Circulating antibodies to mouse laminin in Chagas disease, American cutaneous leishmaniasis, and normal individuals recognize terminal galactosyl(alpha 1-3)-galactose epitopes.

Sera from patients with American cutaneous leishmaniasis and Chagas disease and from monkeys infected with either Trypanosoma cruzi or Trypanosoma rhodesiense show, in RIAs, strong binding to mouse laminin. A distinct although weaker binding activity is also detected in normal human sera. The antibodies recognize a common carbohydrate epitope present on mouse laminin, which was assigned to a terminal galactosyl(alpha 1-3)-galactose group. Distinct crossreactions were observed with some other basement membrane proteins, rabbit glycosphingolipids, defucosylated human B blood group substance and components produced by some human tumor cells. Only little activity was, however, found on laminin obtained from human placenta. The data indicate that the antibodies arising in infectious diseases are stimulated by similar carbohydrate epitopes present on the surface of parasites. Tissue-specific occurrence of such epitopes may exist and explain the involvement of distinct tissues in autoimmune disorders.

Measured and calculated SF-6 collision and swarm ion transport data in SF6 -Ar and SF6 -Xe mixtures.

The measurement of the mobility of SF-6 in the mixtures SF6 -Ar and SF6 -Xe is reported over the density-reduced electric field strength E/N 1-180 Td (1 Townsend = 10(-17) V cm(2)), from a time-resolved pulsed Townsend technique. Simultaneously, the mobility of SF-6 in the same binary mixtures has been calculated from a set of collision cross sections for SF-6 -Ar, SF-6 -Xe, and SF-6 - SF6 using a Monte Carlo simulation procedure for ion transport. The good agreement between measured and calculated mobilities in these gas mixtures has led us to conclude that the validation of our cross section sets is confirmed. The elastic collision cross section, a predominant process for ion energies lower than about 10 eV, was determined from a semiclassical JWKB approximation using a rigid core potential model for the ion-neutral systems under consideration. This elastic cross section was then added to several other inelastic collision cross sections found in the literature for ion conversion, electron detachment of SF-6 and charge transfer. Moreover, the calculations of the mobility and the ratios of the transverse and longitudinal diffusion coefficients to the mobility were extended into a much wider E/N range from 1 to 4000 Td. Additionally, we have also calculated the energy distribution functions and the reaction coefficients for ion conversion and electron detachment. Finally, we have shown that the range of validity for the calculation of the mobility in gas mixtures from Blanc's law is only valid for the low E/N region, where the interaction is dominated by elastic collisions and the ion distribution function remains essentially Maxwellian.

Electron drift velocities in mixtures of helium and xenon and experimental verification of corrections to Blanc's law.

Measurements of electron drift velocities were performed in pure Xe and He and in a number of mixtures ranging up to 70% of Xe. The data were obtained by using a pulsed Townsend technique over the density-normalized electric field strength E/N between 1 and 100 Td . Even for pure gases there are no data in the entire range covered here, and these data represent an extension of accurate drift velocities to higher E/N. A selection of well-established cross sections for low energies, which was extended to higher energies, led to a reasonably good agreement of the calculated transport coefficients with the available data. At the same time we have applied the standard (common E/N) Blanc's law and two forms of common mean energy (CME, due to Chiflykian) procedures. Blanc's law fails for most mixtures at low and moderate E/N, while the CME procedure is capable of following the experimental data for the mixtures much more closely, and even predicting the negative differential conductivity region when such effect does not exist for pure gases. Thus the present paper also represents an experimental test of procedures to correct the standard Blanc's law. Finally, we have used the data for two mixtures to obtain results for the third mixture and in all cases this procedure gave excellent results even though only the standard Blanc's law was used in the process.

Uptake and metabolism of S-adenosyl-L-methionine by Leishmania mexicana and Leishmania braziliensis promastigotes.

