CFTR Influences Beta Cell Function and Insulin Secretion Through Non-Cell Autonomous Exocrine-Derived Factors

Abstract

Although β-cell dysfunction in cystic fibrosis (CF) leads to diabetes, the mechanism by which the cystic fibrosis transmembrane conductance regulator (CFTR) channel influences islet insulin secretion remains debated. We investigated the CFTR-dependent islet-autonomous mechanisms affecting insulin secretion by using islets isolated from CFTR knockout ferrets. Total insulin content was lower in CF as compared with wild-type (WT) islets. Furthermore, glucose-stimulated insulin secretion (GSIS) was impaired in perifused neonatal CF islets, with reduced first, second, and amplifying phase secretion. Interestingly, CF islets compensated for reduced insulin content under static low-glucose conditions by secreting a larger fraction of islet insulin than WT islets, probably because of elevated SLC2A1 transcripts, increased basal inhibition of adenosine triphosphate-sensitive potassium channels (K-ATP), and elevated basal intracellular Ca2+. Interleukin (IL)-6 secretion by CF islets was higher relative to WT, and IL-6 treatment of WT ferret islets produced a CF-like phenotype with reduced islet insulin content and elevated percentage insulin secretion in low glucose. CF islets exhibited altered expression of INS, CELA3B, and several β-cell maturation and proliferation genes. Pharmacologic inhibition of CFTR reduced GSIS by WT ferret and human islets but similarly reduced insulin secretion and intracellular Ca2+ in CFTR knockout ferret islets, indicating that the mechanism of action is not through CFTR. Single-molecule fluorescent in situ hybridization, on isolated ferret and human islets and ferret pancreas, demonstrated that CFTR RNA colocalized within KRT7+ ductal cells but not endocrine cells. These results suggest that CFTR affects β-cell function via a paracrine mechanism involving proinflammatory factors secreted from islet-associated exocrine-derived cell types.

Cystic fibrosis–related diabetes is caused by islet loss and inflammation

“…Our murine models of β cell-specific CFTR deletion allowed us to determine whether the CFTR contributes directly to β cell function without metabolic and pancreatic abnormalities caused by whole-animal CFTR loss. Integrating our data with other reports (20,25,35) indicates that nutrient intolerance and β cell dysfunction in CF models were not caused directly by β cell CFTR loss but by other effects (i.e., insulin resistance, β cell loss, pancreas/islet inflammation) caused by whole-animal CFTR deletion.…”

“…Furthermore, the reported labeling of human pancreatic tissue (15) was not accompanied by clear α and β cell staining, which would be expected given the reported single cell labeling (11,15) and, importantly, did not show the distinct ductal localization of CFTR. Furthermore, our experiments demonstrating the lack of human islet CFTR expression are supported by electrophysiologic and in vitro insulin secretion experiments, the Human Protein Atlas (43), 3 islet endocrine cell transcriptomes (38)(39)(40), and single-molecule fluorescent in situ hybridization (35).…”

“…54), impaired glucose and mixed meal intolerance and impaired islet function have been consistently shown (13,20,35). However, reduced β cell mass (13,20,25), fluctuating insulin resistance (25), pancreatic inflammation (20), and an unidentified exocrine-derived paracrine factor (35) have been reported and linked to nutrient intolerance and β cell dysfunction in these models. In the single report that concluded that β cell CFTR loss in CF mice caused β cell dysfunction and impaired glucose tolerance, β cell mass and insulin sensitivity were not examined (12).…”

“…In murine islets, one study reported complete loss of in vitro GSIS (12), while another reported maintenance of in vitro GSIS with a reduction in pGSIS (11). In ferret islets, CFTR(inh)-172 caused β cell dysfunction in both wild-type and CFTR-knockout ferret islets, indicating nonspecific effects of this CFTR inhibitor (35). One study of β cell CFTR function has used human islets, was dependent on the use of CFTR inhibitors, and found only impaired pGSIS with no effect on basal or GSIS (11).…”

“…islets is limited (11,15,35), we examined CFTR mRNA and protein expression in human islet endocrine cells, whether CFTR regulates human islet insulin secretion, and the hormone secretory profile and transcriptome of human CF islets. Our studies indicate that the CFTR does not intrinsically regulate α or β cell function and that the etiology of CFRD is largely dependent on islet loss and intraislet inflammation in the setting of a complex and progressive multiorgan disease.…”