Insulin treatment increases the transcription of the endogenous prolactin gene and the expression of chimeric plasmids consisting of 5'-flanking DNA from the prolactin gene ligated to the bacterial chloramphenicol acetyl transferase (CAT) gene in GH cells. Thus, this cell culture system accurately reflects the physiological regulation of prolactin by insulin. Defects in prolactin expression due to diabetes have been suggested to account for several disorders including infant respiratory distress syndrome and impotence. The long term goal of this research is to discover all of the components of this regulation and how they interact to cause increased prolactin gene expression. Results to date have identified a consensus insulin response element (IRE). This IRE is the Ets-motif-related sequence CGGAA and it mediates 100 percent of the >10-fold increase in prolactin-CAT expression caused by insulin. The deletion of identical elements in the thymidine kinase and somatostatin promoters renders them insensitive to insulin and establishes that CGGAA is consensus IRE. The transduction of the insulin signal requires insulin receptor with intact kinase function and is dependent on the tyrosine autophosphorylation of the insulin receptor. Experiments to continue these highly successful investigations include: 1) Confirm that GABP mediates 100 percent of the insulin response with knockouts. 2) Evidence that the MAP kinase phosphorylation of GABP alpha may mediate the response to insulin will be further explored. 3) Domains of GABP alpha and GABP beta mutations required to mediate the response to insulin will be determined. GABP alpha and GABP beta mutations will be used to determine if these mutants mediate insulin effects on prolactin gene expression. Chimeric proteins of GABP alpha- Gal4 will be used with an UAS-CAT reporter to investigate functional domains of GABP alpha. GABP-null cells will also be made by homologous recombination and mutants will be tested in a GABP-null environment. 4) Several approaches are described for identifying and cloning the other factors with which GABP might interact to regulate prolactin gene expression.
