Xenopus laevis primary hepatocytes in culture are induced by glucocorticoid hormones to synthesize and secrete fibrinogen. The increase in production of the protein is preceded by a 10- to 30-fold elevation of the mRNAs coding for the three fibrinogen subunits, Aa, Bd, and 7. To analyze the mechanisms underlying this coordinate control of independent genes in a common regulatory network, we show here that the steroid hormone induced simultaneous activation of transcription of the three fibrinogen subunit genes. Using an optimized transcription run-on assay for nuclei from Xenopus primary liver cells, we demonstrate that glucocorticoids rapidly stimulated transcription of the Aafibrinogen subunit gene by 15- to 20-fold, the B/J gene by 5- to 10-fold, and the 7 gene by 5- to 15-fold. The three genes exhibited a highly concerted response to the hormone, in which maximal stimulation occurred by 30 min and was maintained for at least 16 h. Blocking new protein synthesis before hormone treatment reduced total transcription by 45% and partially inhibited specific hormonal induction of all three fibrinogen subunit genes. The effect of glucocorticoids on fibrinogen transcription, therefore, was dependent in part on ongoing protein synthesis, suggesting that hormonal stimulation uses already synthesized stable factors, but also requires labile or newly synthesized factors for the full effect.
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