the babesial parasite. Gas chromatography-mass spectrometry (GC-MS) analysis of fatty acid methyl esters from phospholipids of i-RBC and n-RBC showed the same qualitative composition in both. However, i-RBC had higher ratios of saturated to unsaturated fatty acids and B. bovis cultures did not desaturate [14C] stearate. Cholesterol was the only sterol detected by GC-MS. Phospholipase A2 treatment of i-RBC and n-RBC revealed no enhanced hemolytic effects in i-RBC, suggesting that the erythrocyte membrane phospholipid composition is essentially unaltered by the parasite. Labeling of i-RBC or n-RBC with [125I] Bolton-Hunter resulted in an enhanced phosphatidylserine labeling in i-RBC. This study provides the first data on B. bovis lipid constitution and biosynthesis. They show that phosphatidylcholine formation is the main biosynthetic process in these cells. The striking differences in the contents of phosphatidylcholine between host erythrocytes and the parasite suggests that it may be a useful target for both chemotherapy and immunoprophylaxis against bovine babesiosis.

Evidence for extensive genetic diversity and substructuring of the Babesia bovis metapopulation.

Babesia bovis is a tick-transmitted haemoprotozoan and a causative agent of bovine babesiosis, a cattle disease that causes significant economic loss in tropical and subtropical regions. A panel of nineteen micro- and minisatellite markers was used to estimate population genetic parameters of eighteen parasite isolates originating from different continents, countries and geographic regions including North America (Mexico, USA), South America (Argentina, Brazil), the Middle East (Israel) and Australia. For eleven of the eighteen isolates, a unique haplotype was inferred suggesting selection of a single genotype by either in vitro cultivation or amplification in splenectomized calves. Furthermore, a high genetic diversity (H = 0.780) over all marker loci was estimated. Linkage disequilibrium was observed in the total study group but also in sample subgroups from the Americas, Brazil, and Israel and Australia. In contrast, corresponding to their more confined geographic origin, samples from Israel and Argentina were each found to be in equilibrium suggestive of random mating and frequent genetic exchange. The genetic differentiation (F(ST)) of the total study group over all nineteen loci was estimated by analysis of variance (Θ) and Nei's estimation of heterozygosity (G(ST')) as 0.296 and 0.312, respectively. Thus, about 30% of the genetic diversity of the parasite population is associated with genetic differences between parasite isolates sampled from the different geographic regions. The pairwise similarity of multilocus genotypes (MLGs) was assessed and a neighbour-joining dendrogram generated. MLGs were found to cluster according to the country/continent of origin of isolates, but did not distinguish the attenuated from the pathogenic parasite state. The distant geographic origin of the isolates studied allows an initial glimpse into the large extent of genetic diversity and differentiation of the B. bovis population on a global scale.

In silico predicted conserved B-cell epitopes in the merozoite surface antigen-2 family of B. bovis are neutralization sensitive.

The merozoite surface antigens MSA-2 of Babesia bovis constitute a family of polymorphic GPI-anchored glycoproteins located at the parasite cell surface, that contain neutralization-sensitive B-cell epitopes. These are therefore putative vaccine candidates for bovine babesiosis. It was previously shown that (i) the MSA-2 antigens of the biologically cloned Mo7 strain are encoded by four tandemly organized genes: msa-2a(1), a(2), b and c, and (ii) at least one allele of each of these genes is present in the Argentine R1A strain with a moderate degree of polymorphism. The present work was aimed at defining neutralization-sensitive B-cell epitopes in the MSA-2 family, that are conserved among different B. bovis geographical isolates. To this end, msa-2a, b and c alleles from different isolates from Argentina, USA and Mexico were amplified by PCR, cloned and sequenced. Bioinformatic analysis by ClustalW alignments and B-cell epitope prediction algorithms performed on these sequences allowed the identification of several regions containing putative conserved B-cell epitopes. Four peptides representing these regions: (KDYKTMVKFCN from msa-2a(1); YYKKHIS, from msa-2b; and THDALKAVKQLIKT and ELLKLLIEA from msa-2c) were chemically synthesized, conjugated to keyhole limpet hemocyanin and used to inoculate mice to obtain immune sera. Anti-peptide antibodies recognized B. bovis merozoite extracts in all cases in ELISA tests. In addition, these sera reacted with the surface of merozoites of an Argentine and a Mexican B. bovis strains in immunofluorescence assays, and sera against two of the selected peptides inhibited invasion of erythrocytes by in vitro cultured merozoites. Taken together, the results show that the peptide sequences selected by bioinformatic analysis represent expressed and geographically conserved B. bovis B-cell epitopes that might be strong candidates for development of subunit vaccines.

Molecular characterization of babesia bovis strains using PCR restriction fragment length polymorphism analysis of the msa2-a/b genes.

