Examining the Role of Mitochondrial Respiration in Vanilloid-Induced Apoptosis

Abstract

Background: The vanilloids capsaicin and resiniferatoxin are natural products that contain a vanillyl moiety (4-hydroxy-3-methoxybenzyl). Both vanilloids can induce apoptosis in certain cell types by a mechanism that has not been fully elucidated but may involve plasma membrane or mitochondrial targets. We investigated the role of mitochondrial respiration in vanilloid-induced apoptosis. Methods: Cytofluorometric analysis was used to evaluate the effects of vanilloids on apoptosis, Ca²⁺ mobilization, hydroperoxide generation, and DNA content in cells from two human cutaneous squamous cell carcinoma (SCC) cell lines (parental cells) and in their respiration-deficient clones. Oxygen consumption by the cells was determined polarographically. Results: The majority of the parental SCC cells underwent apoptosis after a 12-hour exposure to 100 μM capsaicin or 10 μM resiniferatoxin. The induction of apoptosis was associated with the mitochondrial permeability transition (i.e., an increase in the permeability of the inner mitochondrial membrane associated with the opening of a nonspecific pore). Exposure of parental cells to either vanilloid was not associated with an increase in intracellular free Ca²⁺ levels but was associated with a rapid increase in hydroperoxide generation and a decrease in oxygen consumption. After vanilloid treatment, the respiration-deficient clones generated less hydroperoxide and were resistant to the mitochondrial permeability transition and the induction of apoptosis. Moreover, vanilloid treatment inhibited cell proliferation in the respiration-deficient clones by promoting G₁ arrest. Conclusions: Vanilloid-induced apoptosis in the parental SCC cells appears to involve the inhibition of mitochondrial respiration. The apoptogenic effects promoted by vanilloid treatment in parental SCC cells, as well as the antiproliferative effects observed in their respiration-deficient clones, suggest that vanilloids may be useful for preventing or treating skin cancers or other hyperproliferative skin disorders.