AB-Free Kava and Dihydromethysticin Inhibit Chronic Hexavalent Chromium Exposure-Induced Cell Malignant Transformation through Down-Regulating RNA Splicing Factor SNRPA1 Expression

Hexavalent chromium [Cr(VI)] is a common occupational and environmental toxicant and an established carcinogen causing lung cancer in humans. The mechanism through which Cr(VI) exposure causes lung cancer remains to be clearly defined. No effective strategies are currently available to prevent lung cancer resulting from chronic Cr(VI) exposure. Kava is traditionally consumed by South Pacific Islanders to reduce anxiety. Epidemiology and experimental studies suggest that kava has anticancer potential and may be used as a preventive agent to reduce the risk of various cancers including lung cancer. The purpose of this study is to determine the effect of AB-Free Kava and its active component dihydromethysticin (DHM) on chronic Cr(VI) exposure-induced cell malignant transformation and the underlying mechanism. The study was performed by pretreating immortalized but nontumorigenic human bronchial epithelial cells (BEAS-2B) with AB-Free Kava (25 μg/mL) or DHM (10 μM) followed by exposing cells to 0.25 μM Cr(VI) (K₂Cr₂O₇) for 20 weeks to determine the impact of AB-Free Kava or DHM on chronic Cr(VI) exposure-induced cell transformation, cancer stem cell (CSC)-like properties, and tumorigenesis. The extent of cell transformation was evaluated by soft agar colony formation assays, Western blot analysis of cancer stemness marker expression levels, and nude mouse xenograft tumorigenesis assays. It was found that AB-Free Kava or DHM pretreatment significantly reduces Cr(VI)-induced cell transformation, CSC-like properties, and tumorigenesis. Mechanistically, it was determined that AB-Free Kava or DHM impairs Cr(VI)-induced cell transformation by down-regulating RNA splicing factor small nuclear ribonucleoprotein polypeptide A′ (SNRPA1) expression to reduce the protooncogene, cancer stemness marker, and driver c-MYC expression. It was further determined that DHM and SNRPA1 regulate c-MYC expression through affecting c-MYC protein stability. It was concluded that AB-Free Kava or DHM inhibits Cr(VI)-induced cell transformation, CSC-like properties, and tumorigenesis by down-regulating RNA splicing factor SNRPA1 expression to increase the protooncogene c-MYC protein degradation.