Growth hormone induces tyrosyl phosphorylation of the transcription factors Stat5a and Stat5b in CMT-U335 canine mammary tumor cells.

It has now been well documented that the normal and tumorous canine mammary glands can be extra-pituitary sites of substantial growth hormone (GH) synthesis. Until now, attempts to reproduce the GH synthesis in-vitro using canine mammary explants or mammary tumor cells have not been successful. Therefore, the response of CMT-U335 canine mammary tumor cells to administered porcine GH (pGH) was investigated as an in-vitro model to study the possible effects of GH synthesis on this site. CMT-U335 cells spontaneously express the growth hormone receptor (GHR) as well as the prolactin receptor (PRLR). Twenty five minutes after administration, GH induced, in a dose-dependent manner, phosphorylation of the transcription factors Stat5a and Stat5b. Clear phosphorylation was induced by 10(-7) M and 10(-8) M pGH, with virtually no phosphorylation at 10(-9) M pGH. A similar dose-dependent phosphorylation of Stat5a by ovine prolactin was found in these cells. Although at high concentrations binding of pGH to the canine PRLR can occur (albeit with a low pKa), the similar dose-dependent effect of oPRL on Stat5a phosphorylation indicated that pGH signaled through the GHR. Remarkably, pGH induced a moderately decreased proliferation of CMT-U335 tumor cells, which may indicate that GH induces differentiation in these tumor cells. The GH-induced activation of Stat5a and Stat5b in these cells, as part of the JAK/Stat signal transduction pathway, is consistent with mammary GH playing a role in autocrine and/or paracrine stimulation of (tumorous) mammary cells.