Promastigotes of Leishmania mexicana and Leishmania braziliensis incorporate S-adenosyl-L-[3H-methyl]methionine (AdoMet) against a concentration gradient through a saturable system. This concentrative uptake requires metabolic energy and is sensitive to temperature and sulfhydryl reagents such as N-ethyl maleimide. Intracellular AdoMet exchanges with external AdoMet. At steady state, unaltered ADoMet in the intracellular pool is at about a 1800-fold concentration in relation to that found in the external medium. Glucose, galactose and ribose did not stimulate uptake rates. Incorporated AdoMet goes into the soluble AdoMet pool, where a small fraction is metabolized, chiefly into methylthioadenosine, decarboxylated AdoMet and methanol. After a 60 min pulse the radioactivity associated with the [3H]AdoMet incorporated disappears with a half-time of 2 h. Transmethylation reactions were analyzed following [3H]AdoMet incorporation. Fractionation experiments indicate that 45-62% and 30-42% of the radioactivity is incorporated into lipids and protein methyl esters respectively, with 5-14% present in the soluble pool of parasites. Sinefungin or its cyclic derivative (1 and 10 micrograms ml-1) in the incubation medium produces 58% and 64% inhibition of AdoMet incorporation into Leishmania promastigotes. Most transmethylation reactions are inhibited, as there is a 50% decrease in the total radioactivity present in both the base-labile and lipidic fraction, with a parallel increase in the percentage of radioactivity in the soluble pool. Previous results give evidence of the importance of AdoMet in American Leishmania promastigote metabolism.

Correlation of sinefungin susceptibility and drug-affinity for protein carboxymethyltransferase activity in American Leishmania species.

Among promastigotes of 22 different American Leishmania strains, a 5000-fold variation in sinefungin susceptibility was found, apparently independent of their taxonomic classification, although L. mexicana strains did tend to be more resistant than L. braziliensis. Protein carboxymethyltransferase (EC 2.1.1.24) and glycine N-methyltransferase (EC 2.1.1.20) activities were not substantially different in sinefungin-susceptible and -resistant American Leishmania strains. However, when [methyl-3H]methionine incorporation into total protein or gamma-glutamyl residues of leishmanial proteins was carried out in the presence or absence of sinefungin, protein carboxymethylating activity was significantly inhibited only in sinefungin-susceptible Leishmania strains. Furthermore, when protein carboxymethyltransferase activity was purified from several leishmanial strains to a state of electrophoretic homogeneity (sp. act. = 240 nmol h-1 (mg protein)-1), the enzyme from the resistant cells showed a higher inhibition constant (mean Ki 55 microM against 2 microM in susceptible cells) for sinefungin. This 28-times stronger affinity of the susceptible cell enzyme towards sinefungin despite normal protein carboxymethyltransferase specific activity seems to be a key element of the resistance mechanism of certain American Leishmania strains.

Differences in allopurinol and 4-aminopyrazolo(3,4-d) pyrimidine metabolism in drug-sensitive and insensitive strains of Trypanosoma cruzi.

Most freshly isolated Trypanosoma cruzi blood trypomastigotes were insensitive to allopurinol (HPP) and 4-aminopyrazolo(3,4-d)pyrimidine (APP). Strains EP and Ya were, however, strongly inhibited by both drugs while strains DS and A-35 were HPP-insensitive but APP-sensitive. In contrast, epimastigotes resulting from one in vitro passage of all eleven T. cruzi strains were highly sensitive to both drugs. While hypoxanthine/guanine and adenine phosphoribosyltransferase and succino-AMP synthetase activities were similar in trypomastigotes of sensitive and insensitive T. cruzi strains, the uptake and metabolism of [14C]HPP and [14C]APP was significantly slower in T. cruzi trypomastigotes of insensitive strains than in sensitive strains. The results suggest the importance of drug uptake rates in determining the pyrazolopyrimidine sensitivity of different T. cruzi strains.

Differential metabolism of allopurinol and derivatives in Trypanosoma rangeli and T. cruzi culture forms.

We have compared the metabolism of allopurinol (4-hydroxypyrazolo (3,4-d)pyrimidine; HPP), by Trypanosoma rangeli, a non-pathogenic American trypanosome, and T. cruzi, the causative agent of Chagas' disease. Our results indicate that T. rangeli was unable to animate allopurinol mononucleotide to 4-aminopyrazolopyrimidine (APP) mononucleotide. Radioactivity was located in the RNA, but not the DNA, only of T. cruzi. Substrate specificity studies showed T. cruzi succino-AMP synthetase activity being 100-fold more active on HPP mononucleotide than the T. rangeli enzyme. These results possibly explain the fact that in conventional LIT medium T. rangeli growth was resistant to high concentrations of HPP. However the incubation of this hemoflagellate in an adenine-depleted LIT medium rendered it sensitive to low concentrations of APP, while remaining resistant to HPP. In contrast, and independently of the culture medium used, T. cruzi was extremely sensitive to both pyrazolopyrimidines.

