B. Overactivation of TGFβ receptor in MSCs by high environmental TGF-β1 down-regulates miR-21 and thus no longer inhibits Smad7 translation and leads to close the TGF-β1 signal circuit. Such adaptive response is disturbed by deliberately changing the expression of components in the signaling pathway, such as (1) silencing TGF-β1 expression in MSCs by TGF-β1 siRNA, (2) blocking TGFβ receptor activity by chemical inhibitor SB431542, (3) silencing Smad7 expression by Smad7 siRNA, (4) over-expression of miR-21 by miR-21 mimic, and (5) knock-down miR-21 by miR-21 inhibitor. 