Coregulation of glucagon-like peptide-1 synthesis with proglucagon and prohormone convertase 1 gene expression in enteroendocrine GLUTag cells

The insulinotropic hormone glucagon-like peptide-1 (GLP-1) is synthesized i n the intestinal L cell by prohormone convertase 1 (PC1)-mediated posttrans lational processing of proglucagon. Previous studies have demonstrated that proglucagon gene transcription in the L cell is stimulated by the protein kinase A (PKA) pathway through a cAMP response element (CRE). Because the P C1 gene contains two functional CREs, the present studies were conducted to investigate whether the PC1 and proglucagon genes are coregulated by PKA, and to elucidate the temporal relationship(s) of PC1 and proglucagon gene e xpression with production of GLP-1, in the intestinal cell. The GLUTag ente roendocrine cell line, which is known to express the proglucagon gene and t o synthesize and secrete GLP-1, was used as a model. Proglucagon and PC1 me ssenger RNA transcript levels were both increased after 12 h (but not 24 h) of treatment of GLUTag cells with forskolin/isobutylmethylxanthine (IBMX), by 2.7 +/- 0.3- and 2.4 +/- 0.8-fold, respectively, compared with controls (P < 0.01-0.001). Activation of PKA resulted in a 2.1 +/- 0.1-fold increas e in PC1 reporter construct expression (P < 0.001) at 12 h, which was depen dent on the presence of the CRE, and a 13- to 24-fold increment in PC1 prot ein levels (P < 0.01) at 12 and 24 h. Similarly, forskolin/IBMX increased s ecretion of GLP-1, by 1.8 +/- 0.2- and 2.2 +/- 0.6-fold at 12 and 24 h, res pectively (P < 0.05-0.01). Although the cell content of GLP-1 was diminishe d after 12 h of treatment (P < 0.001), GLP-1 levels increased back to contr ol values after 24 h of forskolin/IBMX treatment (P < 0.01 vs. 12-h levels) . Thus, PICA-induced secretion of GLP-1 from the L cell is followed by rest oration of the cellular peptide levels through a PKA-mediated, CRE-dependen t up-regulation of proglucagon and PC1 gene expression.