Involvement of mek/erk1/2 and pi3k/akt pathways in the refractory behavior of gh3b6 pituitary tumor cells to the inhibitory effect of tgfβ1

Abstract

Pituitary tumor cells have a poor response to the growth inhibitory effect of TGF1, possibly resulting from the cross talk of TGF/Smads signal with other signaling pathways, an undescribed mechanism in these tumoral cells. To address this hypothesis, we investigated whether the mitogen-activated extracellular signal-regulated kinase (MEK)/ERK1/2 and phosphoinositide-3 kinase/ protein kinase B (PI3K/Akt) pathways were able to regulate the antimitogenic effect of TGF1 on GH3B6 cells. TGF1 treatment decreased the cell proliferation and induced an activation of mothers against decapentaplegic homolog 2/3 (Smad2/3), effects that were potentiated by MEK and PI3K inhibitors, thus indicating the existence of a cross talk between TGF1/Smad with the MEK/ ERK1/2 or PI3K/Akt pathways. In addition, through immunoprecipitation assays, a direct interaction was observed between Smad2/3-ERK1/2 and Smad2/3-Akt, which decreased when the GH3B6 cells were incubated with TGF1 in the presence ofMEKor PI3K inhibitors, thereby suggesting that the ERK1/2-and Akt-Activated states were involved. These Smad2/3-ERK1/2 and Smad2/3-Akt associations were also confirmed by confocal and transmission electron microscopy. These findings indicate that theTGF1-Antimitogenic effect inGH3B6cellswasattenuated by theMEK/ERK1/2and PI3K/Akt pathways via modulating Smad2/3 phosphorylation. This molecular mechanism could explain in part the refractory behavior of pituitary tumor cells to the inhibitory effect of TGF1..