Peptides of the innate immunity as potential anticancer agents: pros and cons

Abstract

Surgical resection was the main approach to cancer therapy, often supplemented by radiation and chemotherapy. The effectiveness of such complex treatment in many cases remains low. In this regard, there is an urgent need to search for new compounds that have selective cytotoxic activity against tumor cells and do not damage normal tissues of the organism. The review discusses mechanisms of antitumor action of cationic antimicrobial peptides (AMPs) of the cathelicidin family - human α-helical cathelicidin (LL-37), and a peptide with β-hairpin conformation – protegrin-1 (PG-1) on lung, breast, pancreas, prostate, squamous skin cancer cells, oral cancer, stomach, ovarian, colorectal cancer, melanoma, leukemia, lymphoma, glioma and neuroblastoma cells. An opportunity of antitumor and pro-oncogenic actions of the peptides and an interplay of these effects with mmunomodulatory action of AMPs on tumor-associated macrophages, natural killer cells and T-lymphocytes is discussed. Possible mechanisms of LL-37 and PG-1 selective action upon tumor cells are presented, including the interaction of LL-37 with G-protein-coupled receptors: the N formylpeptide-2 receptor (FPR2), CXC chemokine-2 (CXCR2), Mas-related gene X2 (MrgX2), purinergic (P2Y11), epidermal (EGFR/ErbB1, ERBb2), insulin-like (IGF1R) growth factors, ligand-gated ion channels (LGIC) and Tolllike (TLR) receptors, with expression varying significantly in different types of tumors, as compared to normal tissues. An increase in the level of LL-37 secretion and expression of its CAMP gene are associated with progression of lung adenocarcinoma, breast, pancreas, and prostate cancer, ovarian cancer, melanoma, and squamous cell carcinoma of the skin. In contrast, CAMP expression and LL-37 secretion are significantly reduced in gastric cancer cells, oral squamous cell cancer, colorectal cancer, leukemia, lymphomas, gliomas, and SH-SY5Y neuroblastoma. Therefore, therapeutic effects of LL-37 can only be used for specific types of tumors. The mechanisms of action of PG-1 on tumor cells are still poorly understood, although the available data indicate that protegrin exhibits a more unidirectional effect, i.e., it damages cell membranes. Protegrin-1 and LL-37 can synergistically enhance the antitumor effects of chemotherapy drugs and have a more pronounced effect on tumor cells, than upon normal cells. Natural AMPs appear to be promising candidates for the role of new antitumor agents, which are also active against malignant metastatic, recurrent multidrug-resistant tumors. On the other hand, peptides such as LL-37, in some cases, exhibit properties that can be considered pro-oncogenic, which indicates a need for further detailed studies on the molecular mechanisms of their action on tumor cells.