Persistent EGF Signaling Serves to Promote Lactotrope Differentiation in GH4 Pituitary Somatolactotrope Tumor Cells, whereas Activation of the PI3K/mTOR/S6K Pathway Promotes Cell Proliferation and the Transformed Phenotype

Background Pituitary somatotropes and lactotropes, expressing GH and PRL, respectively, retain plasticity, allowing rapid cell expansions in response to increased physiological demands. Unfortunately, very little is known about the signaling events that instruct somato‐lactotrope cells to proliferate and then terminally differentiate or that contribute to the transformed phenotype. A number of in vivo transgenic mouse studies revealed that uncontrolled activation of growth factor Ras/MAPK signaling pathways in lactotropes results in lactotrope hyperplasia. Moreover, in vitro studies using GH4 rat pituitary somato‐lactotrope tumor cell lines have shown that activated pMAPK is necessary for both short‐term proliferation and differentiation, with possibly the duration of pMAPK activation dictating differential responses. However, these proliferation studies all relied on short‐term (6–24 hrs) assays. Thus, the specific role of MAPK in durable lactotrope proliferation and differentiation responses remain unknown. Methods In order to directly interrogate the role of EGF and mTOR in lactotrope differentiation, proliferation and transformation, we used persistent EGF addition or pharmacological inhibitors of mTOR, to investigate the role of these pathways in GH4 cell proliferation, clonogenicity, and soft agar growth. Results Using long‐term proliferation, clonogenicity and soft agar growth assays, we find that treating cycling GH4 cells with 5nM EGF and mTOR inhibitors, rapamycin and Torin 1, reduced cell proliferation, clonogenicity, and soft agar growth. Conclusion Collectively, these data reveal that the EGF signaling pathway Ras/Raf/MAPK, promotes lactotrope differentiation, whereas activation of the PI3K/mTOR/S6K pathway regulates lactotrope proliferation and transformed phenotype.