Immunomodulatory and therapeutic activity of curcumin.

Inflammation is a disease of vigorous uncontrolled activated immune responses. Overwhelming reports have suggested that the modulation of immune responses by curcumin plays a dominant role in the treatment of inflammation and metabolic diseases. Observations from both in-vitro and in-vivo studies have provided strong evidence towards the therapeutic potential of curcumin. These studies have also identified a plethora of biological targets and intricate mechanisms of action that characterize curcumin as a potent 'drug' for numerous ailments. During inflammation the functional influence of lymphocytes and the related cross-talk can be modulated by curcumin to achieve the desired immune status against diseases. This review describes the regulation of immune responses by curcumin and effectiveness of curcumin in treatment of diseases of diverse nature.

Dendritic cells and their receptors in antitumor immune response.

DCs are recognized as the pivotal group of lymphocytes, which induce a variety of antitumor immune responses. Enduring professional antigen presenting cells, DCs eminence to induce adaptive antitumor immune response was exploited, which showed promising results in DCs-based phased clinical studies. Nevertheless, DCs also influence other immune cells to induce multiple arms of immune system to cure cancer. Recently, direct cytotoxic capacity of DCs has been demonstrated in several studies. Altogether DCs hold a strong link between innate and adaptive immune responses. DCs are known to kill tumor cells, phagocytose immunogenic substrates and present a wide variety of antigens to prime T cells to induce concerted antitumor responses. These functional aspects of DCs are dependent on the receptors that participate in the stimulation of DCs. In this review, we have discussed these receptors that are known to induce direct cytotoxicity as well as the receptors that greatly influence their antigen presentation functions. Thus DCs are turning out to be an important cell type in cancer immunotherapy.

Immunomodulatory effects of curcumin: in-vivo.

Curcumin specifically exhibits cytostatic and cytotoxic effects against tumors of multiple origin. Previously we have demonstrated apoptotic activity of curcumin against tumor cells with no effect on normal cells in-vitro. Many anti-cancer drugs exhibit deleterious effects on immune cells, which restrict their wide use in-vivo. In the present study, we have evaluated the effect of curcumin on the major functions of T cells, natural killer cells, macrophages and on total splenocytes in-vivo, which insight the role of curcumin on their broad effector functions. This study demonstrates that prolonged curcumin-injections (i.p.) do not impair the cytotoxic function of natural killer cells, the generation of reactive oxygen species and nitric oxide from macrophages and the levels of Th1 regulatory cytokines remained unaltered. Interestingly, curcumin-injections enhanced the mitogen and antigen induced proliferation potential of T cells. We have also evaluated immunomodulatory effects of curcumin in ascites-bearing animals. This study strengthens our belief that curcumin is a safe and useful immunomodulator for the immune system.

The ischemia-responsive protein 94 (Irp94) activates dendritic cells through NK cell receptor protein-2/NK group 2 member D (NKR-P2/NKG2D) leading to their maturation.

Tumor recognition and killing, the uptake of released immunogenic substrate, and the generation of immunity are crucial aspects of dendritic cell (DC)-mediated antitumor immune response. In the context of direct tumoricidal activity, we have recently shown NK cell receptor protein-2 (NKR-P2)/NK group 2 member D (NKG2D) as a potent activation receptor on rat DCs. The activation of DCs with agonistic anti-NKR-P2 mAb, the binding of soluble NKR-P2 to the AK-5 tumor, and DC maturation with fixed AK-5 cells led us to identify a putative NKR-P2 ligand on the AK-5 cell surface. In this study we have shown that the AK-5 tumor-derived ischemia-responsive protein-94 (Irp94, a 110 kDa Hsp family member) acts as a functional ligand for NKR-P2 on DCs and enhances Irp94-NKR-P2 interaction-dependent tumor cell apoptosis via NO. Surface expression of Irp94 was also found on tumors of diverse origin in addition to AK-5. Furthermore, the Th1-polarizing cytokine IL-12, produced from Irp94-ligated BMDCs, augments NK cell cytotoxicity. Irp94-NKR-P2 interaction drives the maturation of BMDCs by up-regulating MHC class II, CD86, and CD1a and also induces autologous T cell proliferation, which displays a crucial state of DCs for adaptive antitumor immune response. These functional properties of Irp94 reside in the COOH terminus subdomain but not in the NH2 terminus ATPase domain of Irp94. We also show the involvement of PI3K, ERK, protein kinase C, phosphatases, and NF-kappaB translocation as downstream mediators of DCs activation upon NKR-P2 ligation with Irp94. Our studies demonstrate for the first time a novel role of a 110-kDa heat shock protein (Irp94) as a ligand for NKR-P2 on DCs, which in turn executes both innate and adaptive immunity.

