Effects of guinea pig PRL and PRLRP1 on proliferation of Nb2 lymphoma cells. A, Guinea pig PRL and PRLRP1 were expressed as FLAG-tagged proteins in a HEK-293 cell expression system. B, Western blot analysis of lysates from genetically engineered HEK-293 cells expressing guinea pig PRL (lane 3) or PRLRP1 (lane 4) resolved in 12% sodium dodecyl sulfate-polyacrylamide gels under reducing conditions and probed with anti-FLAG M2 antibodies. Flag-tagged rat PLP-J (lane 1) and conditioned medium from mock-transfected HEK-293 cells (lane 2) were used as positive and negative controls, respectively. C, Quiescent Nb2 cells were incubated for 72 h with conditioned medium from HEK-293 cells genetically engineered to express guinea pig PRL or PRLRP1. Purified ovine PRL (0.1–10 ng/ml) and conditioned medium from mock-transfected HEK-293 cells were used as positive and negative controls, respectively. Cell numbers were assessed with a nonradioactive cell proliferation assay. Values represent the mean ± sd of six replicates. Asterisks denote significant stimulation above negative control values (P < 0.01). Note that guinea pig PRL stimulates the proliferation of Nb2 lymphoma cells.