JAK1 and breast cancer: To test whether GH signaling in human T47D breast cancer cells, which express low levels of GHR and high levels of PrlR, is affected by extracellular acidosis, we assessed GH induction of phosphorylation of Jak2, Jak1 and Stat5 across a range of zinc ion concentrations at pHe 7.4 and 6.8 (Figure 4E).