ABSTRACT
The phytonutrient ursolic acid (UA), present in apples, rosemary, and other plant sources, has anti-cancer properties in a number of systems, including skin cancers. However, few reports have examined upstream mechanisms by which UA may prevent or treat cancer. Recent reports have indicated UA induces death of cancer cell lines via AMP-activated protein kinase (AMPK), an energy-sensing kinase which possesses both pro-metabolic and anti-cancer effects. Other studies have shown UA activates peroxisome proliferator activated receptor α (PPARα) and the glucocorticoid receptor (GR). Here, we found the cytotoxic effect of UA in skin carcinoma cells required AMPK activation. In addition, two inhibitors of PPARα partially reversed the cytotoxic effects of UA, suggesting its effects are at least partially mediated through this receptor. Finally, inhibition of the GR did not reverse the effects of UA nor did this compound bind the GR under the conditions of experiments performed. Overall, studies elucidating the anti-cancer effects of UA may allow for the development of more potent analogues utilizing similar mechanisms. These studies may also reveal the mediators of any possible side effects or resistance mechanisms to UA therapy.
Subject(s)
AMP-Activated Protein Kinases/metabolism , Antineoplastic Agents, Phytogenic/pharmacology , PPAR alpha/metabolism , Skin Neoplasms/metabolism , Triterpenes/pharmacology , Animals , Cell Line, Tumor , Cell Proliferation/drug effects , Cell Survival/drug effects , Gene Expression Regulation, Neoplastic/drug effects , Humans , Mice , Signal Transduction/drug effects , Skin Neoplasms/drug therapy , Ursolic AcidABSTRACT
UNLABELLED: Ursolic acid, present in apples, rosemary, and other sources, is known to inhibit tumor formation and tumor cell viability in multiple systems, including skin. However, various cancers are resistant to ursolic acid treatment. Herein, skin carcinoma cells (Ca3/7) as compared with skin papilloma cells (MT1/2) displayed more resistance to ursolic acid-induced cytotoxicity. Interestingly, Ca3/7 cells had elevated levels of P-glycoprotein (P-gp), an ATP-dependent efflux pump that mediates resistance to chemotherapy in preclinical and clinical settings, and not only accumulated less but also more rapidly expelled the P-gp substrate rhodamine 123 (Rh123) indicating ursolic acid is transported by P-gp. To determine whether P-gp inhibition can enhance ursolic acid-mediated cytotoxicity, cells were challenged with P-gp inhibitors verapamil or cyclosporin A. Alternatively, cells were pretreated with the natural compound resveratrol, a known chemotherapy sensitizer. Verapamil and resveratrol enhanced the effects of ursolic acid in both cell lines, whereas cyclosporin A only did so in Ca3/7 cells. Similarly, verapamil inhibited Rh123 efflux in both lines, whereas cyclosporin A only inhibited Rh123 efflux in Ca3/7 cells. Resveratrol did not inhibit Rh123 efflux in either line, indicating the synergistic effects of resveratrol and ursolic acid are not manifest by inhibition of P-gp-mediated efflux of ursolic acid. These results indicate that the anti-skin cancer effects of ursolic acid are enhanced with P-gp inhibitors. In addition, resveratrol and ursolic acid interact synergistically, but not through inhibition of P-gp. IMPLICATIONS: Resveratrol and/or p-glycoprotein inhibitors in combination with ursolic acid are an effective anti-skin cancer regimen.
