Transforming growth factor beta-induced expression of chondroitin sulfate proteoglycans is mediated through non-Smad signaling pathways

摘要

The expression of chondroitin sulfate proteoglycans (CSPGs) by reactive astrocytes is a major factor contributing to glial scarring and regenerative failure after spinal cord injury, but the molecular mechanisms underlying CSPG expression remain largely undefined. One contributing factor is transforming growth factor beta (TGF beta), which is upregulated after injury and has been shown to induce expression of CSPGs in vitro. TGF beta typically mediates its effects through the Smad2/3 signaling pathway, and it has been suggested that this pathway is responsible for CSPG expression. However, there is evidence that TGF beta can also activate non-Smad signaling pathways. In this study, we report that TGF beta-induced expression of three different CSPGs - neurocan, brevican, and aggrecan - is mediated through non-Smad signaling pathways. We observed significant increases in TGF beta-induced expression of neurocan, brevican, and aggrecan following siRNA knockdown of Smad2 or Smad4, which indicates that Smad signaling is not required for the expression of these CSPGs. In addition, we show that neurocan, aggrecan, and brevican levels are significantly reduced when TGF beta is administered in the presence of either the PI3K inhibitor LY294002 or the mTOR inhibitor rapamycin, but not the MEK1/2 inhibitor U0126. This suggests that TGF beta mediates this effect through non-Smad-dependent activation of the PI3K-Akt-mTOR signaling pathway, and targeting this pathway may therefore be an effective means of reducing CSPG expression in the injured CNS. (C) 2014 Elsevier Inc. All rights reserved.