The merozoite surface antigen-2 (msa-2) family of Babesia bovis is a group of variable genes that share conserved 5' and 3' ends and encode for membrane-anchored glycoproteins that have been postulated as vaccine candidates. In this work, we analyzed the sequences of three of these genes (msa-2a1, a2, and 2b) from two geographically distant strains and detected a certain degree of genotypic diversity that could be exploited to work out new molecular tools for the discrimination of B. bovis field samples. Here we describe a PCR restriction assay that was developed based on this observation and tested on several B. bovis strains and isolates. The results show a strain-specific band pattern in geographically distant isolates, indicating the presence of differentially located BspMI restriction sites. This approach provides a simple method for the differentiation of American B. bovis strains.

Development of an indirect ELISA for the diagnosis of equine piroplasmosis.

An indirect ELISA (iELISA) for the detection of specific anti-Theileria equi antibodies in horse serum was developed. Its performance showed good concordance (K= 0.79) when compared with a competitive ELISA recommended by the World Organisation for Animal Health. Horse serum samples from two provinces located in the north and east of Argentina (Formosa and Entre Rios, respectively) were analyzed by this iELISA. A high percentage of positive horses were found in Formosa, consistent with the climatic conditions of the region that are apt for the development of tick vectors. Surprisingly, seropositive animals were also detected, although in a lower proportion, in Entre Rios, which has a temperate weather and is presumably tick free. Breeding of thoroughbred racing horses is an important economic asset of Argentina. Since equine piroplasmosis is a constraint for horse export, the epidemiological situation in different regions of the country needs to be assessed for the implementation of control measures. The iELISA developed in this work provides a convenient tool for this type of study.

Lysosomal glycerophosphocholine phosphodiesterase in Tetrahymena.

The purification and characterization of a novel phosphodiesterase (PDE) is presented. The activity was detected in the extracellular medium of Tetrahymena thermophila cultures, by the release of p-nitrophenol from p-nitrophenylphosphocholine (PNPPC) with an acidic pH optimum. In cell homogenates, it is sedimentable, shows a latency similar to that of acid phosphatase and is co-secreted with this enzyme, indicating that it is a lysosomal hydrolase. PNPPC-PDE was purified to homogeneity from the extracellular medium, yielding a single band of 58 kD on SDS polyacrylamide gel electrophoresis. It catalyzed the release of glycerol from glycerophosphocholine (GPC) and GPC competitively inhibits degradation of PNPPC. We present further evidence indicating that the natural substrate for PNPPC-PDE is GPC. Thus, Tetrahymena becomes the first eukaryote in which a lysosomal GPC-PDE is observed. This finding provides a new pathway for the complete breakdown of phosphatidylcholine in a lysosomal medium.

Screening for and characterization of phospholipase A1 hypersecretory mutants of Tetrahymena thermophila.

We have described a procedure for the isolation of mutants of Tetrahymena thermophila with hypersecretion of phospholipase A1 (PLA1). Using random chemical mutagenesis, uniparental cytogamy, genetic crossing and a new, fast and effective screening procedure, four PLA1-hypersecretory mutants were isolated. The screening procedure is based on the formation of a halo appearing around cylindrical holes in a lecithin-containing agar plate filled with cell-free supernatants. About 3,940 clones were tested with this procedure in primary screening for hypersecretory features, of which 60 putative hypersecretory mutants were isolated, subcloned and tested in a secondary screening. Of these, four selected mutants showed 1.8-2.2 more PLA1 activity in the cell-free supernatants compared to the wild-type strain CU 438.1. Hypersecretion was only observable for PLA1; no increased activity for two other lysosomal enzymes could be detected. These hypersecretory mutants of T. thermophila can be very useful for increasing the yield of PLA1 in fermentation processes. This is particularly relevant because, in contrast to other phospholipases, PLA1 is not available on the commercial market for fine chemicals and little is known about the role of PLA1 in cell signaling and metabolism.

Uptake and metabolism of L-serine by Pneumocystis carinii carinii.

Serine is an important amino acid that is utilized in the biosyntheses of proteins and lipids. It is directly incorporated into the head group of phosphatidylserine, which in turn can be converted to other phospholipids. Also, it is required for the formation of long chain bases, precursors of sphingolipids. Uptake and incorporation of radiolabeled serine into both lipids and acid-precipitable material were demonstrated in Pneumocystis carinii carinii organism preparations freshly isolated from infected rat lungs. Radioactivity in proteins was about double that observed in lipids. Liquid scintillation spectrometry of metabolically radiolabeled lipids separated by thin-layer chromatography showed 53% of the total radioactivity were in phosphatidylserine, 12% in phosphatidylethanolamine, 24% in ceramides, and 11% in long chain bases and other compounds. Four long chain bases were detected by thin-layer chromatography in hydrolyzed P. carinii ceramides metabolically labeled with radioactive serine. Phytosphingosine and dihydrosphingosine were tentatively identified by their migrations on thin-layer plates. Radiolabeled ethanolamine was incorporated into P. carinii phosphatidylethanolamine, but relatively low incorporation of radiolabeled choline into phosphatidylcholine occurred. The observations made in this study indicated that P. carinii has the biosynthetic capacity to metabolize phospholipid head groups and to de novo synthesize sphingolipids. L-Cycloserine and beta-Cl-D-alanine, inhibitors of long chain base synthesis, reduced the incorporation of serine into P. carinii long chain bases and ceramides, which supported the conclusion that the pathogen synthesizes sphingolipids.

