Cystic fibrosis related diabetes (CFRD) is a unique form of diabetes which develops in the majority of patients with non-mild cystic fibrosis (CF) by middle age. Even when the degree of hyperglycemia is minimal, CFRD worsens lung disease and shortens life expectancy. The causes of CFRD are multifactorial and include insufficient insulin secretion, hepatic insulin resistance, and exocrine pancreatic insufficiency. While exocrine insufficiency is explained by deficient pancreatic ductal anion transport due to loss of cystic fibrosis transmembrane conductance regulator (CFTR) function, the causes of diminished insulin secretion and hepatic insulin resistance are not fully understood. CF induces significant primary disease not only in the exocrine pancreas, but also in stomach and intestine owing to loss of CFTR function in the gut epithelium. Relatedly, a variety of gut / splanchnic hormones are disrupted in CF, and these alterations have been postulated to contribute to CFRD. The most consistently altered splanchnic hormone in CF is pancreatic polypeptide (PP), a hormone whose metabolic actions have been neglected following the finding that PP-knockout (PP-KO) mice lack a glucose metabolism phenotype. PP levels are universally low in CF and unresponsive to physiological stimuli that normally induce secretion. We find circulating PP deficiency is present even in very young children with CF and in CFTR-KO ferrets despite abundant PP-cells in the pancreas. Using these CF ferrets and newly generated PP-KO ferrets, we find that supplementation and deficiency of PP have broad effects on glucose metabolism, representing the first molecular genetic data confirming classic physiological studies indicating metabolic effects of PP. We hypothesize that PP deficiency in CF represents an underlying parasympathetic neuronal defect in islet function and contributes to insulin resistance in the liver of individuals with CF. To determine the mechanism causing deficient PP secretion in CF, we will test the functional, structural, and neurochemical integrity of parasympathetic neuronal innervation to the PP-secreting cells of the pancreas. Using traced euglycemic hyperinsulinemic clamps, we will determine the mechanism by which PP supplementation, using peptide and genetic approaches, improves glucose tolerance in PP-KO and CF ferrets and determine the PP-receptor involved in this action. We will examine whether PP supplementation, through its pancreatostatic actions, lessens exocrine pancreatic destruction in CF. Finally, through a newly created CFTR-G551D mutant knock-in ferret, where the function of the CFTR mutant can be restored by a small molecule potentiator, we will investigate the connections between early exocrine pancreatic disease in CF and subsequent endocrine pancreas dysfunction including poor PP secretion. Our studies will not only increase understanding of the glucoregulatory actions of PP, but are expected to unlock new understanding of CFRD pathogenesis. The knowledge gained will help open new diagnostic and therapeutic approaches to CFRD and also to other hyperglycemic low-PP states such as diabetes complicating chronic pancreatitis.
For unclear reasons most people who have cystic fibrosis will develop diabetes by middle age, placing them at much higher risk of lung failure and death. Our data suggest that low levels of a hormone called pancreatic polypeptide (PP) raises diabetes risk in patients with cystic fibrosis. The proposed study aims to understand why PP is low in cystic fibrosis and to apply this knowledge to develop new therapies for diabetes in cystic fibrosis patients.
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