Depressed autoantibody synthesis in Trypanosoma cruzi-infected rats born to mothers undergoing this infection during pregnancy.

Earlier work indicated that Trypanosoma cruzi infection in pregnant rats decreased the amount of myocardial damage that developed in their chronically infected offspring. Given the suspected role of autoimmune mechanisms in the generation of chronic myocarditis, we evaluated whether this maternal intervention was likely to affect the synthesis of autoantibodies in infected young. Autoantibodies were investigated against molecules exhibiting cross-reactivity with T. cruzi antigens or not, that is cerebroside sulphate (sulphatide) and actin, respectively. Female '1' rats (75 days old) that had been mated with syngeneic sires were separated into two groups, one challenged with living trypomastigotes at 7, 14 and 21 days following mating, and the other one given physiologic saline at the same intervals. At the time of weaning, offspring were injected with 10(6)/T. cruzi to constitute two infected groups: young born to infected mothers (InMoTc) and young delivered by uninfected mothers (CoMoTc). Serum antibodies were investigated by ELISA at 30 and 60 days post-infection, which represents acute and chronic infection, respectively. T. cruzi infection was associated with the production of anti-sulphatide antibodies, but the phenomenon was significantly less evident in InMoTc young and virtually unnoticeable during their chronic infection. Unlike the anti-sulphatide results, levels of anti-actin antibodies showed no differences between CoMoTc and InMoTc rats when compared during acute or chronic infection. The decreased production of anti-sulphatide autoantibodies of InMoTc offspring may be due to a modification of the immune repertoire of offspring because of the contact with parasite antigens during ontogeny.

Elevated cerebroside antibody levels in human visceral and cutaneous leishmaniasis, Trypanosoma rangeli infection, and chronic Chagas' disease.

A natural cerebroside (antiC) IgM antibody was found at relatively high levels in the serum of every healthy individual studied. The reactivity of the antibody was assessed by using highly purified bovine brain galactocerebroside (galC) or human glucocerebroside (gluC) as antigen. The importance of fatty acid moiety of galC in antigen-antibody reaction was demonstrated by low immunoreactivity using 1-beta-D-galactosyl sphingosine (GS) as antigen and by the absence of absorption to GS-bearing liposomes. The presence of alpha-hydroxy and non-hydroxy fatty acids in galC did not modify its immunoreactivity. Cerebroside antibody binding activity was only partially blocked by 0.5 M galactose or alpha- and beta-methylgalactopyranoside, suggesting poor specificity of antiC for a specific glycosidic residue or linkage. In fact, liposome-bearing gluC absorbed galC. AntiC did not adsorb on rabbit, guinea pig, or human erythrocytes (RBC), but absorbed strongly on rat RBC. Elevated antibody levels were found in 57% of Kala azar patients, 56% of Trypanosoma rangeli-infected patients, 30% of chronic chagasic patients, and 20% of cutaneous leishmaniasis patients, but were not found in 16 other inflammatory or infectious diseases studied. This suggests an association between Kinetoplastida infection and elevated levels of antiC, with parasitic galC acting probably as a highly immunogenic antigen. A possible role of anti-galC in the neuropathological symptoms of Chagas' disease and in the control of parasitemia levels in T. rangeli-infected individuals is discussed.

Inhibitory effects of sinefungin and its cyclic analog on the multiplication of Trypanosoma cruzi isolates.

