Cysteamine-induced reduction in tissue somatostatin immunoreactivity is associated with alterations in somatostatin mRNA.

The drug cysteamine (CHS) induces a profound loss of somatostatin-14 (SS-14) biological and immunological (SS-14 LI) activity from somatostatin cells in vivo and in vitro. The present study was designed to determine (i) whether CHS induced loss of somatostatin is accompanied by secondary increases in SS-mRNA perhaps through loss of autoinhibition of somatostatin cells; (ii) whether CHS exerts additional direct effects on SS gene regulation. CHS was administered to rats in vivo or applied in vitro to primary cultures of rat islet cells, rat islet somatostatin-producing tumor cells (1027 B2), and endogenous or in vitro synthesized SS-mRNA. In vivo administration of CHS led to 80% reduction in tissue SSLI by 4 h. These changes were accompanied by significant alterations in SS-mRNA that were both tissue-specific and time-dependent. The pattern in brain and intestine was typified by a significant 60% increase in SS-mRNA at 2 h followed by a gradual reduction to approximately 55% of control at 8 h. Stomach showed a significant 95% increase in SS-mRNA at 4 h followed by a 37% decrease by 8 h. Pancreatic SS-mRNA displayed a sustained 25-65% reduction for 8 h. Pretreatment of islet cell cultures with CHS reproduced the in vivo findings with pancreas viz. decreased SSLI (80-90% of control) accompanied by a parallel reduction in SS-mRNA (40-50% of control) sustained from 2-72 h. CHS also induced a reduction in immunoreactive insulin and insulin mRNA in cultured islet cells. As with normal islet cells, CHS treatment of 1027 B2 islet tumor cells led to a profound and sustained decrease in SSLI and SS-mRNA. These changes occurred in the absence of any alteration in intracellular cAMP levels. CHS was without effect when incubated directly with SS-mRNA isolated from 1027 B2 cells or with in vitro synthesized SS-mRNA. We conclude that in addition to its effect on SSLI, CHS also induces time- and tissue-dependent alterations in SS-mRNA. The mechanism of CHS action on SS-mRNA is complex and may involve both an indirect effect secondary to loss of somatostatin autoinhibition (to account for SS-mRNA increases) and/or a direct inhibition of SS gene expression (to explain SS-mRNA reduction). The precise site of direct CHS action on SS gene regulation remains to be defined.(ABSTRACT TRUNCATED AT 400 WORDS)