Role of non-Smad signaling pathways in transforming growth factor beta (TGFβ)-induced expression of chondroitin sulfate proteoglycans (CSPGs) by reactive astrocytes
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Chondroitin sulphate proteoglycans (CSPGs) from the glial scar inhibit axonal regeneration following spinal cord injury. CSPG expression can be induced by transforming growth factor β (TGFβ), which suggests that inhibition of TGFβ may reduce CSPG levels. Astrocytes were treated with cyclic AMP (cAMP), which reduced TGFβ signaling protein Smad2 in astrocytes. However, cAMP-treated astrocytes showed strong neurocan expression following TGFβ treatment, which suggests that TGFβ may mediate CSPG expression through non-Smad pathways. Smad2 or Smad4 were knocked down in astrocytes using siRNA and TGFβ-induced neurocan, brevican and aggrecan expression were still observed, indicating that Smad signaling is not required for CSPG expression. Administration of a PI3K/Akt inhibitor produced significant reductions in neurocan, brevican and aggrecan expression in astrocytes, which suggests that PI3K/Akt pathway mediates CSPG expression. Erk1/2 inhibitor treatment did not reduce CSPG expression significantly. Targeting non-Smad signaling pathways may therefore be effective strategies to reduce CSPG expression following injury.