Radiation-induced alterations in rat mesangial cell Tgfb1 and Tgfb3 gene expression are not associated with altered secretion of active Tgfb isoforms.

Despite evidence of selective radiation-induced modulation of expression of rat mesangial cell Tgfb gene isoforms, it is unclear whether these changes in gene expression are accompanied by changes in protein secretion. To address this issue, primary cultures of rat mesangial cells (passage number 6- 11) were placed in serum-free medium 24 h prior to irradiation with single doses of 0.5-20 Gy of (137)Cs gamma rays. After irradiation, cells were maintained in serum-free medium for a further 24 h. Irradiation of quiescent mesangial cells resulted in a significant (P </= 0.05) dose-independent increase in steady-state levels of Tgfb1 mRNA 24 h postirradiation. In contrast, steady-state levels of Tgfb3 mRNA exhibited a dose-dependent reduction after irradiation; this reduction was statistically significant after doses of 5 and 10 Gy compared to control cells (P </= 0.05). These radiation-induced changes in Tgfb gene expression were associated with modest increases in Tgfb protein as determined using mink lung epithelial cells transfected with the Pai1 promoter-luciferase construct. Twenty-four hours after a single dose of 5 Gy, the total Tgfb protein secreted by the mesangial cells was 181 +/- 2.0% of that secreted by unirradiated control cells (P </= 0.01). However, this increase was seen in terms of latent Tgfb protein; radiation failed to increase significantly the amount of active Tgfb protein secreted by mesangial cells. Both quiescent and irradiated rat mesangial cells secreted active Tgfb as primarily the Tgfb3 isoform. These data reinforce the need to interpret changes in Tgfb gene expression with caution.