Expression and regulation of the rat cholecystokinin gene

Abstract

Cholecystokinin (CCK) is a polypeptide hormone which is produced in both the upper intestine and the central nervous system. Upon release from the intestinal mucosa, CCK stimulates pancreatic enzyme secretion and gallbladder emptying. Although CCK is present in unusually high concentrations in several brain regions, its function remains unclear. The expression and regulation of the rat CCK gene was studied by linking variable amounts of the 5\sp\prime-flanking region of the rat gene to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene. Transfection assays indicate that sequences within 102 base pairs of the cap site are required for the expression from this promoter. This region contains a sequence that is homologous with both the c-fos and polyoma enhancers and the cAMP- and TPA-responsive elements described for several genes. In addition, the $-$119 to $-$81 fragment of the CCK promoter acts as a transcriptional enhancer in front of a heterologous promoter. Sequences located within 155 bp of the transcription initiation site were found to be responsive to the phorbol ester TPA and to elevated levels of cAMP induced by forskolin. These data indicate that the region homologous to known cAMP- and TPA-responsive elements might be responsible for the regulation of expression by these agents. However, additional studies will be required to identify and characterize the element(s) responsible for the transcriptional regulation by these substances. The element that confers tissue specificity on the rat gene was found to reside within a 2 kb fragment more than 800 bp upstream of the transcription initiation site. These studies utilized a stable cell line that was established from a rat medullary thyroid carcinoma known to produce high levels of immunoreactive CCK. Additional experiments will be necessary to define the boundaries of the tissue-specific element