Photosensitized methyl paraben induces apoptosis via caspase dependent pathway under ambient UVB exposure in human skin cells

Methyl paraben (MP), is a widely used preservative in pharmaceutical, food and cosmetic products. Its molecular mechanism under ambient ultraviolet radiation is not well understood. We investigated photosensitizing mechanism of MP under ambient UVB (0.6 mW/cm²) intensity. MP showed dose dependent decrease in cell viability of human keratinocyte cell line (HaCaT) by MTT and NRU assays. Study showed 40% reduction in antimicrobial activity of UVB irradiated MP through E. coli culture. Photosensitized MP (25 μg/ml) significantly enhanced lipid peroxidation, intracellular ROS generation and disrupted mitochondrial membrane integrity. MP induced loss of lysosomal membrane integrity and endoplasmic reticulum (ER) mediated stress evident from Ca⁺² release. Phototoxicity of MP showed nuclear fragmentation, phosphatidylserine translocation, 30% tail DNA and micronuclei formation. Study showed mitochondria mediated apoptosis via upregulation of Bax, Apaf-1, Cytochrome C and Caspase-3. Upregulation of Caspase-12 (2 folds) specifically showed role of ER in apoptosis. Specific caspase inhibitor, Z-VAD-FMK showed involvement of caspase cascade pathway in apoptosis. Results indicate that photosensitive MP leads to oxidative stress mediated DNA damage and apoptosis through mitochondria and ER. MP causes deleterious effects and its long term exposure to human skin may promote skin diseases. Therefore, MP should be replaced by other photosafe preservatives for humans.