Cross-linking a mAb to NKR-P2/NKG2D on dendritic cells induces their activation and maturation leading to enhanced anti-tumor immune response.

NKR-P2/NKG2D is the chief tumor recognition receptor of NK cells and some T cells, which recognizes stress inducible ligands on tumors and mediates cell activation. We have recently reported the involvement of NKR-P2 in rat dendritic cell (DC) activation. We demonstrate the potential of agonistic anti-NKR-P2 mAb (1A6), which mimics the NKR-P2 ligand and induces activation and maturation of DCs. Interaction of DCs with 1A6 enhances nitric oxide-mediated apoptosis in tumor cells. Cross-linking of NKR-P2 with mAb1A6 up-regulates MHC II, CD86, CD1a, antigen-presentation function and decreases endocytic activity of DC, thus drives DCs to play a pivotal role in adaptive immune responses. NKR-P2 cross-linking with 1A6 also induced the secretion of inflammatory cytokines, IL-1beta, tumor necrosis factor-alpha, IFN-gamma and IL-12 by DCs. Blocking of 1A6-mediated activation and maturation with inhibitors of PI3K, p38K and ERK1/2K suggests involvement of MAP kinase in signal transduction. 1A6 cross-linking activates nuclear factor kappa B, which acts as key executioner of DC activation. Administration of 1A6 antibody induces rapid regression and protective immune responses against transplantable tumors, suggesting mAb induced activation and maturation of DCs, leading to enhanced anti-tumor immune response.

Heat stress induced apoptosis in BC-8 cells derived from AK-5 tumor involves downregulation of Bcl-2 and generation of reactive oxygen species.

BC-8, a rat histiocytoma undergoes apoptosis after heat shock, which is due to lack of an effective heat shock response. Heat shock induced generation of free radicals, which in turn are involved in the induction of apoptotic death in BC-8 cells. Treatment of BC-8 cells with N-acetylcysteine partially inhibited the heat induced apoptosis. Introduction of Bcl-2 gene in these cells did not protect them from apoptotic death, whereas transfection with hsp-70 gene did render these cells resistant to heat induced apoptosis transiently. Heat shock also downregulated the expression of Bcl-2 and p53 in these cells. These observations suggested that the heat shock induced apoptosis was mediated through reactive oxygen species and controlled upstream of Bcl-2 check point.

Biological effects of curcumin and its role in cancer chemoprevention and therapy.

Curcumin, a natural component of the rhizome of curcuma longa has emerged as one of the most powerful chemopreventive and anticancer agents. Its biological effects range from antioxidant, anti-inflammatory to inhibition of angiogenesis and is also shown to possess specific antitumoral activity. The molecular mechanism of its varied cellular effects has been studied in some details and it has been shown to have multiple targets and interacting macromolecules within the cell. Curcumin has been shown to possess anti-angiogenic properties and the angioinhibitory effects of curcumin manifest due to down regulation of proangiogenic genes such as VEGF and angiopoitin and a decrease in migration and invasion of endothelial cells. One of the important factors implicated in chemoresistance and induced chemosensitivity is NFkB and curcumin has been shown to down regulate NFkB and inhibit IKB kinase thereby suppressing proliferation and inducing apoptosis. Cell lines that are resistant to certain apoptotic inducers and radiation become susceptible to apoptosis when treated in conjunction with curcumin. Besides this it can also act as a chemopreventive agent in cancers of colon, stomach and skin by suppressing colonic aberrant crypt foci formation and DNA adduct formation. This review focuses on the various aspects of curcumin as a potential drug for cancer treatment and its implications in a variety of biological and cellular processes vis-à-vis its mechanism of action.

A transmembrane-anchored rat RAE-1-like transcript as a ligand for NKR-P2, the rat ortholog of human and mouse NKG2D.

