Kalkitoxin Inhibits Angiogenesis, Disrupts Cellular Hypoxic Signaling, and Blocks Mitochondrial Electron Transport in Tumor Cells

Abstract

The biologically active lipopeptide kalkitoxin was previously isolated from the marine cyanobacterium Moorea producens (Lyngbya majuscula). Kalkitoxin exhibited N-methyl-d-aspartate (NMDA)-mediated neurotoxicity and acted as an inhibitory ligand for voltage-sensitive sodium channels in cultured rat cerebellar granule neurons. Subsequent studies revealed that kalkitoxin generated a delayed form of colon tumor cell cytotoxicity in 7-day clonogenic cell survival assays. Cell line- and exposure time-dependent cytostatic/cytotoxic effects were previously observed with mitochondria-targeted inhibitors of hypoxia-inducible factor-1 (HIF-1). The transcription factor HIF-1 functions as a key regulator of oxygen homeostasis. Therefore, we investigated the ability of kalkitoxin to inhibit hypoxic signaling in human tumor cell lines. Kalkitoxin potently and selectively inhibited hypoxia-induced activation of HIF-1 in T47D breast tumor cells (IC50 5.6 nM). Mechanistic studies revealed that kalkitoxin inhibits HIF-1 activation by suppressing mitochondrial oxygen consumption at electron transport chain (ETC) complex I (NADH-ubiquinone oxidoreductase). Further studies indicate that kalkitoxin targets tumor angiogenesis by blocking the induction of angiogenic factors (i.e., VEGF) in tumor cells.

Mitochondrial Dysfunction Inhibits Hypoxia-Induced HIF-1α Stabilization and Expression of Its Downstream Targets

“…For instance it has been shown that BAY 87-2243, a potent inhibitor of HIF-1α, reduced tumor growth, potentially through targeting mitochondrial complex I (CI) (51,52). Similarly, the CI inhibitor Kalkitoxin reduced tumor cell proliferation under hypoxia, but was also capable of reducing HIF1 stabilization (53). The CI inhibitor AG311 also reduced HIF-1α stabilization and in combination with an inhibitor of the pyruvate dehydrogenase kinase, a key regulatory enzyme of oxidative metabolism, resulted in a reduced tumor growth (54).…”

AG311, a small molecule inhibitor of complex I and hypoxia-induced HIF-1α stabilization

“…In contrast to the biguanides, BAY-87-2243, a new complex I inhibitor, has nanomolar potency, but its major limitation is its specificity for human complex I, limiting its preclinical evaluation using other species [59]. A series of marine toxins including mycothiazole, furospongolide, lehualide B, and kalkitoxin have recently been identified as complex I inhibitors and disruptors of HIF-1α signaling in hypoxic cancer cells [60]. Limitations of these toxins, as the moniker implies, are potential toxic off-target effects such as N-methyl-D-aspartate (NMDA)-mediated neurotoxicity by kalkitoxin [60].…”

“…A series of marine toxins including mycothiazole, furospongolide, lehualide B, and kalkitoxin have recently been identified as complex I inhibitors and disruptors of HIF-1α signaling in hypoxic cancer cells [60]. Limitations of these toxins, as the moniker implies, are potential toxic off-target effects such as N-methyl-D-aspartate (NMDA)-mediated neurotoxicity by kalkitoxin [60]. In comparison to these complex I inhibitors, the IC 50 of AG311 for cancerous cells is in the lower micromolar range and thus falls between the concentrations for biguanides (mM) and BAY-87-2243/toxins (nM).…”

Marine Cyanobacteria and Microalgae Metabolites—A Rich Source of Potential Anticancer Drugs

“…to decrease HCT116 colon cancer cell survival [131]. Somocystinamide A (66) (Figure 10), a lipopeptide obtained from the same cyanobacterium, exhibited antiproliferative activity against Jurkat (T cell leukemia), CEM leukemia, A549 lung carcinoma, Molt4T leukemia, M21 melanoma and U266 myeloma cells.…”