Abstract
Mast cells proliferate in vivo in areas of active fibrosis, during parasite infestations, in response to repeated immediate hypersensitivity reactions and in patients with mastocytosis. We investigated how progesterone reduces the proliferation of HMC‐1^560^ mast cells that proliferate spontaneously in culture. Cells were incubated with 1 µM to 1 nM progesterone for 24–48 h. Progesterone (1 µM) reduced the spontaneous proliferation of HMC‐1^560^ mast cells to half that of cells cultured without hormone. [^3^H] thymidine incorporation was only 50% of control; there were fewer cells in G2/M and more cells in G0/G1. The amounts of phospho‐Raf‐1 (Tyr 340–341) and phospho‐p42/p44 MAPK proteins were also reduced. In contrast progesterone had no effect on MAP kinase‐phosphatase‐1. The Raf/MAPK pathway, which depends on Src kinase activity, is implicated in the control of cell proliferation. HMC‐1^560^ cells incubated with the tyrosine kinase inhibitor PP1 proliferated more slowly than controls and had less phospho‐Raf‐1 (Tyr 340–341) and phospho‐p42/p44 MAPK. The Csk homologous kinase (CHK), an endogenous inhibitor of Src protein tyrosine kinases, was also enhanced in progesterone‐treated cells. In contrast, progesterone had no effect on the growth of cells transfected with siRNA CHK. We conclude that progesterone increases the amount of csk homologous kinase, which in turn reduces HMC‐1^560^ mast cell proliferation. This effect parallels decreases in the phosphorylated forms of Raf‐1 and p42/44 MAPK, as their production depends on Src kinase activity. J. Cell. Biochem. 102: 1271–1280, 2007. © 2007 Wiley‐Liss, Inc.