NKG2D is a potent activating immunoreceptor that has been extensively characterized in NK cells and shown to lend costimulatory functions in CD8(+) T cells under certain conditions. The list is growing of ligands for NKG2D, which are distantly related to MHC class I molecule and are often up-regulated during cellular distress. Here, we describe the cloning of a novel (and incidentally the first) ligand for NKR-P2, the rat ortholog of human and mouse NKG2D, termed as rat RAE-1-like transcript (RRLT). The newly identified ligand is homologous to mouse RAE-1 family of proteins, but differs from them in being transmembrane anchored. The protein localizes to cell surface, and is expressed in a variety of tumor cell lines and tissues. Ectopic expression of the ligand induces NK cell activation, and renders the target cells susceptible to NK cell killing. In addition, the role of RRLT-NKR-P2 interaction in tumor killing has been demonstrated.

Role of STAT3 and NFkappaB signaling in the serum factor-induced apoptosis in AK-5 cells.

AK-5, a rat histiocytoma, is rejected in about 70% of the syngeneic animals when injected subcutaneously. The sera from the tumor rejecting animals possess a potent factor, referred to as serum factor (SF) that induces apoptosis in AK-5 tumor cells. In the present study, we show that treatment with SF or JAK/STAT inhibitors AG490 and Piceatannol induces apoptosis to a similar extent in BC-8 (a single cell clone of AK-5) cells. Our results demonstrate downregulation of a transcription factor, STAT3, as a critical regulator of SF-induced apoptosis in BC-8 cells. SF treatment enhanced the activity of NFkappaB, another transcription factor that regulates both pro- and antiapoptotic genes. The enhanced NFkappaB activity resulted in the elevation of TRAIL and its receptor DR4, both known to induce apoptosis. Activation of death receptors in turn enhances caspase-8 activity and stimulates the downstream pathways regulating BC-8 cell apoptosis. SF induced apoptosis in BC-8 cells mediated through downregulation of STAT3 and elevated NFkappaB activity is abrogated by treatment with MAPK inhibitors-PD98059 and SB203580. Our studies therefore indicate that modulation of MAPK activity plays a central role in SF-induced death signaling pathways in BC-8 cells.

Differential cytotoxic effects of Annona squamosa seed extracts on human tumour cell lines: role of reactive oxygen species and glutathione.

Annonaceous acetogenins are a new class of compounds that have been reported to have potent pesticidal, parasiticidal, anti-microbial, cell growth inhibitory activities. In this study, organic and aqueous extracts from the defatted seeds of Annona squamosa (custard apple) were tested on different human tumour cell lines for antitumoural activity. While organic and aqueous extracts induced apoptosis in MCF-7 and K-562 cells, they failed to do so in COLO-205 cells. Treatment of MCF-7 and K-562 cells with organic and aqueous extracts resulted in nuclear condensation, DNA fragmentation, induction of reactive oxygen species (ROS) generation and reduced intracellular glutathione levels. In addition downregulation of Bcl-2 and PS externalization by Annexin-V staining suggested induction of apoptosis in MCF-7 and K-562 cells by both the extracts through oxidative stress. On the contrary, COLO-205 cells showed only PS externalization but no change in ROS and glutathione levels. These observations suggest that the induction of apoptosis by A. squamosa extracts can be selective for certain types of cancerous cells.

Differential apoptotic and redox regulatory activities of curcumin and its derivatives.

We have synthesized different bioconjugates of curcumin, which were tested for their pro- and antioxidant properties. In the present study five representative derivatives of curcumin, i.e., 4,4'-di-(O-acetyl) curcumin, 4,4'-di-(O-glycinoyl) curcumin, 4,4'-di-(O-glycinoyl-di-N-piperoyl) curcumin, 4,4'-di-(O-piperoyl) curcumin, and 4,4'-(O,O-cystinoyl)-3,3'-dimethoxydiphenyl-1,6-heptadiene-3,5-dione, were used for testing their apoptotic potential on tumor cells. Dipiperoyl and diglycinoyl derivatives showed higher apoptotic activity at lower concentrations, whereas diacetyl curcumin had slightly lower apoptotic activity on tumor cells. On the other hand, diglycinoyl-dipiperoyl and cystinoyl heptadiene derivatives had lost their apoptotic potential significantly. The apoptotic activity of these derivatives correlated very well with the generation of ROS by the tumor cells, whereas GSH levels remained unaltered. Our studies also indicate downregulation of Bcl-2 and participation of caspase-3 in the apoptotic death of tumor cells.

