Sandip Bhattacharyya

Glucocorticoids suppress bone formation via the osteoclast.

The pathogenesis of glucocorticoid-induced (GC-induced) bone loss is unclear. For example, osteoblast apoptosis is enhanced by GCs in vivo, but they stimulate bone formation in vitro. This conundrum suggests that an intermediary cell transmits a component of the bone-suppressive effects of GCs to osteoblasts in the intact animal. Bone remodeling is characterized by tethering of the activities of osteoclasts and osteoblasts. Hence, the osteoclast is a potential modulator of the effect of GCs on osteoblasts. To define the direct impact of GCs on bone-resorptive cells, we compared the effects of dexamethasone (DEX) on WT osteoclasts with those derived from mice with disruption of the GC receptor in osteoclast lineage cells (GRoc-/- mice). While the steroid prolonged longevity of osteoclasts, their bone-degrading capacity was suppressed. The inhibitory effect of DEX on bone resorption reflects failure of osteoclasts to organize their cytoskeleton in response to M-CSF. DEX specifically arrested M-CSF activation of RhoA, Rac, and Vav3, each of which regulate the osteoclast cytoskeleton. In all circumstances GRoc-/- mice were spared the impact of DEX on osteoclasts and their precursors. Consistent with osteoclasts modulating the osteoblast-suppressive effect of DEX, GRoc-/- mice are protected from the steroids inhibition of bone formation.

Leishmania donovani Suppresses Activated Protein 1 and NF-κB Activation in Host Macrophages via Ceramide Generation: Involvement of Extracellular Signal-Regulated Kinase

ABSTRACT In vitro infection of murine peritoneal macrophages with the protozoan Leishmania donovani has been found to alter the signaling parameters of the host. The present study indicates that the enhancement of intracellular ceramide level in macrophages after infection is a major event relating to macrophage dysfunction. We have previously demonstrated that increased ceramide synthesis in host macrophages was involved in the dephosphorylation of extracellular signal-regulated kinase (ERK). In the present study, we further show that downregulation of ERK by ceramide was found to be associated with the inhibition of activated protein 1 (AP-1) and NF-κB transactivation. Pharmacological inhibition of ceramide synthesis by Fumonisin B1 restored the induction of AP-1 and NF-κB DNA-binding activities in infected BALB/c macrophages. On the contrary, in the case of macrophages from the leishmaniasis-resistant C.D2 mice, L. donovani failed to induce sustained ceramide synthesis. Enhanced mitogen-activated protein kinase phosphorylation, AP-1 and NF-κB DNA-binding activity, and the generation of nitric oxide (NO) were observed in L. donovani-infected C.D2 macrophages. ERK activation was necessary for the activation of transcription factors AP-1 and NF-κB, NO generation, and restriction of the parasite burden in the resistant murine host macrophages. Hence, the induction of ceramide synthesis in host macrophages appears to be instrumental and one of the turning points leading to silencing of the macrophage antileishmanial responses.

Immunomodulatory Role of Interleukin-10 in Visceral Leishmaniasis: Defective Activation of Protein Kinase C-Mediated Signal Transduction Events

ABSTRACT Leishmania donovani, an intracellular protozoan parasite, challenges host defense mechanisms by impairing the signal transduction of macrophages. In this study we investigated whether interleukin-10 (IL-10)-mediated alteration of signaling events in a murine model of visceral leishmaniasis is associated with macrophage deactivation. Primary in vitro cultures of macrophages infected with leishmanial parasites markedly elevated the endogenous release of IL-10. Treatment with either L. donovani or recombinant IL-10 (rIL-10) inhibited both the activity and expression of the Ca2+-dependent protein kinase C (PKC) isoform. However, preincubation with neutralizing anti-IL-10 monoclonal antibody (MAb) restored the PKC activity in the parasitized macrophage. Furthermore, we observed that coincubation of macrophages with rIL-10 and L. donovani increased the intracellular parasite burden, which was abrogated by anti-IL-10 MAb. Consistent with these observations, generation of superoxide (O2−) and nitric oxide and the release of murine tumor necrosis factor-α were attenuated in response to L. donovani or rIL-10 treatment. On the other hand, preincubation of the infected macrophages with neutralizing anti-IL-10 MAb significantly blocked the inhibition of nitric oxide and murine tumor necrosis factor-α release by the infected macrophages. These findings imply that infection with L. donovani induces endogenous secretion of murine IL-10, which in turn facilitates the intracellular survival of the protozoan and orchestrates several immunomodulatory roles via selective impairment of PKC-mediated signal transduction.

