Exocrine pancreas glutamate secretion help to sustain enterocyte nutritional needs under protein restriction.

Glutamine (Gln) is the most concentrated amino acid in blood and considered conditionally essential. Its requirement is increased during physiological stress, such as malnutrition or illness, despite its production by muscle and other organs. In the malnourished state, Gln has been suggested to have a trophic effect on the exocrine pancreas and small intestine. However, the Gln transport capacity, the functional relationship of these two organs, and the potential role of the Gln-glutamate (Glu) cycle are unknown. We observed that pancreatic acinar cells express lower levels of Glu than Gln transporters. Consistent with this expression pattern, the rate of Glu influx into acinar cells was approximately sixfold lower than that of Gln. During protein restriction, acinar cell glutaminase expression was increased and Gln accumulation was maintained. Moreover, Glu secretion by acinar cells into pancreatic juice and thus into the lumen of the small intestine was maintained. In the intestinal lumen, Glu absorption was preserved and Glu dehydrogenase expression was augmented, potentially providing the substrates for increasing energy production via the TCA cycle. Our findings suggest that one mechanism by which Gln exerts a positive effect on exocrine pancreas and small intestine involves the Gln metabolism in acinar cells and the secretion of Glu into the small intestine lumen. The exocrine pancreas acinar cells not only avidly accumulate Gln but metabolize Gln to generate energy and to synthesize Glu for secretion in the pancreatic juice. Secreted Glu is suggested to play an important role during malnourishment in sustaining small intestinal homeostasis. NEW & NOTEWORTHY Glutamine (Gln) has been suggested to have a trophic effect on exocrine pancreas and small intestine in malnourished states, but the mechanism is unknown. In this study, we suggest that this trophic effect derives from an interorgan relationship between exocrine pancreas and small intestine for Gln-glutamate (Glu) utilization involving the uptake and metabolism of Gln in acinar cells and secretion of Glu into the lumen of the small intestine.

A heterologous hormone response element enhances expression of rat beta-casein promoter-driven chloramphenicol acetyltransferase fusion genes in the mammary gland of transgenic mice.

Previous studies have demonstrated that the entire rat beta-casein (R beta C) gene and a -524/+490 R beta C fragment-chloramphenicol acetyltransferase (CAT) fusion gene are expressed preferentially in the mammary gland of transgenic mice in a developmentally regulated fashion. However, transgene expression was infrequent, less than 1% of that observed for the endogenous gene, and varied as much as 500-fold, presumably due to the site of chromosomal integration. To determine whether a heterologous hormone-responsive enhancer could be used to increase both the level and frequency of expression in the mammary gland, a fragment derived from the mouse mammary tumor virus long terminal repeat containing four hormone response elements (HREs) was inserted into the R beta C promoter at a site not known to contain transcriptional regulatory elements. Transgenic mice generated which carried HRE-enhanced R beta C-CAT fusion genes expressed CAT activity in the mammary glands of all founder lines examined at levels that were on average 13-fold greater than for lines generated with similar constructs not carrying HREs. In the highest expressing line, the level of HRE-enhanced transgene expression was found to be developmentally regulated, increasing 14-fold in the mammary gland from virgin to day 10 of lactation. In this line, expression was also observed in the thymus and spleen; however, the level of CAT activity was 4-fold lower than in the mammary gland and was not developmentally regulated. In adrenalectomized mice, the administration of dexamethasone stimulated CAT expression in the mammary gland but not in the thymus and spleen. These studies demonstrate that in the context of the R beta C promoter, the HRE functions in the mammary gland to increase both the frequency and level of transgene expression.