LL13-37

Multidrug resistance (MDR) of cancer cells to a wide spectrum of anticancer drugs is a major obstacle to successful chemotherapy. It is usually mediated by the overexpression of one of the three major ABC transporters actively pumping cytotoxic drugs out of the cells. There has been great interest in the search for inhibitors toward these transporters with an aim to circumvent resistance. This is usually achieved by screening from natural product library and the subsequent structural modifications. This study reported the reversal of ABCG2-mediated MDR in drug-selected resistant cancer cell lines by a class of host defense antimicrobial peptides, the human cathelicidin LL37 and its fragments. The effective human cathelicidin peptides (LL17-32 and LL13-37) were found to increase the accumulation of mitoxantrone in cancer cell lines with ABCG2 overexpression, thereby circumventing resistance to mitoxantrone. At the effective concentrations of the cathelicidin peptides, cell proliferation of the parental cells without elevated ABCG2 expression was not affected. Result from drug efflux and ATPase assays suggested that both LL17-32 and LL13-37 interact with ABCG2 and inhibit its transport activity in an uncompetitive manner. The peptides were also found to downregulate ABCG2 protein expression in the resistant cells, probably through a lysosomal degradation pathway. Our data suggest that the human cathelicidin may be further developed for sensitizing resistant cancer cells to chemotherapy.

Human cathelicidin LL37 and its fragments LL13-37 and LL17-32 exhibited similar potencies in inhibiting growth of the yeast Candida albicans. After treatment with 0.5 μM and 5 μM LL13-37, the hyphae changed from a uniformly thick to an increasingly slender appearance, with budding becoming less normal in appearance and cell death could be detected. Only the yeast form and no hyphal form could be observed following exposure to 50 μM LL13-37. LL13-37 at a concentration of 5 μM was able to permeabilize the membrane of yeast form as well as hyphal form of C. albicans since the nuclear stain SYTOX Green was localized in both forms. Mycelia treated with LL13-37 stained with SYTOX Green, but did not stain with MitoTracker deep red, indicating that the mitochondria were adversely affected by LL13-37. Bimane-labeled LL13-37 was able to enter some of the hyphae, but not all hyphae were affected, suggesting that LL37 impaired membrane permeability characteristics in some of the hyphae. Reactive oxygen species was detectable in the yeast form of C. albicans cells after treatment with LL13-37 but not in the untreated cells. The results suggest that the increased membrane permeability caused by LL13-37 might not be the sole cause of cell death. It might lead to the uptake of the peptide, which might have some intracellular targets.

Cathelicidins exhibit anti-HIV activity but it is not known if they reduce the activity of enzymes crucial to the life cycle of the retrovirus. It is shown in this investigation that human cathelicidin LL37 and its fragments LL13-37 and LL17-32 inhibited HIV-1 reverse transcriptase dose-dependently with an IC50 value of 15μM, 7μM, and 70μM, respectively. The three peptides inhibited HIV-1 protease with a weak potency, achieving 20-30% inhibition at 100μM. The mechanism of inhibition was protein-protein interaction as revealed by surface plasmon resonance. The peptides were devoid of the ability to inhibit translocation of HIV-1 integrase, which has been labeled with green fluorescent protein, into the nucleus. The peptides did not exert toxicity on human peripheral blood mononuclear cells.