The E1a gene prevents inhibition of keratinocyte proliferation by dexamethasone.

Glucocorticoids have inhibitory effects on the proliferation of several cell types. In this study, we found that dexamethasone, a synthetic steroid with glucocorticoid activity, inhibits proliferation of established mouse Pam 212 keratinocytes. Transfection with the adenoviral early region 1a (E1a) gene confers a strong resistance to the inhibition by dexamethasone. Two deletion E1a mutants, one whose product lacks the ability to bind the cellular proteins p60/p105/p107 and another that is unable to bind p300, were shown to induce a resistance similar to that associated with the intact E1a gene. These results differ from those previously observed with two other growth inhibitory signals, transforming growth factor beta 1 and adenosine 3',5'-cyclic monophosphate, in which the mutated E1a genes confer only partial or no resistance, indicating that a different mechanism mediates resistance against glucocorticoids.

Metabolic fate of plasma membrane diacylglycerols in NIH 3T3 fibroblasts.

We have examined the metabolism of three radiolabeled 1,2-diacylglycerols (DGs) in NIH 3T3 fibroblasts. Since the lipids used are not appreciably taken up by the cells, we used a phosphatidylserine (PS)-based liposome fusion system to rapidly associate the lipid species with the plasma membrane. When 1,2-[1-14C]dioleoyl-sn-3-glycerol ([14C]DOG) is delivered in this way, it is rapidly converted predominantly to phosphatidylcholine (PC) and triacylglycerol (TG) and to a lesser extent, to monoacylglycerol (MG) and fatty acids (FA), as well as phosphatidic acid (PA) and phosphatidylinositol (PI). We present evidence that [14C] DOG is largely utilized as an intact molecule rather than being broken down to FA and then incorporated to cell lipids. Examination of the metabolism of 1-stearoyl-2-[1-14C]myristoyl-sn-3-glycerol ([14C]SMG) and 1-stearoyl-2-arachidonoyl-sn-3-glycerol ([14C]SAG) reveal important differences. Both produce substantial labeling of PC but [14C]SMG gives rise to the highest proportion of TG and the lowest of PA and PI, whereas [14C]SAG yields the opposite pattern. When phosphatidic acid labeled on its glycerol backbone (1,2-dioleoyl-sn-[U-14C] glycero-3-phosphate) was supplied to the cells via the liposomes, rapid appearance of labeled DG was found which then decreased with concomitant labeling of cellular PC and TG. Only small amounts of the glycerol backbone were recovered in PI. Our experiments identify three types of processes involved in the metabolism of plasma membrane DGs: (i) transferase-catalyzed conversions to PC and TG, (ii) lipolytic breakdown to MG and FA, and (iii) phosphorylation to PA and then conversion to PI. The relative proportions of each DG species converted to these different products are strongly dependent on the fatty acyl composition of the particular DG molecular species, even though formation of PC is the major event in all cases. Since DGs are important second messengers, our study supports the view that conversion to PC and TG can play a key role in DG signal attenuation.

Diversity of roles of protein kinase C alpha in the proliferation of Swiss 3T3 cells.

We examined the role of protein kinase C alpha (PKC alpha ) in the stimulation of DNA synthesis of Swiss 3T3 cells induced by bombesin, platelet-derived growth factor (PDGF) and phorbol 12-myristate 13-acetate (PMA). We found that cells in which this kinase had been down-regulated showed a partially abrogated mitogenic response to bombesin. The response to PDGF was unaltered; however, the response to PMA was completely suppressed. The mitogenic effect of maximal doses of bombesin and PMA combined was greater than that of either agent alone, suggesting that bombesin does not fully activate the PKC pathway. Accordingly, bombesin-induced PKC alpha translocation from cytosol to membranes was partial, while that observed with PMA was essentially complete. Moreover, exposure to Ro-31-8220, a PKC inhibitor, had significantly greater effects on the response to PMA than on that to bombesin. Our findings point out different roles that PKC alpha may play in diversely activated cells: while, in the case of PMA, stimulation of this kinase may be necessary and sufficient to induce proliferation, it appears to be necessary only for a full response to bombesin, and redundant among the mechanisms triggered by PDGF.