PT - JOURNAL ARTICLE AU - Chuan-Chang Chuang AU - Sai-Koong Tan AU - Lung-Kuo Tai AU - Jin-Ping Hsin AU - Fung-Fang Wang TI - Evidence for the Involvement of Protein Kinase C in the Inhibition of Prolactin Gene Expression by Transforming Growth Factor<strong>-</strong>β<sub>2</sub> DP - 1998 Jun 01 TA - Molecular Pharmacology PG - 1054--1061 VI - 53 IP - 6 4099 - http://molpharm.aspetjournals.org/content/53/6/1054.short 4100 - http://molpharm.aspetjournals.org/content/53/6/1054.full SO - Mol Pharmacol1998 Jun 01; 53 AB - We investigated the mechanisms by which transforming growth factor (TGF)-β2 inhibited prolactin mRNA expression in GH3 rat pituitary tumor cells. Maximal inhibition was observed with cells exposed to 5 ng/ml TGF-β2 for 24 hr. Continuous presence of the hormone during the entire period was not necessary because exposure of cells to TGF-β2 for 20 min was sufficient to trigger the same extent of prolactin mRNA inhibition at 24 hr as with its persistent presence. The action of TGF-β2 could be abolished by cycloheximide or EGTA, suggesting the requirement of a newly synthesized protein and extracellular Ca2+. The response of prolactin mRNA to TGF-β2 was inhibited by preincubation of cells with phorbol-12-myristate-13-acetate, which down-regulated protein kinase C (PKC). The activities of both the cytosolic and membrane PKC were significantly reduced at 20 min after TGF-β2 addition, and inhibition continued to 24 hr, the last time point analyzed. However, the ratio of cytosolic to membrane PKC was not altered by TGF-β2. Inhibition of PKC did not require the sustained presence of TGF-β2. In vitro kinase assays of the immunoprecipitated PKC demonstrated that the activities of α, ε, μ, and ζ isozymes were significantly decreased in the TGF-β2-treated cells, whereas that of PKCλ was not affected. Western blotting did not reveal any change in PKCε steady state protein levels, suggesting TGF-β2 inhibits PKC activity through a post-translational mechanism. Our results support that inhibition of PKC activity is an early event mediating TGF-β2-inhibited prolactin mRNA expression in GH3 cells. The American Society for Pharmacology and Experimental Therapeutics