Inhibition of polyamine synthesis by alpha-difluoromethylornithine and its effects on pancreatic secretion and growth in the rat

Animal disease models generated by genetic engineering of polyamine metabolism

The polyamines putrescine, spermidine and spermine are natural components of all living cells. Although their exact cellular functions are still largely unknown, a constant supply of these compounds is required for mammalian cell proliferation to occur. Studies with animals displaying genetically altered polyamine metabolism have shown that polyamines are intimately involved in the development of diverse tumors, putrescine apparently has specific role in skin physiology and neuroprotection and the higher polyamines spermidine and spermine are required for the maintenance of pancreatic integrity and liver regeneration. In the absence of ongoing polyamine biosynthesis, murine embryogenesis does not proceed beyond the blastocyst stage. The last years have also witnessed the appearance of the first reports linking genetically altered polyamine metabolism to human diseases.

Genetic approaches to the cellular functions of polyamines in mammals

The polyamines putrescine, spermidine and spermine are organic cations shown to participate in a bewildering number of cellular reactions, yet their exact functions in intermediary metabolism and specific interactions with cellular components remain largely elusive. Pharmacological interventions have demonstrated convincingly that a steady supply of these compounds is a prerequisite for cell proliferation to occur. The last decade has witnessed the appearance of a substantial number of studies, in which genetic engineering of polyamine metabolism in transgenic rodents has been employed to unravel their cellular functions. Transgenic activation of polyamine biosynthesis through an overexpression of their biosynthetic enzymes has assigned specific roles for these compounds in spermatogenesis, skin physiology, promotion of tumorigenesis and organ hypertrophy as well as neuronal protection. Transgenic activation of polyamine catabolism not only profoundly disturbs polyamine homeostasis in most tissues, but also creates a complex phenotype affecting skin, female fertility, fat depots, pancreatic integrity and regenerative growth. Transgenic expression of ornithine decarboxylase antizyme has suggested that this unique protein may act as a general tumor suppressor. Homozygous deficiency of the key biosynthetic enzymes of the polyamines, ornithine and S-adenosylmethionine decarboxylase, as achieved through targeted disruption of their genes, is not compatible with murine embryogenesis. Finally, the first reports of human diseases apparently caused by mutations or rearrangements of the genes involved in polyamine metabolism have appeared.

Stimulation of pancreatic growth. Distal small bowel resection mediated by increased levels of cholecystokinin

SUMMARY BACKGROUND DATA

Distal, but not proximal, resection of the small bowel induces growth of rat pancreas, but the mechanism of this phenomenon is poorly clarified. The release of cholecystokinin (CCK), a trophic hormone for the pancreas, is regulated by a negative-feedback control of bile salts. The ileum is a major site for reabsorption of bile salts. Thus, unsuppressed release of CCK due to deleted reabsorption of bile salts after distal small bowel resection may be a cause of pancreatic growth. In this study, the authors have examined whether pancreatic growth after distal small bowel resection was mediated by endogenous CCK and have determined whether the mechanism of this pancreatic growth required biosynthesis of polyamine.

METHODS

Male Fischer 344 rats underwent 70% distal small bowel resection or transection of the ileum. Beginning 48 hours after surgery, CR1409 (a CCK-receptor antagonist) or saline was injected subcutaneously every 8 hours. All animals were pair-fed and killed 14 days after surgery. The pancreas from each rat was excised, weighed, and assayed for DNA, RNA, protein, and polyamine content.

RESULTS

Distal small bowel resection increased pancreatic weight, DNA, RNA, and protein, as well as polyamine levels; all of these increases were significantly suppressed by CR1409. Postprandial release of CCK into the circulation was significantly increased after distal small bowel resection.

CONCLUSIONS

Pancreatic growth after distal small bowel resection was associated with the stimulation of polyamine biosynthesis; growth appeared to be mediated by endogenous CCK.