Glycyrrhizic acid facilitates anti-tumor immunity by attenuating Tregs and MDSCs: An immunotherapeutic approach

•GA inhibits melanoma growth by normalizing progressive tumor microenvironment.•GA induces apoptosis and modulates inflammatory cytokines from Th2 to Th1 in melanoma.•GA inhibits pSTAT3 mediated immune suppression of Tregs and MDSCs in melanoma.•Combinatorial immunotherapy of GA and Mw restricts advanced stage melanoma growth. Melanoma is one of the most aggressive malignancies and its treatment remains challenging due to its highly metastatic property and availability of limited effective drugs. In addition, immunosuppresive tumor microenvironment (TME) has been identified as major barrier to evoke anti-tumor response in melanoma. Recent studies revealed that immunosuppressive TME is directly correlated with heightened activations of T regulatory cells (Tregs) and Myeloid derived suppressor cells (MDSCs) functions. In this study, we investigated the anti-cancer effect of a triterpenoid, glycyrrhizic acid (GA) on melanoma. Our study revealed that GA not only exhibited anti-proliferative effects on melanoma cells it significantly restricted progression of melanoma tumor. However, the therapeutic efficacy of GA in impressive regression of tumor was found to be directly correlated with induction of apoptosis and modulation of cytokines from Th2 to Th1 type. To unravel the mechanism of anti-melanoma effect of GA, it has been delineated that GA inhibits pSTAT3 to evade anti-tumor suppressive function of Tregs and MDSCs. Downregulation of FOXP3, GITR and CTLA4 in tumor-infiltrating Tregs and inhibition of Cox2, PGE2 and Arginase 1 in intra-tumoral MDSC were evidenced as some of the key events during therapeutic intervention of GA in melanoma management. Moreover, GA effectively restricted advanced stage solid tumor while used in combination with Mycobacterium indicus pranii, a known immunomodulator, which alone is reported to be ineffective to restrict advanced stage solid tumor. Thus, our findings may open up a novel insight of GA as a promising agent in cancer immunotherapy or adjuvant therapy in future.