Supercritical CO2 extraction of velutinol A from Mandevilla velutina (Apocynaceae) cultured cells and MALDI-TOF MS analysis

MALDI-TOF MS analysis of supercritical CO₂ extracted samples obtained from Mandevilla velutina cell cultures allowed the detection of the anti-bradykinin pregnanic steroid, velutinol A, using low amount of sample (1 g lyophilized cells), with minimum analyte isolation.     

Stimulation and inhibition of lipolysis in isolated rat adipocytes: evidence for age-related changes in responses to forskolin and PGE1

The ability of a number of hormones to activate cellular responses in a variety of cells declines with age. The mechanisms responsible for these alterations are complex and incompletely understood. Rat adipocytes have served as an important model to study blunted responses to stimulatory hormones which function by activating cAMP accumulation. We have previously found that the blunted lipolytic response of adipocytes from older rats to the beta adrenergic receptor agonist isoproterenol appeared to be due to a lessened ability of isoproterenol to activate cAMP accumulation. Further, the blunted response to isoproterenol was apparently caused by an accentuated inhibition of lipolysis, mediated by adenosine receptors activated by endogenously released adenosine. The present studies were designed to test and extend those conclusions. We have utilized forskolin to augment the cAMP accumulation that occurs in the presence of isoproterenol. Isoproterenol-activated lipolysis was greater in adipocytes from 2 month old rats compared with those from 12 month old rats (603 +/- 32 vs 450 +/- 29 nmol/10(5) cells/hr, P less than 0.01). However, in the presence of forskolin (10(-6) M), there was no significant difference in the response to isoproterenol between the two groups (646 +/- 23 vs 615 +/- 29 nmoles/10(5) cells/hr). As we had seen previously, the adenosine receptor agonist phenylisopropyladenosine more effectively inhibited lipolysis in the adipocytes from older rats. We now also find that PGE1 more efficaciously inhibits lipolysis in the cells from older rats. These data confirm that diminished cAMP accumulation in adipocytes from older rats appears to be a rate-limiting alteration in the regulation of lipolysis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tachykinin-induced vasodilatation in rat skin measured with a laser-Doppler flowmeter: evidence for receptor-mediated effects

Vasodilatation was induced by perfusion of the tachykinins substance P (SP), neurokinin A and neurokinin B and the analogues [Glp6, D-Pro9]SP-(6-11) and [Glp6, L-Pro9]SP-(6-11) over the base of vacuum-induced blisters on the rat footpad. Vasodilatation was measured as change in blood flow using a laser-Doppler flowmeter. The tachykinins induced vasodilatation in a dose-response manner with a threshold of approximately 3 pmol and pD2's of 6.48, 6.13 and 6.21 for SP, neurokinin A and neurokinin B respectively. The D- and L-Pro analogues of [Glp6, Pro9]SP-(6-11) also induced vasodilatation in a dose-dependent manner. The L-Pro analogue was more potent than the D-Pro analogue (D/L ratio of the EC50's = 21) which suggests the involvement of an NK-1 type receptor in the mediation of small vessel vasodilatation. The vasodilatation to SP was reduced by 64% and 59% in capsaicin- and antihistamine-pretreated animals respectively, demonstrating the involvement of capsaicin-sensitive primary afferent nerves and mast cells in the vasodilatation component of the neurogenic inflammatory response.     

Probing the molecular basis for potent and selective protein-tyrosine phosphatase 1B inhibition

Protein-tyrosine phosphatases (PTPs) are important for the control of proper cellular tyrosine phosphorylation. Despite the large number of PTPs encoded in the human genome and the emerging roles played by PTPs in human diseases, a detailed understanding of the role played by PTPs in normal physiology and in pathogenic conditions has been hampered by the absence of PTP-specific inhibitors. Such inhibitors could serve as useful tools for determining the physiological functions of PTPs and may constitute valuable therapeutics in the treatment of several human diseases. However, because of the highly conserved nature of the active site, it has been difficult to develop selective PTP inhibitors. By taking an approach to tether together two small ligands that can interact simultaneously with the active site and a unique proximal noncatalytic site, we have recently acquired Compound 2 (see Fig. 1), the most potent and selective PTP1B inhibitor identified to date, which exhibits several orders of magnitude selectivity in favor of PTP1B against a panel of PTPs. We describe an evaluation of the interaction between 2 and its analogs with PTP1B and its site-directed mutants selected based on hydrogen/deuterium exchange of PTP1B backbone amides in the presence and absence of 2. We have established the binding mode of Compound 2 and identified 12 PTP1B residues that are important for the potency and selectivity of Compound 2. Although many of the residues important for Compound 2 binding are not unique to PTP1B, the combinations of all contact residues differ between PTP isozymes, which suggest that the binding surface defined by these residues in individual PTPs determines inhibitor selectivity. Our results provide structural information toward understanding of the molecular basis for potent and selective PTP1B inhibition and further establish the feasibility of acquiring potent, yet highly selective, PTP inhibitory agents.     

