CELL-SPECIFIC EXPRESSION AND REGULATION OF A GLUCOKINASE GENE LOCUS TRANSGENE

Transgenic mice containing one or more extra copies of the entire gluc okinase (GK) gene locus were generated and characterized. The GK trans gene, an 83-kilobase pair mouse genomic DNA fragment containing both p romoter regions, was expressed and regulated in a cell-specific manner , and rescued GK null lethality when crossed into mice bearing a targe ted mutation of the endogenous GK gene. Livers from the transgenic mic e had elevated GK mRNA, protein, and activity levels, compared with co ntrols, and the transgene was regulated in liver by dietary manipulati ons. The amount of GK immunoreactivity in hepatocyte nuclei, where GK binds to the GK regulatory protein, was also increased. Pancreatic isl ets displayed increased GK immunoreactivity and NAD(P)H responses to g lucose, but only when isolated and cultured in 20 mM glucose, as a res ult of the hypoglycemic phenotype of these mice (Niswender, K. D., Shi ota, M., Postic, C., Cherrington, A. D., and Magnuson, M. A. (1997) J. Biol. Chem. 272, 22604-22609). Together, these results indicate that the region of the gene from -55 to +28 kilobase pairs (relative to the liver GK transcription start site) contains all the regulatory sequen ces necessary for expression of both GK isoforms, thereby placing an u pper limit on the size of the GK gene locus.