The cyclic adenosine 3',5'-monophosphate-responsive factor CREB is constitutively activated in human somatotroph...

Oncogenic gsp proteins appear to stimulate the transformation of pituitary somatotrophs by inducing the constitutive activation of adenyl cyclase. Previous work implicating the cAMP-responsive transcription factor CREB as a biochemical intermediate in the proliferative response to cAMP led us to examine whether CREB activity was correspondingly elevated in human somatotroph adenomas. In a series of 15 human GH-secreting tumors, we found that each of these contained elevated levels of Ser133-phosphorylated and, hence, activated CREB compared with nonfunctioning pituitary tumors. Four of the GH-secreting adenomas contained an oncogenic gsp gene by polymerase chain reaction analysis, and two additional adenomas expressed wild-type G alpha s protein at 5- to 10-fold higher levels than nonfunctioning pituitary tumors. As both oncogenic gsp and overexpressed G alpha s proteins can induce Ser133 phosphorylation and cAMP-responsive gene expression in transfected GC somatotroph cells, our studies indicate that these proteins may promote somatotroph transformation in part by inducing the transcription of specific CREB-dependent target genes.