Sclareol, a fragrant natural compound, suppresses P-glycoprotein activity and sensitizes resistant cancer cells to doxorubicin

Abstract

Multidrug resistance (MDR) is one of the major obstacles to successful cancer treatment. How to overcome cancer MDR is still an unsolved issue in clinical practice although several generations of MDR transporters’ inhibitors have been developed and widely investigated so far. Nature is an important source of potential anticancer agents capable to suppress the activity of membrane transporters implicated in MDR such as P-glycoprotein (P-gp). In this study, we evaluated the effects of sclareol (SC), a naturally occurring labdane type diterpene, on the P-gp activity and its potential to sensitize different human cancer cell lines to doxorubicin (DOX). To that end, we used several human cancer cell lines (colorectal carcinoma, DLD1, and its MDR variant DLD1-TxR, non-small cell lung carcinoma NCI-H460, and its MDR variant NCIH460/R, glioblastoma U251, U87, and its MDR variant U87-TxR) and normal human embryonic lung fibroblasts (MRC-5). The effects of SC alone and in combination with DOX on cell viability were assessed by MTT, while the effects on DOX and rhodamine 123 (Rho 123) accumulation as determinants of P-gp activity were assessed by flow cytometry. The efficient concentrations of SC that significantly decreased cell viability (IC50 values) ranged between 20 µM for DLD1 and 60 µM for MRC-5. The presence of MDR phenotype did not diminish the SC effect on cell viability, even more, SC was more potent in U87-TxR than in U87 cells. The effects of 72 h simultaneous treatment of SC (10 and 20 µM) with DOX (20, 50, 100, 200 and 500 nM) demonstrated the considerable potential of SC to sensitize DLD1, DLD1-TxR, NCI-H460/R, U87-TxR and U251 cells to DOX. However, the observed sensitization was not due to the P-gp inhibition in all MDR cancer cell lines. Only in NCI-H460/R the obvious suppression of P-gp was observed due to the significant increase in the accumulation of both P-gp substrates (DOX and Rho 123). SC did not affect the P-gp activity in DLD1 and DLD1-TxR cells. On the contrary, DOX and Rho123 accumulation increased in U87 and U87-TxR albeit the fact that U87 cells do not express P-gp. Results obtained in this study showed a considerable potential of SC to sensitize cancer cells to DOX. However, the effects of SC are cancer type-specific and not solely dependent on the suppression of P-gp activity. Further investigations are envisioned to determine molecular mechanisms of SC in different cancer cell types.