Glucagon gene transcription in an islet cell line is regulated via a protein kinase C-activated pathway

Proglucagon is a polyprotein precursor containing not only glucagon and glicentin, but glucagon-like peptides-I and -II and an intervening peptide (IP-II). The glucagon gene is expressed in both pancreatic islets and neuroendocrine L-cells of the gastrointestinal tract. We have recently cloned an islet cell line from a rat pancreatic islet cell tumour that simultaneously expresses the glucagon, insulin, somatostatin, and angiotensinogen genes. We investigated the potential role of "second messenger" pathways in the regulation of glucagon gene expression. Both the tumor promoter agent phorbol myristate acetate (PMA) and a diacylglycerol analog, 1,2-dioctanoylglycerol, induced a 2.7- and 2.5-fold increase in steady-state glucagon mRNA levels at 24 h, respectively. The increase was progressive up to 24 h and was specific for glucagon mRNA; the insulin and somatostatin mRNA levels remained unchanged. An inactive phorbol ester, 4 beta-phorbol 12,13,20-triacetate, was without effect. The glucagon mRNA increase induced by PMA was mediated through an increase in glucagon gene transcription reaching maximal stimulation at 30-60 min. Glucagon mRNA half-life was similar in both control and PMA-treated cells, approximating 12 h. The stimulation of glucagon gene transcription was accompanied by a corresponding 3-fold increase in proglucagon biosynthesis. Neither dibutyryl cAMP nor glucocorticoids affected glucagon mRNA levels, while inducing a 5-fold increase in somatostatin mRNA levels and 4.8-fold stimulation in angiotensinogen mRNA at 24 h, respectively. We conclude that expression of the glucagon gene in this islet cell line is regulated at the level of transcription through a protein kinase C (Ca2+/phospholipid-dependent enzyme)-activated pathway.