Effect of receptor inhibition on inducible nitric oxide synthase (iNOS) expression. A, Chondrocytes were treated with 10 ng/ml interleukin-1β (IL-1β) along with 3 μM sphingosine 1-phosphate (S1P), 100 ng/ml pertussis toxin (PTX) (inhibitor of Gαi/S1P1), 10 μM JTE-013 (inhibitor of S1P2), or 5 μM suramin (inhibitor of S1P3) for 3 hours. Expression of iNOS mRNA was quantified by real-time reverse transcription–polymerase chain reaction (RT-PCR). B, Chondrocytes were transfected with small interfering RNA (siRNA) against S1P2 or a scrambled control (scr), and 24 hours after transfection, chondrocytes were treated with 10 ng/ml IL-1β and 3 μM S1P for 3 hours. Expression of iNOS mRNA was quantified by real-time RT-PCR. Results are the mean ± SEM arbitrary units, normalized to the values for the housekeeping genes, in 3 cartilage samples. ∗ = P < 0.05; ∗∗ = P < 0.01 versus IL-1β–treated vehicle control.