In the adrenal gland, adrenocorticotropin (ACTH) acting through the cAMP protein kinase (PKA) transduction pathway is the main regulator of genes involved in glucocorticoid synthesis. The prolactin (PRL) receptor is expressed in the adrenal cortex of most mammals, but experimental proof that PRL ensures direct control on glucocorticoid synthesis in rodents remains elusive. To unravel the physiological importance of PRL in adrenocortical functions, we measured steroidogenic capacity of Prlr-deficient mice (Prlr ؊/؊ ) and explored the influence of JAK/STAT signaling, the major PRL transduction pathway, on the steroidogenic activity of adrenocortical cell cultures. We demonstrate that lack of Prlr does not affect basal (nor stress-induced) corticosterone levels in mice. PRL triggers JAK2/STAT5-dependent transcription in adrenal cells, but this does not influence corticosterone release. In contrast, pharmacological or siRNA-mediated inhibition of JAK2 reveals its essential role in both basal and ACTH/cAMP-induced steroidogenesis. We demonstrate that nuclear JAK2 regulates the amount of active transcription factor CREB (cAMP response element-binding protein) through tyrosine phosphorylation and prevention of proteasomal degradation, which in turn leads to transcriptional activation of the rate-limiting steroidogenic Star gene. Hence, we describe a novel link between PKA and JAK2 by which nuclear JAK2 signaling controls adrenal steroidogenesis by increasing the stability of CREB.Acute and chronic adrenal cortex steroidogenesis is regulated mainly through the adrenocorticotropin (ACTH) activation of the cAMP-dependent protein kinase A (PKA) signaling pathway. Chronic response to ACTH results in increased transcription of genes encoding steroidogenic enzymes and the proteins responsible for cholesterol mobilization and transport, such as the steroidogenic acute regulatory protein (STAR), 4 which mediates the rate-limiting and regulated step in steroidogenesis. cAMP-induced PKA activations result at least in the phosphorylation of the transcription factors GATA-4-binding protein, steroidogenic factor 1 (SF1), CAAT enhancerbinding protein (C/EBP) and cAMP response element-binding protein (CREB). These in turn stimulate transcription of genes involved in steroidogenesis (1, 2). The cAMP-responsive unit of the mouse Star proximal promoter (Ϫ110 and Ϫ67 bp) is a prototypical sequence that involves multiple DNA binding motifs for these transcription factors including three CRE halfsites that can bind CREB (2-4).Although cAMP is an essential inducer of steroidogenesis, other mechanisms triggered by paracrine and endocrine signals have been involved in the regulation of steroid synthesis in basal or stress-induced conditions. They can either act independently or in synergy with the PKA signaling (5). Prolactin (PRL) is a stress-related protein that has been reported to enhance aldosterone and glucocorticoid production in various mammalian species including rat, guinea pig, mouse, pig, and human (6 -12). A direct action of PRL o...