Cytotoxic and Antitumor Activity of Lactaptin in Combination with Autophagy Inducers and Inhibitors

Autophagy is a degradative process in which cellular organelles and proteins are recycled to restore homeostasis and cellular metabolism. Autophagy can be either a prosurvival or a prodeath process and remains one of the most fundamental processes for cell vitality. Thus autophagy modulation is an important approach for reinforcement anticancer therapeutics. Earlier we have demonstrated that recombinant analog of human milk protein lactaptin (RL2) induced apoptosis of various cultured cancer cells and activated lipidation of microtubule-associated protein 1 light chain 3 (LC3). In this study we investigated whether autophagy inhibitors—chloroquine (CQ), Ku55933 (Ku), and 3-methyladenine (3MA)—or inducer—rapamycin (Rap)—can enhance cytotoxic activity of lactaptin analog in cancer cells and its anticancer activity in the mice model. Western Blot analysis revealed that RL2 induced short-term autophagy in MDA-MB-231 and MCF-7 cells at early stages of incubation and that these data were confirmed by the transmission electron microscopy of autophagosome/autophagolysosome formation. RL2 stimulates reactive oxygen species (ROS) production, autophagosomes accumulation, upregulation of ATG5 with processing of LC3I to LC3II, and downregulation of p62/sequestosome 1 (p62). We have shown that autophagy modulators, CQ, Ku, and Rap, synergistically increased cytotoxicity of RL2, and RL2 with CQ induced autophagic cell death. In addition, CQ, Ku, and Rap in combination with RL2 decreased activity of lysosomal protease Cathepsin D. More importantly, combining RL2 with CQ, we improved antitumor effect in mice. Detected synergistic cytotoxic effects of both types of autophagy regulators, inhibitors, and inducers with RL2 against cancer cells allow us to believe that these combinations can be a basis for the new anticancer approach. Finally, we suppose that CQ and Rap promoting of short-term RL2-induced autophagy interlinks with final autophagic cell death.