Immunomodulatory effects of curcumin: In-vivo

Abstract

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.

Introduction

Cancer chemotherapy is often associated with the side-effects on immune cells. Thus, the prerequisites for anti-cancer drugs are to ensure no damaging effects on the immune cells, failing which the drug may completely terminate the subsided immune response in tumor-bearing host.

Curcumin is an illustrious dietary ingredient of the Indian subcontinent [1]. In solution, curcumin [1,7-bis (4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3, 5-dione] exists as a keto-enol tautomer, which is extracted from the rhizome of Curcuma longa[2]. Curcumin is a yellow, hydrophobic fluorescent molecule, which rapidly impregnates in cellular membranes [3], [4].

Overwhelming evidences have confirmed that curcumin is an anticancerous compound and executes its function by modulating multiple targets in tumor cells [5], [6]. Inhibition of NF-κB activation before IκBα phosphorylation [7], ROS (Reactive Oxygen Species) generation in tumors is the major target of curcumin, which induce tumor cell apoptosis [8]. On the other hand, carcinogen mediated tumor initiation is prevented by curcumin, involving ROS, reactive nitrogen species; NF-κB, (NF-E2)-related factor 2, heme oxygenase-1, glutathione S-transferase and glutathione reductase [9]. In-vivo curcumin inhibited the generation of intestinal tumors by modulating intestinal immune cell profile [10].

In-vitro the apoptotic activity of curcumin on tumor cells differs with dose. We have reported that induction of stress response rendered tumor cell lines resistant to curcumin-mediated apoptosis, which was dependent on ROS intermediates [8]. Curcumin dose directly determines ROS generation capacity, intracellular ATP levels, apoptosis or necrosis in osteoblast [11]. Curcumin has also been shown to modulate the reversal of multi-drug resistance [12].

We have previously reported the involvement of ROS in curcumin-mediated tumor cell apoptosis leading to the regression of ascitic tumors [13]. Curcumin also accelerated the spontaneous regression of solid tumors involving modulation of innate immune response [14]. In addition, depletion of endogenous glutathione enhanced the sensitivity of tumor cells to curcumin and curcumin had no effect on normal hepatocytes [15].

The direct role for curcumin has also been demonstrated in the cure of various autoimmune disorders, curcumin inhibited IL-12 mediated Th1 dependent neuronal demyelination in murine model of multiple sclerosis by targeting Janus kinase 2, tyrosine kinase 2, STAT3 and STAT4 [16]. Curcumin also enhanced the clearance of amyloid-β (plaques) in the brain by Mϕs (macrophages) in Alzheimer's patients [17]. Under severe conditions of infection, curcumin attenuated LPS-mediated endotoxemia [18]. Curcumin targets TLR-adapter-MD-2 and inhibits homodimerisation of TLR4 to exhibit anti-inflammatory response [19], [20]. Curcumin has been shown to attenuate the expression of IL-1β, IL-6, and cyclin E in TNF-α treated human keratinocytes [21]. Curcumin also prevented rheumatoid arthritis, by inducing apoptosis and inhibiting prostaglandin E2 production in synovial fibroblasts of rheumatoid arthritis patients [22], [23]. Curcumin controls allergic responses by attenuating Th2 inflammatory responses [24].

In plasma 2.25 µg/ml of curcumin concentration can be achieved within 15 min, by injecting 0.1 g/kg of curcumin (i.p.) in mice. Similarly 26.06 µg/g of curcumin was found in spleen after 1 h of injections [25]. In circulation the major curcumin biotransformants are curcumin-glucuronide, sulfate, hexahydrocurcumin, hexahydrocurcuminol, and hexahydrocurcumin glucuronide. Synthetic curcumin derivatives differ in their apoptotic and redox regulatory activities [26].

It is plausible that customary consumption of curcumin does not exert health disorders in humans; moreover it exhibits a promising role in various pathological conditions ranging from cancer to autoimmune disorders and inflammation. Owing to multiple mechanisms and distinct responses on various cell types, it becomes imperative to assess its effect on immune cells in-vivo. This study was carried out to investigate the role of curcumin on the major functions of the immune cells in-vivo. We have studied the effect of curcumin on T cell proliferation, NK cell mediated cytotoxicity, production of cytokines, generation of NO and ROS by Mϕs, which are the major effectors of anti-tumor immune response in-vivo.