Ingenol mebutate (Picato®) is a recently FDA approved topical
agent for the treatment of actinic keratosis with as little as a 2 or 3 day treatment.1, 2 This drug has also been shown to be effective in treating superficial basal-cell cancer and is potentially effective in killing melanoma cells.3-5 How it works is still speculative, but this natural product, derived from the sap of the tree Euphorbia peplus, has been used extensively for its medicinal properties.2 In this issue of the Journal, Stahlhut et al. reveal for the first time evidence for the role of apoptosis and mitochondrial permeability as a possible mechanism of ingenol mebutate-mediated cytotoxicity. The authors demonstrate that ingenol mebutate elicits a strong and sustained increase in intracellular calcium that involves both ER-associated and mitochondrial-associated calcium stores. Interestingly, cancer cells take up ingenol mebutate and have a more robust calcium release promoting cell death upon treatment as compared to the same dose of ingenol mebutate in cultured donor keratinocytes.
These findings are consistent with previous reports showing that ingenol mebutate specifically targets rapidly dividing
cells in the basal cell layer (ie, dysplastic keratinocytes).
The findings in this study are the first to evaluate the intracellular
mechanisms involved in how ingenol mebutate may promote dysplastic and neoplastic keratinocyte cell death and elimination while sparing normal keratinocytes. This study employed an in vitro method and utilized a model of reconstituted skin to demonstrate
the localization of this novel compound. Future studies are needed to evaluate how this novel compound functions in vivo, especially considering how this compound implicates apoptosis and alternative death pathways in promoting cell death.
Ingenol mebutate treatment was shown to promote death of dysplastic and neoplastic skin cells and subsequently promoted keratinocyte proliferation in a mouse model of UVB-induced actinic keratosis and skin cancer.6 The authors of this earlier study suggest that early inflammation and neutrophil
infiltration were the initiating events for regenerating keratinocytes at the basal layer. Additionally, Le et al. have further shown an immunostimulatory effect of ingenol mebutate
resulting in an increase in anti-tumor CD8+ cells.7 Ingenol mebutate was also shown to differentially regulate apoptosis and TNF-α related apoptosis ligand (TRAIL) induced apoptosis in melanoma cancer cells.4 However, the apoptosis regulators are yet to be determined. Cozzi et al., showed that ingenol mebutate specifically targeted cells with mutant regulation of p53.6 Indeed, p53 expression and function are altered in cancer cells and thus ingenol mebutate may be acting on regulators of apoptosis. For instance, novel regulators such as the anti-apoptotic protein Fortilin are known to regulate p53, yet little else is known of this protein in relation to how apoptosis is modulated based on Fortilin-mediated regulation of p53.8 B-cell lymphoma-extra large (Bcl-xl) has further been shown to differentially regulate mitochondrial and ER-calcium stores.9 Other studies suggest a role for apoptotic regulators including
Bcl proteins and the Protein Kinase C regulated Mitogen Activated Protein Kinase pathways (PKC-MAPK) in ingenol mebutate-mediated cell death.8-10 Additionally, the authors of the current study show a differential regulation of mitochondrial
and ER stores of calcium. Apoptosis regulators in the Bcl-2 family, including Bcl-xL and Bax, are known to regulate calcium-mediated cell death yet the mechanisms are still to
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