Elevated transforming growth factor β signaling activation in β-actin-knockout mouse embryonic fibroblasts enhances myofibroblast features.

Signaling by the transforming growth factor-β (TGF-β) is an essential pathway regulating a variety of cellular events. TGF-β is produced as a latent protein complex and is required to be activated before activating the receptor. The mechanical force at the cell surface is believed to be a mechanism for latent TGF-β activation. Using β-actin null mouse embryonic fibroblasts as a model, in which actin cytoskeleton and cell-surface biophysical features are dramatically altered, we reveal increased TGF-β1 activation and the upregulation of TGF-β target genes. In β-actin null cells, we show evidence that the enhanced TGF-β signaling relies on the active utilization of latent TGF-β1 in the cell culture medium. TGF-β signaling activation contributes to the elevated reactive oxygen species production, which is likely mediated by the upregulation of Nox4. The previously observed myofibroblast phenotype of β-actin null cells is inhibited by TGF-β signaling inhibition, while the expression of actin cytoskeleton genes and angiogenic phenotype are not affected. Together, our study shows a scenario that the alteration of the actin cytoskeleton and the consequent changes in cellular biophysical features lead to changes in cell signaling process such as TGF-β activation, which in turn contributes to the enhanced myofibroblast phenotype.