Sinefungin and its cyclic analog were evaluated in vitro for activity against the multiplication of Trypanosoma cruzi. When either drug was tested for eight days on twelve T. cruzi epimastigote isolates, an 800-fold difference in drug sensitivity was found. Both drugs were trypanostatics at a concentration range from 0.1 micrograms/ml to 300 micrograms/ml. The 50% effective concentration (EC50) of sinefungin and its cyclic analog at which the growth of a given isolate was inhibited was 0.38 micrograms/ml for sinefungin and 0.31 micrograms/ml for the cyclic analog against the Ma, Marin, OPS-86, Y, and Ya isolates, and > 300 micrograms/ml for sinefungin and 217 micrograms/ml for the cyclic analog against the A-35, Bertoldo, DS, EP, ES, OPS-58, and FL isolates. Incubation of drug-sensitive isolates for more than 10 days in glucose-saline (GS) medium, but not in minimal essential medium, in the presence of a 30-fold EC50 concentration of the drug induced an increase in the drug-resistant population, which maintained this phenotype for several passages in drug-free culture medium. Growth curves were analyzed as a function of parasite inoculum; it was observed that with sinefungin-sensitive T. cruzi epimastigote isolates grown in GS medium in the presence of 10 micrograms/ml of the drug, the inhibitory effects of the drug were dependent on the initial inoculum: 1 x 10(3)-1 x 10(4) parasites/ml were strongly inhibited even after 16 days. Significant impairment of thymidine incorporation into the DNA of parasites by both drugs was observed only in drug-sensitive epimastigote isolates.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a natural human antibody with anti-galactosyl(alpha 1-2)galactose specificity that is present at high titers in chronic Trypanosoma cruzi infection.

An antibody reactive with the galactosyl(alpha 1-2)galactose [gal(alpha 1-2)gal] epitope was characterized in human sera by enzyme-linked immunosorbent assay, red blood cell (RBC) and laminin absorption, and oligosaccharide inhibition. This antibody was found evenly distributed between the IgG and IgM classes and was present at high titers in the serum of all normal adults studied, but in 75% of children less than three years of age, it was observed at the lower limit of detection, and gradually increased to adult levels by the age of six. Although this antibody bound to gal(alpha 1-3)gal-linked synthetic antigens, it did not bind to the same residues present in rabbit, rat, and guinea pig RBC or in murine laminin or nidogen. These latter results, plus the fact that antigen-antibody binding was strongly blocked by gal(alpha 1-2)gal but not by methyl-alpha-galactopyranoside or melibiose, suggest that this antibody is indeed different from anti-gal(alpha 1-3)gal antibody. Anti-gal(alpha 1-2)gal antibody levels were significantly elevated in 66% of patients with chronic chagasic cardiomyopathy, but were not elevated in patients with different clinical forms of leishmaniasis, Trypanosoma rangeli-infected patients, or in patients with 15 other infectious and inflammatory diseases. Gal(alpha 1-2)gal antibodies did not absorb to intact T. cruzi parasites, but absorbed strongly to trypomastigote and epimastigote sonicates, suggesting some masking of reactive epitopes.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemotherapy with ethidium bromide-DNA complex in established Chagas' disease.

Two different strains of mice (AKR and NMRI-IVIC) were inoculated intraperitoneally with the virulent Y strain of Trypanosoma cruzi, and then treated with the lysosomotropic ethidium bromide-DNA complex, according to several different treatment schedules. When animals were treated 48 hours after intraperitoneal inoculation with three intraperitoneal doses of EB-DNA no parasitemia was detected, even after 11 weeks, confirming previous results. However, when infection was allowed to become fully established, that is 3-4 weeks after inoculation, and then challenged with several different treatment schedules (with varied doses and timing of administration) we failed to cure established Chagas' disease, suggesting that the claim of effectiveness for this EB-DNA complex is limited to early Chagas' disease.

Effect of allopurinol on different strains of Trypanosoma cruzi.

Groups of mice were inoculated with six Trypanosoma cruzi strains and then treated with 32 mg/kg body weight allopurinol for 10 consecutive days. Effects of the drug on mortality rates were closely evaluated and repeated fresh blood examinations were done. Infected mice showed at least four parasitemia patterns with varied mortality rates and parasitemia levels. Evidence is provided that, independently of the parasitemia pattern or level and strain origin, there are marked differences in the sensitivity to allopurinol between the several T. cruzi strains studied. These differences in drug response seem to be related to biological characteristics of the strains and pose further challenges in the rational therapy of Chagas' disease.

Increase in asialoganglioside- and monosialoganglioside-reactive antibodies in chronic Chagas' disease patients.