Migration of antigen presenting cells from periphery to the peritoneum during an inflammatory response: role of chemokines and cytokines.

We demonstrate the migration of antigen presenting cells (APCs), macrophages, and dendritic cells from the subcutaneous site to the peritoneum after they have picked up the antigen, using cell tracking dye. The migration of the APCs is more universal as it was also observed after injection of MethA tumor, DH-5alpha cells, and leishmania parasites, in addition to AK-5 tumor cells. Cellular migration is mediated by several chemokines and cytokines that also induce heavy influx of immune cells into the peritoneum. MIP-3beta secreted by the mesothelial cells is involved in the cellular influx into the peritoneum, whereas IL-12 and IFN-gamma produced by the APCs induced activation of immune cells in the peritoneum. Our results suggest an antigen presentation function for the APCs in the peritoneum as studied by lymphoproliferation assays. These studies indicate antigen presentation function of the activated migratory APCs from the distant subcutaneous site to the peritoneum, suggesting it acts as an important lymphoid organ involved in the enhancement of effector cell function.

Effect of curcumin on normal and tumor cells: role of glutathione and bcl-2.

Curcumin, a well-known dietary pigment derived from Curcuma longa, inhibited growth of several types of malignant cells both in vivo and in vitro. However, its mechanism of action still remains unclear. In this study, we have focused primarily on the cytotoxic effects of curcumin on three human tumor cell lines and rat primary hepatocytes. Curcumin induced apoptosis in MCF-7, MDAMB, and HepG2 cells in a dose-dependent and time-dependent manner. Apoptosis was mediated through the generation of reactive oxygen species. Attempts were made to establish the role played by endogenous glutathione on the apoptotic activity of curcumin. Depletion of glutathione by buthionine sulfoximine resulted in the increased generation of reactive oxygen species, thereby further sensitizing the cells to curcumin. Interestingly, curcumin had no effect on normal rat hepatocytes, which showed no superoxide generation and therefore no cell death. These observations suggest that curcumin, a molecule with varied actions, could be developed into an effective chemopreventive and chemotherapeutic agent.

Involvement of NKR-P2/NKG2D in DC-mediated killing of tumor targets: indicative of a common, innate, target-recognition paradigm?

DC are the most efficient antigen-presenting cells that regulate the immune response. Here, we demonstrate the expression of NK cell receptor protein-2 (NKR-P2) on rat and mouse DC, and we show that NKR-P2 gets reorganized upon antigen contact. DC activated with anti-NKR-P2 mAb exhibit enhanced apoptotic killing of tumor targets, whereas blocking the interaction between NKR-P2 and its ligand with rNKR-P2 abrogated apoptotic killing, suggesting NKR-P2 to function as an activating molecule on DC. In vivo injection of anti-NKR-P2 mAb augmented DC activity and delayed tumor progression. NKR-P2 signaling involved Ca(2+ )influx, culminating in the expression of the apoptosis-inducing molecule, TNF-alpha. Taken together, these observations suggest that NKR-P2 (the rat orthologue of human NKG2D) acts as a target-recognition molecule on DC.

Suppression of macrophage function in AK-5 tumor transplanted animals: role of TGF-beta1.

Transforming growth factor-beta 1 (TGF-beta1), a multifunctional cytokine secreted by various cell types has been implicated in diverse physiologic and pathophysiologic functions, including immunological, inflammatory, and neoplastic processes. AK-5 tumor cells when injected intraperitoneally grow as ascites, causing 100% mortality. Peritoneal macrophages cocultured with AK-5 cells, in vitro showed specific suppression of tumoricidal function. We report that AK-5 cells secrete the latent form of TGF-beta1, which is converted to its active homodimeric form, causing suppression of the secretion of cytocidal molecule by macrophages thereby inhibiting their cytotoxic capabilities. When macrophages are cocultured with AK-5 cells in the presence of antibodies against TGF-beta1 or the receptor II for TGF-beta, there is significant recovery in the secretion of nitric oxide and macrophage cytotoxic potential. These results suggest a major role for TGF-beta produced by the tumor cells, in the immune escape mechanisms adopted by the AK-5 tumor cells.

Interleukin-12 secreted by mature dendritic cells mediates activation of NK cell function.