Induction of selective blood-tumor barrier permeability and macromolecular transport by a biostable kinin B1 receptor agonist in a glioma rat model

Treatment of malignant glioma with chemotherapy is limited mostly because of delivery impediment related to the blood-brain tumor barrier (BTB). B1 receptors (B1R), inducible prototypical G-protein coupled receptors (GPCR) can regulate permeability of vessels including possibly that of brain tumors. Here, we determine the extent of BTB permeability induced by the natural and synthetic peptide B1R agonists, LysdesArg(9)BK (LDBK) and SarLys[dPhe(8)]desArg(9)BK (NG29), in syngeneic F98 glioma-implanted Fischer rats. Ten days after tumor inoculation, we detected the presence of B1R on tumor cells and associated vasculature. NG29 infusion increased brain distribution volume and uptake profiles of paramagnetic probes (Magnevist and Gadomer) at tumoral sites (T(1)-weighted imaging). These effects were blocked by B1R antagonist and non-selective cyclooxygenase inhibitors, but not by B2R antagonist and non-selective nitric oxide synthase inhibitors. Consistent with MRI data, systemic co-administration of NG29 improved brain tumor delivery of Carboplatin chemotherapy (ICP-Mass spectrometry). We also detected elevated B1R expression in clinical samples of high-grade glioma. Our results documented a novel GPCR-signaling mechanism for promoting transient BTB disruption, involving activation of B1R and ensuing production of COX metabolites. They also underlined the potential value of synthetic biostable B1R agonists as selective BTB modulators for local delivery of different sized-therapeutics at (peri)tumoral sites.     

Allosteric inhibition of PTP1B activity by selective modification of a non-active site cysteine residue

The fluorogenic reagent 4-(aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (ABDF) attenuates the functional activity of the protein tyrosine phosphatase PTP1B by reacting selectively with a single cysteine residue, leaving other cysteines in the protein unmodified. This modification reduces Vmax without substantially affecting substrate binding (Km), indicative of an allosteric mode of inhibition. Consistent with this, the cysteine residue modified by ABDF, Cys 121, lies outside the catalytic site but makes interactions with residues that contact His 214, which has been shown to be important for catalysis. Cys 121 is highly conserved among phosphatases, and ABDF also inhibits TC-PTP and LAR. These findings illustrate that targeting cysteine residues outside catalytic sites may be exploited in allosterically regulating enzymes. Moreover, these results suggest a new strategy for inhibiting a promising diabetes target.     

Different cellular sources and different roles of adenosine: A1 receptor-mediated inhibition through astrocytic-driven volume transmission and synapse-restricted A2A receptor-mediated facilitation of plasticity

Adenosine is a prototypical neuromodulator, which mainly controls excitatory transmission through the activation of widespread inhibitory A1 receptors and synaptically located A2A receptors. It was long thought that the predominant A1 receptor-meditated modulation by endogenous adenosine was a homeostatic process intrinsic to the synapse. New studies indicate that endogenous extracellular adenosine is originated as a consequence of the release of gliotransmitters, namely ATP, which sets a global inhibitory tonus in brain circuits rather than in a single synapse. Thus, this neuron-glia long-range communication can be viewed as a form of non-synaptic transmission (a concept introduced by Professor Sylvester Vizi), designed to reduce noise in a circuit. This neuron-glia-induced adenosine release is also responsible for exacerbating salient information through A1 receptor-mediated heterosynaptic depression, whereby the activation of a particular synapse recruits a neuron-glia network to generate extracellular adenosine that inhibits neighbouring non-tetanised synapses. In parallel, the local activation of facilitatory A2A receptors by adenosine, formed from ATP released only at high frequencies from neuronal vesicles, down-regulates A1 receptors and facilitates plasticity selectively in the tetanised synapse. Thus, upon high-frequency firing of a given pathway, the combined exacerbation of global A1 receptor-mediated inhibition in the circuit (heterosynaptic depression) with the local synaptic activation of A2A receptors in the activated synapse, cooperate to maximise salience between the activated and non-tetanised synapses.     

