  Transforming growth factor beta (TGF$\beta$) plays a central role in
morphogenesis, growth, and cell differentiation. This cytokine is particularly
important in cartilage where it regulates cell proliferation and extracellular
matrix synthesis. While the action of TGF$\beta$ on chondrocyte metabolism has
been extensively catalogued, the modulation of specific genes that function as
mediators of TGF$\beta$ signalling is poorly defined. In the current study,
elements of the Smad component of the TGF$\beta$ intracellular signalling
system and TGF$\beta$ receptors were characterised in human chondrocytes upon
TGF$\beta$1 treatment. Human articular chondrocytes were incubated with
TGF$\beta$1. Then, mRNA and protein levels of TGF$\beta$ receptors and Smads
were analysed by RT-PCR and western blot analysis. The role of specific protein
1 (Sp1) was investigated by gain and loss of function (inhibitor, siRNA,
expression vector). We showed that TGF$\beta$1 regulates mRNA levels of its own
receptors, and of Smad3 and Smad7. It modulates TGF$\beta$ receptors
post-transcriptionally by affecting their mRNA stability, but does not change
the Smad-3 and Smad-7 mRNA half-life span, suggesting a potential
transcriptional effect on these genes. Moreover, the transcriptional factor
Sp1, which is downregulated by TGF$\beta$1, is involved in the repression of
both TGF$\beta$ receptors but not in the modulation of Smad3 and Smad7.
Interestingly, Sp1 ectopic expression permitted also to maintain a similar
expression pattern to early response to TGF$\beta$ at 24 hours of treatment. It
restored the induction of Sox9 and COL2A1 and blocked the late response
(repression of aggrecan, induction of COL1A1 and COL10A1). These data help to
better understand the negative feedback loop in the TGF$\beta$ signalling
system, and enlighten an interesting role of Sp1 to regulate TGF$\beta$
response.