Antibodies reactive with the core glycan of asialoganglioside (GA1), monosialoganglioside (GM1), and disialoganglioside (GD1a) were studied in human sera. In healthy individuals, GA1-, GM1-, and GD1a-reactive antibodies were mainly of the IgM class, but also of the IgA and IgG classes, and were present at low titers in the serum of 68%, 79%, and 91% of the individuals studied, respectively. Levels of anti-GA1 and anti-GM1 antibodies, mainly of the IgA and IgG classes, were significantly elevated (P < 0.001) in 62% and 72% of subjects, respectively, chronically infected with Trypanosoma cruzi, with no association found with the degree of myocardial damage. No significant increase in anti-GA1 and anti-GM1 antibodies was found in dilated cardiomyopathy patients. The level of anti-GD1a antibody was not significantly different between healthy controls and chronic chagasic or dilatatory cardiomyopathy patients. Since the peripheral nervous system is very rich in gangliosides, it is possible that the increases in GA1- and GM1-specific antibodies that develop during chronic T. cruzi infection are involved in the pathology of peripheral neuropathy in Chagas' disease.

Parasitic oligosaccharide residues recognized by patients with mucocutaneous and localized cutaneous leishmaniasis.

Humoral immune responses were studied in 118 Venezuelan patients with either active mucocutaneous (MCL) or localized cutaneous leishmaniasis (LCL). Most patients had elevated antibody levels to the six promastigote oligosaccharide residues studied: galactosyl(alpha 1-2)galactose, galactosyl(alpha 1-3)galactose, galactosyl(alpha 1-6)galactose, galactosyl(alpha 1-3)mannose, galactofuranosyl(beta 1-3)mannose, and galactocerebroside. Significantly higher antibody levels were found in patients with MCL against galactosyl(alpha 1-3)galactose and Leishmania tropica glycoinositol phospholipid (GIPL)-1, GIPL-2, and GIPL-3 compared with patients with LCL. For both clinical forms of American cutaneous leishmaniasis (ACL), the most reactive antigen was galactosyl(alpha 1-3)galactose, with elevated levels found in 63% and 79% of MCL and LCL patients, respectively. In patients with MCL and LCL, no significant relationship was found between antibody levels against a given oligosaccharide residue and clinical parameters such as age, leishmanin diameter, number of skin lesions, or time of evolution. It is noteworthy that 33% and 15% of MCL and LCL patients, respectively, did not have elevated antibody levels against the six different oligosaccharide residues studied. This suggests the presence of a subpopulation of non-humoral immunoreactive ACL patients. The relationship between abnormal levels of oligosaccharide antibodies and the final outcome of the disease remains to be established.

Specific inhibitory effect of 3-deazaneplanocin A against several Leishmania mexicana and L. braziliensis strains.

The growth inhibitory effect of 3-deazaneplanocin A (c3NpcA) was tested against some pathogenic members of the family of American Trypanosomatidae. Under our culture conditions, c3NpcA displayed a strongly and uniformly leishmanistatic effect on all 23 American Leishmania (L. mexicana and L. brasiliensis) strains in the study (mean dose producing 50% inhibition compared with control parasite growth [ID50] = 96 ng/ml, 0.32 microM), but showed no inhibition against the several T. cruzi and T. rangeli strains tested with concentrations up to 10,000 ng/ml. This compound also induced a substantial expansion of the intracellular pools of both S-adenosylhomocysteine (AdoHcy) and S-adenosylmethionine (AdoMet), as well as a significant diminution of the AdoMet:AdoHcy ratio. Strong AdoHcy hydrolase activity was detected in American Leishmania promastigotes. At at a dose of 200 ng/ml, c3NpcA inhibited S-adenosyl-L-3H-methylmethionine and 3-thymidine incorporation by promastigotes after four days incubation in the presence of the drug. At a dose of 100 ng/ml, c3NpcA eliminated approximately 56% of the L. mexicana and L. brasiliensis from infected human macrophages, compared with simultaneously cultivated controls. Two schedules of 10 consecutive intraperitoneal injections of c3NpcA, with doses ranging from 0.5 to 1.5 mg/kg/day, significantly reduced development of cutaneous leishmanial infection produced in inbred BALB/c mice by L. b. guyanensis inoculation, although a few parasites remained at the inoculation site.

Cutaneous leishmaniasis in mice: resistance to glucan immunotherapy, either alone or combined with chemotherapy.