Interleukin 10 production in patients undergoing cardiopulmonary bypass: evidence of inhibition of Th-1-type responses

The cardiopulmonary bypass (CPB) procedure has long been associated with a generalized immunosuppression. To understand further the cytokine-mediated regulation of the complex physiological and immunological changes induced by CPB, the authors decided to investigate whether CPB affects the release of interleukin (IL)-10, as well as other cytokines, in correlation to the inhibition of T cell responses. Using phytohaemagglutinin (PHA) as mitogen and peripheral blood mononuclear cells (PBMC) isolated from patients undergoing CPB, we investigated whether this procedure has an effect on the secretion of different patterns of cytokines (Th1- and Th2-type) and PBMC proliferation. In all patients, CPB significantly enhances IL-10 and IL-6 production in resting and PHA-stimulated PBMC. On the other hand, IL-2 production, in response to PHA, was significantly diminished. Reduced IL-2 and enhanced IL-10 production were associated with a significant decrease in PBMC proliferation. Immunosuppression was also associated to lymphopenia, while neutrophil counts were significantly enhanced. These results show that after CPB there is a transient but clear unbalanced immune response demonstrated by a differentiated production of Th1- and Th2-type cytokines. The release of different patterns of cytokines observed after CPB may be helpful in understanding and preventing the development of infectious and immune complications in surgical procedure employing CPB.     

IL-4 (B cell stimulatory factor 1) overcomes Fc gamma receptor-mediated inhibition of mouse B lymphocyte proliferation without affecting inhibition of c-myc mRNA induction

Mouse B cells are stimulated to proliferate by Fab'2 fragments of rabbit anti-mouse Ig antibodies. Proliferation is inhibited, however, in the presence of IgG anti-mouse Ig. We have previously shown that this inhibition is mediated by binding of the IgG anti-Ig to receptors for Fc gamma R on B cells. This report describes conditions under which IgG anti-mu or anti-delta will induce proliferation despite Fc gamma R engagement. Culture supernatants of Con A-stimulated, Il-4-secreting Th cell lines, but not of Il-2-secreting Th cell lines, will co-stimulate with IgG anti-Ig to induce small B cells to incorporate [3H]TdR. This co-mitogenic activity is inhibitable by anti-IL-4 antibodies and can also be induced by Il-4 affinity purified from the T cell supernatants or by supernatants containing rIl-4. B cells precultured with Il-4 for 18 h, while still expressing normal levels of Fc gamma R, also proliferate to IgG anti-Ig. We have previously shown that Fc gamma R-mIg cross-linking will inhibit mIg-dependent increases in c-myc mRNA levels. We investigated whether Il-4 allows B cells to respond to IgG anti-Ig by elevating c-myc. The data show that Il-4 has little effect on c-myc mRNA levels in either IgG or Fab'2 anti-Ig-containing cultures.     

Internalization of V2-vasopressin receptors in LLC-PK1-cells: evidence for receptor-mediated endocytosis.

The mechanism of internalization of the vasopressin-receptor (V2-subtype) of LLC-PK1-cells, a pig renal tubular cell line, is unknown. We studied internalization utilizing a novel, highly specific vasopressin analogue ((125I)-[8-p(OH)-phenylpropionyl]-LVP, 2000 Ci/mmol). Scatchard analysis performed with membranes of LLC-PK1-cells revealed a Kd of 0.8 +/- 0.2 nM and a Bmax of 366 +/- 41 fmol/mg of protein. Degradation of the ligand was excluded by RP-HPLC-analysis. Internalization was proven by the acid-wash technique, quantitative light-microscopic autoradiography and electron microscopy. The ligand was internalized in a time- and temperature-dependent manner. At 4 degrees C, no uptake was found; at 22 degrees C, after 30 min of incubation, more than 50% of the radioligand was found inside the cell. Electron microscopy demonstrated that plasma-membrane bound vasopressin receptors are internalized by receptor-mediated endocytosis via coated pits.     

