Thapsigargin-induced Ca²⁺ increase inhibits TGFβ1-mediated Smad2 transcriptional responses via Ca²⁺/calmodulin-dependent protein kinase II

Transforming growth factor β (TGFβ) signalling plays important roles in a variety of tissues and cell types. Impaired TGFβ signalling contributes to several pathologies, including cancer, fibrosis as well as neurodegenerative diseases. TGFβ receptor type I-mediated phosphorylation of Smad2, the formation of the Smad2-Smad4 complex and translocation to the nucleus are critical steps of the TGFβ signalling pathway. Here, we demonstrate that thapsigargin-mediated increase of intracellular Ca²⁺ concentrations inhibited TGFβ1-induced Smad2 transcriptional activity in the oligodendroglial cell line OLI-neu. We provide evidence that thapsigargin treatment dramatically reduced the nuclear translocation of Smad2 after TGFβ1 treatment but had no effect on its phosphorylation at Ser465/467. Moreover, using Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitors and a constitutively active CaMKII mutant, we provide evidence that the observed inhibition of TGFβ signalling in OLI-neu cells was strongly dependent on Ca²⁺-mediated CaMKII activation. In summary, this study clearly shows that the TGFβ1-induced Smad2 nuclear translocation is negatively regulated by intracellular Ca²⁺ in OLI-neu cells and that increased intracellular Ca²⁺ concentrations block Smad2-mediated transcription of TGFβ target genes. These results underline the importance of intracellular Ca²⁺ for the regulation of TGFβ signalling.