Influence of letrozole on mRNA concentrations of cytochrome P4501A1 and estrogen receptors ER-α and ER-β in cultured C-28/I2 chondrocytes analyzed by quantitative RT-PCR and on estrone concentration of these cells assessed by enzyme-linked immunosorbent assay. (A) Compared to controls, chondrocytes cultured without aromatase inhibitor incubated with 10−11 M to 10−7 M letrozole show significantly increased expression of cytochrome P4501A1 (CYP1A1) mRNA after 48 hours with 10−7 M letrozole. We also observed a tendency toward this effect after incubation for 12 and 24 hours using concentrations of 10−9 M and 10−7 M letrozole. (B) Quantitative RT-PCR (qRT-PCR) shows significantly increased expression of estrogen receptor ER-β mRNA following incubation with 10−11 M to 10−7 M letrozole for 48 hours. No regulatory influence was observed after incubation with 10−11 M to 10−7 M letrozole for 12 and 24 hours. (C) qRT-PCR revealed significantly increased expression of ER-α mRNA following incubation with 10−7 M and 10−11 M letrozole for 24 and 48 hours. (D) In comparison to control chondrocytes cultured without aromatase inhibitor, incubation with 10−11 M to 10−7 M letrozole for 48 hours led to significantly downregulated expression of estrone (E1) protein as revealed by enzyme-linked immunosorbent assay. All experiments were performed in triplicates (n = 3).