Glucocorticoids target suppressor of cytokine signaling 1 (SOCS1) and type 1 interferons to regulate Toll-like receptor–induced STAT1 activation

Endogenous and pharmacologic glucocorticoids (GCs) limit inflammatory cascades initiated by Toll-like receptor (TLR) activation. A long-standing clinical observation has been the delay between GC administration and the manifestation of GCs anti-inflammatory actions. We hypothesized that the GCs would have inhibitory effects that target late temporal pathways that propagate proinflammatory signals. Here we interrogated signal transducer and activator of transcription 1 (STAT1) regulation by GC and its consequences for cytokine production during activation of macrophages with TLR-specific ligands. We found that robust STAT1 activation does not occur until 2–3 h after TLR engagement, and that GC suppression of STAT1 phosphorylation first manifests at this time. GC attenuates TLR4-mediated STAT1 activation only through induction of suppressor of cytokine signaling 1 (SOCS1), which increases throughout the 6-h period after treatment. Inhibition of TLR3-mediated STAT1 activation occurs via two mechanisms, impairment of type I IFN secretion and induction of SOCS1. Our data show that SOCS1 and type I interferons are critical GC targets for regulating STAT1 activity and may account for overall GC effectiveness in inflammation suppression in the clinically relevant time frame.

Glucocorticoids and the osteoclast.

Abstract:  Glucocorticoid (GC)‐induced bone loss is the most common cause of secondary osteoporosis but its pathogenesis is controversial. GCs clearly suppress bone formation in vivo but the means by which they impact osteoblasts is unclear. Because bone remodeling is characterized by tethering of the activities of the two cells, the osteoclast is a potential modulator of the effect of GCs on osteoblasts. To address this issue we compared the effects of dexamethasone on wild‐type (WT) osteoclasts with those derived from mice with disruption of the GC receptor in osteoclast lineage cells and found that the bone‐degrading capacity of GC‐treated WT cells is suppressed. The inhibitory effect of dexamethasone on bone resorption reflects failure of osteoclasts to organize their cytoskeleton in response to M‐CSF. Dexamethasone specifically arrests M‐CSF activation of RhoA, Rac, and Vav3, each of which regulate the osteoclast cytoskeleton. In all circumstances, mice lacking the GC receptor in osteoclast lineage cells are spared the impact of dexamethasone on osteoclasts and their precursors. Consistent with osteoclasts modulating the osteoblast‐suppressive effect of dexamethasone, GC receptor‐deficient mice are protected from the steroids inhibition of bone formation.

CXC Chemokine–Mediated Protection against Visceral Leishmaniasis: Involvement of the Proinflammatory Response

Visceral leishmaniasis, caused by the protozoan parasite Leishmania donovani, is characterized by the loss of ability of the host to generate an effective immune response. In the present study, the comparative potential of CXC chemokines, interferon-gamma-inducible protein-10 (IP-10) and interleukin-8 (IL-8) in restricting Leishmania donovani infection via the release of nitric oxide and proinflammatory cytokines was studied in an in vitro model. Nitric oxide, a crucial mediator for IP-10-mediated leishmanicidal activity, was found to be dependent on inducible nitric oxide synthase 2 (iNOS2) expression and was linked to the mitogen-activated protein kinases (MAPK) signaling pathway. Further, IP-10 was also able to abrogate the survival of Leishmania in an in vivo model of visceral leishmaniasis by restoration of Th1 cytokines and nitric oxide. Thus, this study strongly demonstrates that IP-10, like CC chemokines, is involved in rendering a protective response in visceral leishmaniasis via up-regulation of proinflammatory mediators.