Characterization of dual agonists for kinin B1 and B2 receptors and their biased activation of B2 receptors

Kinin B1 and B2 receptors (kB1R and kB2R) play important roles in many physiological and pathological processes. In some cases, kB1R or kB2R activation can have overlapping or complementary beneficial effects, thus an activator of both receptors might be advantageous. We found that replacement of the C-terminal Arg in the natural kB2R activators bradykinin (BK) or kallidin (KD) with Lys (K(9)-BK or K(10)-KD) resulted in agonists that effectively stimulate the downstream signaling of both the kB1R and kB2R as measured by increased inositol turnover, intracellular calcium, ERK1/2 phosphorylation, arachidonic acid release and NO production. However, K(9)-BK and K(10)-KD displayed some characteristics of biased agonism for kB2Rs as indicated by the rapid kinetics of ERK1/2 phosphorylation induced by K(9)-BK or K(10)-KD compared with the prolonged response mediated by BK or KD. In contrast, kinetics of ERK phosphorylation stimulated by K(10)-KD activation of the kB1R was the same as that induced by known kB1R agonist des-Arg(10)-KD. Furthermore, the endocytosis of kB2Rs mediated by K(9)-BK and K(10)-KD was remarkably less than that induced by BK and KD respectively. K(10)-KD stimulated kB1R and kB2R-dependent calcium responses and ERK1/2 phosphorylation in bovine endothelial cells. In cytokine-treated human endothelial cells, K(10)-KD stimulated ERK1/2 phosphorylation and a transient peak of NO production that was primarily kB2R-dependent. K(10)-KD also stimulated prolonged NO production that was both kB1R and kB2R-dependent. These data provide the first examples of dual agonists of kB1R and kB2R, and a biased agonist of kB2R and may provide useful clues for developing dual modulators of kB1Rs and kB2Rs for potential therapeutic use.     

Skeletal effects of serotonin (5-hydroxytryptamine) transporter inhibition: evidence from clinical studies

The discovery of a functional serotonin (5-hydroxytryptamine; 5-HT) transporter (5-HTT) in bone has given rise to questions about the physiologic role of 5-HT in bone, and the possible clinical implications for humans. 5-HT is known to play a role in the pathophysiology of depression, and many antidepressant medications function by inhibiting the 5-HTT. Among the antidepressants, those that selectively block the 5-HTT (namely, selective serotonin reuptake inhibitors; SSRIs) appear to have skeletal effects. Several studies have demonstrated lower bone density, increased rates of bone loss at the hip, and increased rates of fracture among older individuals taking SSRIs. However, there remains uncertainty about whether it is the antidepressant medications themselves or the reason for their use (depression) that is responsible for these observed bone changes. This paper reviews the epidemiologic literature that explores the role of the 5-HTT in bone health, by looking at questions about how depression, antidepressant therapy and SSRIs impact bone health in humans. Further research will be important to better understand how these factors interact to influence skeletal status, and to characterize the biochemical mechanism through which 5-HT may mediate bone turnover and metabolism.     

Identification of a buried pocket for potent and selective inhibition of Chk1: prediction and verification

Inhibition of the Chk1 kinase by small molecules binding to its active site is a strategy of great therapeutic interest for oncology. We report how computational modelling predicted the binding mode of ligands of special interest to the Chk1 ATP site, for representatives of an indazole series and debromohymenialdisine. These binding modes were subsequently confirmed by X-ray crystallography. The binding mode of a potent indazole derivative involves non-conventional C-H...O and N-H...pi-aromatic interactions with the protein. These interactions are formed in a buried pocket at the periphery of the ATP-binding site, the importance of which has previously been overlooked for ligand design against Chk1. It is demonstrated that filling this pocket can confer ligands with dramatically enhanced affinity for Chk1. Structural arguments in conjunction with assay data explain why targeting this pocket is also advantageous for selective binding to Chk1. Structural overlays of known inhibitors complexed with Chk1 show that only the indazole series utilizes the pocket of interest. Therefore, the analysis presented here should prove helpful in guiding future structure-based ligand design efforts against Chk1.     

