Polypyrimidine tract-binding protein promotes insulin secretory granule biogenesis

Abstract

Pancreatic beta-cells store insulin in secretory granules that undergo exocytosis upon glucose stimulation. Sustained stimulation depletes beta-cells of their granule pool, which must be quickly restored. However, the factors promoting rapid granule biogenesis are unknown. Here we show that beta-cell stimulation induces the nucleocytoplasmic translocation of polypyrimidine tract-binding protein (PTB). Activated cytosolic PTB binds and stabilizes mRNAs encoding proteins of secretory granules, thus increasing their translation, whereas knockdown of PTB expression by RNA interference (RNAi) results in the depletion of secretory granules. These findings may provide insight for the understanding and treatment of diabetes, in which insulin secretion is typically impaired.

Mechanisms of action of glucagon-like peptide 1 in the pancreas

“…Both glucose and GLP-1 stimulate nucleocytoplasmic translocation of PTB1 in INS-1 cells (Knoch et al, 2006). Cytosolic PTB1 quickly upregulates the expression of insulin and the secretory vesicle protein ICA512 a receptor tyrosine proteinphosphatase-like protein associated with insulin secretory vesicles (Knoch et al, 2004). Knoch and colleagues also uncovered a PKA-dependent phosphorylation of PTB1 in INS-1 cells.…”

“…Polypyrimidine tract binding protein (PTB) binds to the U-rich polypyrimidine tract of mRNAs encoding insulin and insulin secretory vesicle proteins thereby stabilizing them (Knoch et al, 2004;Knoch et al, 2006;Tillmar et al, 2002). Both glucose and GLP-1 stimulate nucleocytoplasmic translocation of PTB1 in INS-1 cells (Knoch et al, 2006).…”

Knockdown of polypyrimidine tract-binding protein suppresses ovarian tumor cell growth and invasiveness in vitro

“…It is now widely believed that PTB functions as a negative regulator of pre-mRNA splicing, blocking the inclusion of numerous alternative exons into mRNA (Black, 2003), including its own exon . Besides roles in splicing, PTB has also been implicated in the regulation of other aspects of RNA metabolism, such as premRNA polyadenylation (Castelo-Branco et al, 2004), mRNA stability (Kosinski et al, 2003;Knoch et al, 2004), mRNA export from the nucleus (Zang et al, 2001) and mRNA localization in the cytoplasm (Cote et al, 1999). In addition, PTB is involved in the control of cap-independent translation driven by the internal ribosomal entry site (IRES).…”

The importance of RNA binding proteins in preproinsulin mRNA stability

“…Here it was shown that glucose stimulated PTB phosphorylation by PKA is mediated by increased cAMP levels and that this was potentiated by the glucagon-like peptide 1. (Knoch et al, 2004;Knoch et al, 2006). In one of these studies more or less the entire pool of PTB was exported into the cytoplasm in response to stimulation with a high glucose concentration.…”

“…Indeed, PTB was recently shown to bind this sequence thereby mediating increased preproinsulin mRNA stability and levels . Interestingly, PTB has been demonstrated to bind similar pyrimidine-rich sequences located to the 3'-UTR regions of other insulin granule proteins, such as ICA512, PC1/3 and PC2, indicating that the biosynthesis of secretory granule proteins in general is controlled by one unifying mechanism (Knoch et al, 2004). …”