Dendritic cells (DCs) are known to modulate immune response by activating effector cells of both the innate and the adaptive immune system. In the present study, we demonstrate that co-culture of DCs with paraformaldehyde-fixed tumor cells augments the secretion of interleukin (IL)-12 by DCs and these activated DCs upon co-culture with naive NK cells enhance the cytolytic activity of NK cells against NK-sensitive target YAC-1. Similarly, DCs isolated from tumor-bearing animals also activated NK cells in vitro. For efficient activation of NK cells, the ratio of activated DCs to NK cells is crucial. Addition of anti-IL-12 antibody to the culture system completely abolished activation of NK cells by DCs, suggesting that IL-12 secreted by DCs is an essential factor in NK cell activation. Adoptive transfer of DCs isolated from tumor-bearing animals into normal rats also induced activation of NK cells in normal animals.

Antitumour activity of Annona squamosa seed extracts is through the generation of free radicals and induction of apoptosis.

The plant, Annona squamosa traditionally known as custard apple possesses potent bioactive principles in all its parts. The effect of aqueous and organic extracts from defatted seeds of A. squamosa was studied on a rat histiocytic tumour cell line, AK-5. Both the extracts caused significant apoptotic tumour cell death with enhanced caspase-3 activity, down regulation of antiapoptotic genes Bcl-2 and Bcl(XL), and enhanced the generation of intracellular ROS, which correlated well with the decreased levels of intracellular GSH. In addition, DNA fragmentation and annexin-V staining confirmed that the extracts induced apoptosis in tumour cells through the oxidative stress. Aqueous extracts of A. squamosa seeds possessed significant antitumor activity in vivo against AK-5 tumor.

Phycocyanin-mediated apoptosis in AK-5 tumor cells involves down-regulation of Bcl-2 and generation of ROS.

C-phycocyanin, which is a major biliprotein of the blue-green algae, has been shown to possess cyclooxygenase-2 inhibitory activity. We have studied the effect of phycocyanin on a rat histiocytic tumor line. AK-5 cells are induced into apoptotic death program when treated with phycocyanin, which involves the activation of caspase-3. Phycocyanin-mediated apoptotic death is induced through the generation of reactive oxygen radicals. Free radical scavengers inhibited phycocyanin-induced apoptotic death in AK-5 cells. Bcl-2, an inhibitor of apoptosis, is shown to regulate ROS generation. Bcl-2 gene-transfected AK-5 cells are resistant to phycocyanin-induced death. Overexpression of Bcl-2 inhibited the production of ROS in phycocyanin-treated AK-5 cells. Thus, our observations demonstrate phycocyanin-induced apoptotic death in AK-5 cells, which is inhibited by Bcl-2 expression through the regulation of free radical generation. Phycocyanin, a natural product, could therefore be a possible chemotherapeutic agent through its apoptotic activity against tumor cells.

Protection conferred by Bcl-2 expression involves reduced oxidative stress and increased glutathione production during hypothermia-induced apoptosis in AK-5 tumor cells.

Hypothermia is known to retard mammalian cell growth, however, BC-8 cells, which have originated from AK-5 tumor after single cell cloning, were triggered into apoptotic pathway when grown at 30 degrees C. Cell death process showed typical apoptotic features like DNA fragmentation, cytochrome c release, etc. Introduction of Bcl-2 gene in BC-8 cells inhibited hypothermia-induced apoptotic process, which is ascribed to reduced ROS generation and higher glutathione production. Thus, Bcl-2 seems to control the apoptotic induction process at the level of redox regulation, in addition to its known effects at the mitochondrial dysregulation. These observations suggest that tumors, which are low in Bcl-2 expression, are sensitive to hypothermic shock and make hypothermia an interesting inducer of apoptosis in tumor cells.

Baculoviral p35 inhibits oxidant-induced activation of mitochondrial apoptotic pathway.

In this study we report that the baculovirus p35 anti-apoptotic protein prevents cell death by quenching free radicals at a very upstream step in the apoptotic pathway. Mitochondria of activated rat peritoneal macrophages as well as Spodoptera frugiperda (Sf9) insect cells, following treatment with oxidants, H(2)O(2)/UVB irradiation, release cytochrome c followed by activation of caspase-3. Transfection of macrophages/Sf9 cells with a construct carrying the p35 gene under the CMV/HSP promoters resulted in p35 expression and consequent arrest of oxidative stress-induced apoptosis. p35 expression also inhibited cytochrome c release from the mitochondria of oxidant-exposed cells and blocked caspase-3 activation.