A novel inflammatory pathway involved in leukocyte recruitment: role for the kinin B1 receptor and the chemokine CXCL5

The kinin B1 receptor is an inducible receptor not normally expressed but induced by inflammatory stimuli and plays a major role in neutrophil recruitment, particularly in response to the cytokine IL-1beta. However, the exact mechanism involved in this response is unclear. The aim of this study was to dissect the molecular mechanism involved, in particular to determine whether specific ELR-CXCL chemokines (specific neutrophil chemoattractants) played a role. Using intravital microscopy, we demonstrated that IL-1beta-induced leukocyte rolling, adherence, and emigration in mesenteric venules of wild-type (WT) mice, associated with an increase in B1 receptor mRNA expression, were substantially attenuated (>80%) in B1 receptor knockout mice (B1KO). This effect in B1KO mice was correlated with a selective down-regulation of IL-1beta-induced CXCL5 mRNA and protein expression compared with WT mice. Furthermore a selective neutralizing CXCL5 Ab caused profound suppression of leukocyte emigration in IL-1beta-treated WT mice. Finally, treatment of human endothelial cells with IL-1beta enhanced mRNA expression of the B1 receptor and the human (h) CXCL5 homologues (hCXCL5 and hCXCL6). This response was suppressed by approximately 50% when cells were pretreated with the B1 receptor antagonist des-Arg9-[Leu8]-bradykinin while treatment with des-Arg9-bradykinin, the B1 receptor agonist, caused a concentration-dependent increase in hCXCL5 and hCXCL6 mRNA expression. This study unveils a proinflammatory pathway centered on kinin B1 receptor activation of CXCL5 leading to leukocyte trafficking and highlights the B1 receptor as a potential target in the therapeutics of inflammatory disease.     

Growth inhibition of a B cell leukemia: evidence implicating an anti-idiotype immune response for protective tumor immunity

BCL1, a spontaneous surface IgM (mu lambda)-positive (sIgM+) B cell leukemia of BALB/c (Igha) origin rarely grows in the Ig heavy chain (Igh) congenic mouse C.B-20 (Ighb) but is highly metastatic and lethal in the host strain of origin. Previous studies indicated that BCL1 tumor immunity in C.B-20 mice was associated with a T cell-mediated immune response against H-40, a minor histocompatibility (H) antigen controlled by a gene linked to the Igh locus. However, we observed that BCL1 leukemia grew progressively in BAB-14 (Igha/b) mice, a strain capable of generating an anti-H-40 immune response. This suggested that anti-H-40 immunity was insufficient for protection and implied that an Igh-V (variable) region gene product was also important for BCL1 growth inhibition. We therefore evaluated the role of two possible Igh-V region-linked gene products in BCL1 growth inhibition; namely, an Igh-V region-linked minor H antigen or alternatively the BCL1 IgM idiotype (Id). We could find no evidence for an Igh-V region-linked minor H antigen because immunosuppressed (500 R) CB-20 mice reconstituted with C.B-20 anti-BAB-14 splenocytes were susceptible to BCL1 growth, whereas recipients reconstituted with C.B-20 anti-BALB/c splenocytes were resistant to BCL1 challenge. In contrast, C.B-20 mice immunized against purified BCL1 IgM protein could adoptively confer BCL1 tumor immunity. C.B-20 mice immunized against other BALB/c IgM myeloma proteins containing either lambda or kappa light chains failed to protect C.B-20 mice suggesting that recognition of a unique determinant (Id) and not an allotype was crucial for tumor immunity. The BCL1 mu-chain appeared to make the major contribution to the idiotypic determinant because a hybridoma product composed of BCL1 mu-chains and BALB/c kappa-chains still elicited BCL1 immunity. Adoptive transfer of C.B-20 anti-BCL1 Id splenocytes into irradiated recipients that prevented an anti-H-40 response due to H-40 tissue expression failed to adoptively confer BCL1 immunity. Thus, these data suggest that BCL1 growth inhibition requires a T cell-mediated response against both H-40 and the BCL1 Id; these responses must be elicited concurrently in the tumor-bearing host to achieve protective BCL1 immunity.     

Identification of a key region of kinin B(1) receptor for high affinity binding of peptide antagonists

