Kenneth Call

Abstract In the present study we have examined the ability of 8‐bromoadenosine cyclic 3′, 5′‐phosphate (8‐bromo‐cAMP; the membrane permeant analog of cAMP which can activate protein kinase A) to mimic hormone action and stimulate glucose transport and glucose transporter (GLUT‐1) gene expression as well as the expression of several growth‐related protooncogenes in quiescent 3T3‐L1 fibroblasts. 8‐Bromo‐cAMP induced a rapid and prolonged increase in the rate of hexose transport. Early activation of hexose transport (within 30 min) was associated with increased plasma membrane immunoreactive glucose transporters, which corresponded to a doubling in the number of D‐glucose‐displaceable, plasma membrane cytochalasin B binding sites. The time course for 8‐bromo‐cAMP‐induced hexose transport preceded the accumulation of GLUT‐1 mRNA, which peaked between 4 and 8 h after exposure to the agent, and subsequently declined to approach basal (control) levels. Expression of the immediate‐early genes c‐fos and jun‐B was induced by 8‐bromo‐cAMP on a rapid, but sustained time course, whereas induction of c‐jun expression was delayed. Alterations in specific mRNAs following exposure to 8‐bromo‐cAMP were due to increased gene transcription (as judged by nuclear transcription run‐on assays), although with respect to GLUT‐1, an increase in mRNA stability was also observed. Treatment of the cells with forskolin resulted in the induction of GLUT‐1 expression as well as expression of the immediate early genes. Exposure of quiescent 3T3‐L1 fibroblasts to 8‐bromo‐cAMP resulted in a substantial increase in rates of total protein and RNA synthesis, but had little effect on DNA synthesis. The results demonstrate that 8‐bromo‐cAMP initiated a G 0 /G 1 transition, but did not permit progression into S‐phase. The results further suggest that increased cytosolic cAMP results in the stimulation of glucose transport by three distinct mechanisms to include translocation of pre‐existing transporters, increased transcription of the GLUT‐1 gene and increased stability of GLUT‐1 mRNA.