Specific protein 1 mediates transforming growth factor beta (TGFβ)-induced modulation of TGFβ receptors. (a) Subconfluent cultures of chondrocytes were treated for 24 hours in the presence or absence of mithramycin (150 nM). TGFβ receptor type I (TβRI), TGFβ receptor type II (TβRII), Smad3 and Smad7 expression was analysed at the mRNA level by real-time RT-PCR. (b) Human articular chondrocytes (HAC) were also nucleofected with specific protein 1 (Sp1) siRNA oligonucleotides or control sequence. Thereafter, the medium was replaced with DMEM + 10% FCS for 24 hours. Total RNA was then extracted and real-time RT-PCR analysis was performed. Histograms represent the relative TβRI, TβRII, Smad3 or Smad7 mRNA levels versus GAPDH. (c) HAC were transfected overnight with pEVR2-Sp1 (or with insertless plasmid as controls). Thereafter, media were replaced with DMEM + 2% FCS for 24 hours in the absence or the presence of transforming growth factor beta 1 (TGFβ1) (5 ng/ml). Therefore, TβRI, TβRII, Smad3 or Smad7 mRNA levels were analysed and expressed as relative expression versus GAPDH. *, P < 0.05, **, P < 0.01, ***, P < 0.001.