Characterization of the Mouse Islet-Specific Glucose-6-Phosphatase Catalytic Subunit–Related Protein Gene Promoter by In Situ Footprinting: Correlation With Fusion Gene Expression in the Islet-Derived βTC-3 and Hamster Insulinoma Tumor Cell Lines

Glucose-6-phosphatase (G6Pase) is a multicomponent system located in the endoplasmic reticulum comprising a catalytic subunit and transporters for glucose-6-phosphate, inorganic phosphate, and glucose. We have recently cloned a novel gene that encodes an islet-specific G6Pase catalytic subunit–related protein (IGRP) (Ebert et al., Diabetes 48:543–551, 1999). To begin to investigate the molecular basis for the islet-specific expression of the IGRP gene, a series of truncated IGRP–chloramphenicol acetyltransferase (CAT) fusion genes were transiently transfected into the islet-derived mouse βTC-3 and hamster insulinoma tumor cell lines. In both cell lines, basal fusion gene expression decreased upon progressive deletion of the IGRP promoter sequence between −306 and −66, indicating that multiple promoter regions are required for maximal IGRP-CAT expression. The ligation-mediated polymerase chain reaction footprinting technique was then used to compare trans -acting factor binding to the IGRP promoter in situ in βTC-3 cells, which express the endogenous IGRP gene, and adrenocortical Y1 cells, which do not. Multiple trans -acting factor binding sites were selectively identified in βTC-3 cells that correlate with regions of the IGRP promoter identified as being required for basal IGRP-CAT fusion gene expression. The data suggest that hepatocyte nuclear factor 3 may be important for basal IGRP gene expression, as it is for glucagon, GLUT2, and Pdx-1 gene expression. In addition, binding sites for several trans -acting factors not previously associated with islet gene expression, as well as binding sites for potentially novel proteins, were identified.