POU Homeodomain Protein Oct-1 Functions as a Sensor for Cyclic AMP

Abstract

Cyclic AMP is a fundamentally important second messenger for numerous peptide hormones and neurotransmitters that control gene expression, cell proliferation, and metabolic homeostasis. Here we show that cAMP works with the POU homeodomain protein Oct-1 to regulate gene expression in pancreatic and intestinal endocrine cells. This ubiquitously expressed transcription factor is known as a stress sensor. We found that it also functions as a repressor of Cdx-2, a proglucagon gene activator. Through a mechanism that involves the activation of exchange protein activated by cyclic AMP, elevation of cAMP leads to enhanced phosphorylation and nuclear exclusion of Oct-1 and reduced interactions between Oct-1 or nuclear co-repressors and the Cdx-2 gene promoter, detected by chromatin immunoprecipitation. In rat primary pancreatic islet cells, cAMP elevation also reduces nuclear Oct-1 content, which causes increased proglucagon and proinsulin mRNA expression. Our study therefore identifies a novel mechanism by which cAMP regulates hormone-gene expression and suggests that ubiquitously expressed Oct-1 may play a role in metabolic homeostasis by functioning as a sensor for cAMP.Many peptide hormones and neurotransmitters use the second messengers, such as cyclic AMP (cAMP), to exert their biological functions, including regulation of gene expression and metabolic homeostasis (1-7). Extensive studies have shown that in addition to the activation of protein kinase A (PKA), 5 cAMP is able to trigger intracellular signaling events via other mechanisms, including Epac (the activation of exchangeprotein directly activated by cAMP) (8 -13). As nonkinase effectors of cAMP, Epac molecules are evidently involved in regulating gene expression, cell adhesion, and pancreatic peptide hormone secretion (4,10,12,14). In pancreatic islet and intestinal endocrine L cells, cAMP elevation is associated with increased expression of proglucagon (gcg) or proinsulin genes (15, 16). Expression of these two hormone-encoding genes is also controlled by transcriptional activators, including certain homeodomain proteins such as the caudal homeodomain protein Cdx-2 (17-22). We have demonstrated previously that in both PKA-active and PKA-deficient pancreatic and intestinal proglucagon-producing endocrine cell lines, cAMP elevation leads to increased Cdx-2 expression (23). Furthermore, Cdx-2 expression in a PKA-deficient pancreatic islet InR1-G9 cell line can be activated by an Epac pathway-specific cAMP analogue 8-pMeOPT-2Ј-O-Me-cAMP (23). More recently, we have observed expression of Epac2 in pancreatic and intestinal proglucagon-producing cells and demonstrated that Epac signaling serves as the mediator of cAMP in regulating the expression of gcg (14).In this study, we further explored mechanistically how cAMP-Epac signaling activates Cdx-2 expression in pancreatic and intestinal endocrine cells. Our observations suggest the existence of a novel mechanism by which cAMP regulates pancreatic and intestinal hormone-gene expression. It is likely that this r...

Oct-1 functions as a sensor for metabolic and stress signals

“…The binding leads to reduced Cdx-2 and proglucagon gene expression. 7 We then examined the effect of cAMP on Oct-1 expression and revealed that Oct-1 is a sensor of cAMP. In contrary to the effect of H 2 O 2 , cAMP elevation reduced Oct-1 nuclear content.…”

Octamer-binding transcription factors: genomics and functions

“…In pancreatic islet cells, Oct1 senses intracellular cAMP levels (132). Elevation of cAMP levels enhances Oct1 phosphorylation and shuttles Oct1 from the nucleus to the cytoplasm.…”

Oct1 Is a Switchable, Bipotential Stabilizer of Repressed and Inducible Transcriptional States

“…These data suggest that MAPK signals impinge on Oct1 to mediate switching from repression to antirepression through a mechanism requiring ERK1/2. Repression by PMA-Cdx2 is a homeodomain transcription factor critical for mammalian development (43) and a known Oct1 regulatory target (44,45). Cdx2 expression is mostly confined to the gastrointestinal tract in adults, where it acts as a tumor suppressor (46), but inappropriate expression of Cdx2 is observed in murine leukemia models and in most cases of pediatric acute myeloid leukemia and adult lymphoid leukemia (47)(48)(49)(50), where it promotes self-renewal (49).…”