Leishmania braziliensis, L. mexicana, and L. garnhami were studied for their ability to produce American cutaneous leishmaniasis, using C57BL/6 female mice as the animal model. No significant difference in the clinical course of mouse foot pad infection was found between the three American Leishmania species studied. In general, the incubation period varied from 2-4 weeks. Mice developed only local swelling and sometimes ulceration at the sites of inoculation. After 4 weeks of progress lesions began to decrease without obvious impact on the general health of the mice. When glucan immunotherapy (120 - 240 mg/kg body weight) was initiated previous to, or simultaneously with, infection the development of foot pad lesions was not significantly inhibited, this despite clear evidence of generalized stimulation of the reticuloendothelial system. On the other hand, pentavalent antimony at high doses (1,000 - 1,500 mg/kg) induced only a significant lengthening of the latent period. However, different combinations of glucan and pentavalent antimony (various doses of each drug, timing of administration, or changes in the sequence of use of both drugs) did not significantly alter the clinical course of American Leishmania infection as compared with pentavalent antimony alone. Thus, not only were glucan or glucantime alone unable to cure the infection (as evidenced by some animals which showed rapidly growing lesions some time after the end of treatment), but no potentiation was observed.

Sinefungin as treatment for American Leishmania in sensitive BALB/c and resistant C57BL/6 mice.

Using inbred BALB/c and C57BL/6 mice as animal models for American cutaneous leishmaniasis, we evaluated the inhibitory effect of sinefungin on foot-pad infection produced by 5 different Leishmania isolates. When treatment was initiated a few days, or even 2 weeks, after infection, an obvious leishmanicidal effect was detected on mice infected with Leishmania mexicana or L. braziliensis isolates, which lasted at least 50 weeks for all isolates studied. The optimal dose schedule was 4 mg/kg body weight/day, injected ip for 10 consecutive days; lower doses produced only a short leishmanistatic effect. The optimal dose found was 50-fold lower than the LD50. In vitro studies using Leishmania-infected murine peritoneal macrophages showed sinefungin as a powerful inhibitory drug, mean ED50 for the several Leishmania isolates studied being 50 ng/ml. No correlation was found between in vitro sensitivity of culture promastigotes and in vivo sensitivity to sinefungin of an American Leishmania isolate.

Ganglion cysts associated with cruciate ligaments of the knee: a possible cause of recurrent knee pain.

Intraarticular ganglia of the knee are uncommon; however, these ganglion cysts may produce knee discomfort without a clear etiology. We present the cases of 10 patients with ganglion cysts arising from cruciate ligaments of the knee joint who underwent arthroscopic excision after MR examination. The MR findings, clinical features and arthroscopic findings were evaluated comparatively. Diagnoses were confirmed by means of a histological study after arthroscopic excision. The cysts were fluid-filled, with low T1-weighted signal intensity and high T2-weighted signal intensity. Except for two patients with recent accidents, the remaining eight presented chronic pain without any history of trauma. Pain was the most frequent clinical sign. It was associated with knee extension in 3 cases and with flexion in 3 cases. In 7 cases, cysts were exclusively associated with the anterior cruciate ligament (ACL). Only in one case was a cyst associated with an ACL rupture. Four patients presented meniscal lesions. All ganglia appeared solitary in each knee. Postarthroscopy evolution was painless in 8 patients. Histologic diagnoses corresponded to ganglion cysts. The tissues from the patient with the ACL rupture presented a fibrous reaction with myxoid degeneration forming intraligamentary ganglion cysts.

The Rac1 splice form Rac1b promotes K-ras-induced lung tumorigenesis.

Rac1b, an alternative splice form of Rac1, has been previously shown to be upregulated in colon and breast cancer cells, suggesting an oncogenic role for Rac1b in these cancers. Our analysis of NSCLC tumor and matched normal tissue samples indicates Rac1b is upregulated in a significant fraction of lung tumors in correlation with mutational status of K-ras. To directly assess the oncogenic potential of Rac1b in vivo, we employed a mouse model of lung adenocarcinoma, in which the expression of Rac1b can be conditionally activated specifically in the lung. Although expression of Rac1b alone is insufficient to drive tumor initiation, the expression of Rac1b synergizes with an oncogenic allele of K-ras resulting in increased cellular proliferation and accelerated tumor growth. Finally, we show that in contrast to our previous findings demonstrating a requirement for Rac1 in K-ras-driven cell proliferation, Rac1b is not required in this context. Given the partially overlapping spectrum of downstream effectors regulated by Rac1 and Rac1b, our findings further delineate the signaling pathways downstream of Rac1 that are required for K-ras driven tumorigenesis.