To investigate the molecular basis for the specificity of ligand recognition in human kinin B(1) (B(1)R) and B(2) (B(2)R) receptors, we constructed a series of chimeric receptors by progressively replacing, from the N to the C terminus, the human B(2)R domains by their B(1) counterparts. The chimeric construct possessing the C-terminal tail and the transmembrane domain VII (TM VII) of the B(2)R (construct 6) displayed 7- and 20- fold decreased affinities for the B(1) agonist [(3)H]desArg(10)-kallidin (desArg(10)-KD) and the B(1) antagonist [(3)H]desArg(10)-[Leu(9)]-KD respectively, as compared with the wild-type B(1)R. Moreover, the substitution of the B(1) TM VII by its B(2) homologue TM increased the affinity for the pseudopeptide antagonists, Hoe140 and NPC 567. High affinity for desArg(10)-KD binding was fully regained when the B(2) residue Thr(287) was replaced in construct 6 by the corresponding B(1) Leu(294) residue. When the B(2) residue Tyr(295) was exchanged with the corresponding B(1) Phe(302), high affinity binding for both agonist and antagonist was recovered. Moreover, the L294T and F302Y mutant B(1)R exhibited 69- and 6.5-fold increases, respectively, in their affinities for the B(2) receptor antagonist, Hoe140. Therefore we proposed that Leu(294) and Phe(302) residues, which may not be directly involved in the binding of B(1)R ligands and, hence, their Thr(287) and Tyr(295) B(2) counterparts, are localized in a receptor region, which plays a pivotal role in the binding selectivity of the peptide or pseudopeptide kinin ligands.     

Serotonin-1B receptor-mediated inhibition of [3H]GABA release from rat ventral tegmental area slices

In order to assess a role of 5-HT(1B) receptors for regulation of GABA transmission in the ventral tegmental area (VTA), VTA slices from the rat were incubated with [(3)H]GABA and beta-alanine, and superfused in the presence of nipecotic acid and aminooxyacetic acid. [(3)H]GABA release was induced by exposures to the medium containing 30 mM potassium for 2 min. The results showed that high potassium-evoked [(3)H]GABA release was sensitive to calcium withdrawal or blockade of sodium channels by tetrodotoxin, suggesting that tritium overflow induced by high potassium derived largely from neuronal stores. Administration of CP 93129 (0.15 and 0.45 microM), a 5-HT(1B) receptor agonist, or RU 24969 (0.15 and 0.45 microM), a 5-HT(1B/1A) receptor agonist, but not 8-OH-DPAT (0.45 microM), a 5-HT(1A) receptor agonist, inhibited high potassium-evoked [(3)H]GABA release in a concentration-related manner. The RU 24969-induced inhibition of [(3)H]GABA release was antagonized by either SB 216641, a 5-H(1B) receptor antagonist, or cyanopindolol, a 5-HT(1B/1A) receptor antagonist, but not by WAY 100635, a 5-HT(1A) receptor antagonist. Pre-treatment with SB 216641 also antagonized CP 93129-induced inhibition of [(3)H]GABA release. The results support the hypothesis that 5-HT(1B) receptors within the VTA can function as heteroreceptors to inhibit GABA release.     

The CD40-inducible Bcl-2 family member A1 protects B cells from antigen receptor-mediated apoptosis

CD40 activation is necessary for thymus-dependent humoral immune responses and rescuing both phenotypically immature WEHI-231 B lymphoma cells from B cell antigen receptor-induced cell death and germinal center B cells from spontaneous apoptosis. As some effects of CD40 are probably mediated by differences in gene expression, cDNA expression arrays and RNase protection assays were used to identify the anti-apoptotic Bcl-2 homolog A1 as a CD40-inducible gene in B cell lines and purified germinal center B cells. Sustained CD40-induced A1 upregulation correlated with CD40-mediated rescue of WEHI-231 cells from anti-IgM-induced apoptosis. Moreover, overexpression of A1 specifically protected WEHI-231 cells from anti-IgM-induced apoptosis but not cell death triggered by certain other stimuli.     

When the selfish suffer: evidence for selective prosocial emotional and physiological responses to suffering egoists

Prosociality is fundamental to social relationships, but providing it indiscriminately risks exploitation by egoists. Past work demonstrates that individuals avoid these risks through a more selective form of prosociality, cooperating less and sharing fewer resources with egoists (e.g. Axelrod & Hamilton, 1981). The evolution of cooperation. Science, 211(4489), 1390–1396). We extend this work to explore whether individuals experience reduced prosocial affective and physiological responses to egoists in situations where they are suffering. In two studies, participants learned of a target’s egoistic or non-egoistic traits, and then encountered the target suffering. Suffering egoists evoked less compassion in others than non-egoists and elicited physiological responses that diverged from patterns associated with compassion and social engagement (reduced heart rate and greater respiratory sinus arrhythmia activity). Participants' feelings of distrust toward egoists explained these attenuated emotional and physiological responses. These results build upon studies of prosocial behavior by suggesting that individuals experience reduced prosocial emotional and physiological responses toward suffering egoists.