Transcriptional activation of the human growth hormone gene by ras oncogene

Abstract

ras Oncogenes play an important role in causing cellular transformation and proliferation. They have been implicated in the formation of many human tumors but only rarely been identified in pituitary adenomas. We studied the effect of ras activation on growth hormone (GH) production. Transcriptional regulation of human GH was investigated by transient transfections in a pituitary cell line GH 4 using different promoter fragments cloned 5′ of the luciferase reporter gene (−344 to −83). Co-transfection of the constitutively active valine 12 mutant ras oncogene (V-12 ras) resulted in a selective and dose-dependent stimulation of −344-GH/Luc activity. This effect is pituitary-cell specific as activation of the human GH promoter by ras was absent in a human chorion carcinoma cell line JEG3. Co-transfection of protein kinase inhibitor did not influence ras mediated stimulation of the human GH promoter. Investigations of several deletion constructs of the human GH promoter revealed that elements between −145 and −83 are sufficient to transduce ras signaling. This region contains two Pit-1 bindings sites as well as a Zn-15 binding site. These studies demonstrate transduction of ras signaling to the human GH promoter through a protein kinase A (PKA) independent signaling pathway. This separate transduction mechanism may convey regulation by yet unknown factors.