Use of an Attenuated Leishmanial Parasite as an Immunoprophylactic and Immunotherapeutic Agent against Murine Visceral Leishmaniasis

ABSTRACT The ability of the leishmanial parasite UR6 to act as an immunoprophylactic and immunotherapeutic agent against Leishmania donovani infection in BALB/c mice was investigated. Unlike the virulent L. donovani AG83 (MOHOM/IN/1983/AG83), UR6 given through intracardiac route failed to induce visceral infection, but when it was injected subcutaneously, UR6 induced a short-lived and localized self-healing skin lesion. Priming of peritoneal macrophages with UR6 in vitro induced superoxide (O2−) generation, whereas similar experiments with virulent AG83 inhibited O2− generation. It was observed that priming of mice with either live or sonicated UR6 in the absence of any adjuvant provided strong protection against subsequent virulent challenge. Further, UR6-primed infected mice not only displayed a strong antileishmanial delayed-type hypersensitivity (DTH) response but also showed an elevated level of the serum antileishmanial immunoglobulin G2a (IgG2a) isotype, whereas infected mice failed to mount any antileishmanial DTH response and showed an elevated level of IgG1. This indicates that UR6 priming and subsequent L. donovani infection allowed the expansion of Th1 cells. Our studies indicate that UR6 has potential to be used as an immunoprophylactic and immunotherapeutic agent against experimental visceral leishmaniasis.

Regulation of impaired protein kinase C signaling by chemokines in murine macrophages during visceral leishmaniasis.

ABSTRACT The protein kinase C (PKC) family regulates macrophage function involved in host defense against infection. In the case of Leishmania donovani infection, the impairment of PKC-mediated signaling is one of the crucial events for the establishment of parasite into the macrophages. Earlier reports established that C-C chemokines mediated protection against leishmaniasis via the generation of nitric oxide after 48 h. In this study, we investigated the role of MIP-1α and MCP-1 in the regulation of impaired PKC activity in the early hours (6 h) of infection. These chemokines restored Ca2+-dependent PKC activity and inhibited Ca2+-independent atypical PKC activity in L. donovani-infected macrophages under both in vivo and in vitro conditions. Pretreatment of macrophages with chemokines induced superoxide anion generation by activating NADPH oxidase components in infected cells. Chemokine administration in vitro induced the migration of infected macrophages and triggered the production of reactive oxygen species. In vivo treatment with chemokines significantly restricted the parasitic burden in livers as well as in spleens. Collectively, these results indicate a novel regulatory role of C-C chemokines in controlling the intracellular growth and multiplication of L. donovani, thereby demonstrating the antileishmanial properties of C-C chemokines in the disease process.

Dexamethsone Suppresses Bone Formation via the Osteoclast

Glucocorticoids are central to treating inflammatory and immune disorders. These steroids, however, profoundly impact the skeleton, particularly when administered for prolonged periods. In fact, high-dose glucocorticoid therapy is almost universally associated with bone loss, prompting among the most common forms of crippling osteoporosis. Despite the frequency and severity of glucocorticoid-induced osteoporosis, its treatment is less than satisfactory, suggesting that its pathogenesis is incompletely understood. Net bone mass represents the relative activities of osteoblasts and osteoclasts and there is little question that glucocorticoids suppress the bone-forming cells, in vivo, via a process involving accelerated apoptosis (Weinstein 2001; Weinstein, Jilka, Parfitt, et al. 1998). Surprisingly, however, addition of glucocorticoids to cultures of osteoprogenitor cells actually increases their capacity to form mineralized bone nodules (Aubin 1999; Purpura, Aubin, and Zandstra 2004). This paradox raises the possibility that glucocorticoid suppression of bone formation, in vivo, reflects, at least in part, the steroids targeting intermediary cells, which in turn inhibit the osteoblast. Bone remodeling is an ever-occuring event in mammals which is characterized by tethering of osteoclast and osteoblast function. The process is initiated by osteoclasts (OCs) resorbing a packet of bone, which in turn leads to osteoblasts being recruited to the site of resorption. This process establishes that osteoclastic bone resorption, in some manner, promotes osteoblastic bone formation at the same location. Consequently, pathologically or pharmacologically inhibited resorption eventuates in arrested osteoblast activity.