Pituitary tumor development involves clonal expansion stimulated by
hormones and growth factorscytokines. Using
mRNA differential display, we found that the
bone morphogenetic protein (BMP) inhibitor noggin is down-regulated in
prolactinomas from
dopamine D2-receptor-deficient mice. BMP-4 is overexpressed in
prolactinomas taken from
dopamine D2-receptor-deficient female mice, but expression of the highly homologous BMP-2 does not differ in normal pituitary tissue and
prolactinomas. BMP-4 is overexpressed in other
prolactinoma models, including
estradiol-induced rat
prolactinomas and human
prolactinomas, compared with normal tissue and other
pituitary adenoma types (Western blot analysis of 48
tumors). BMP-4 stimulates, and noggin blocks, cell proliferation and the expression of c-Myc in human
prolactinomas, whereas BMP-4 has no action in other human
pituitary tumors. GH3 cells stably transfected with a dominant negative of Smad4 (Smad4dn; a BMP signal cotransducer) or noggin have reduced tumorigenicity in nude mice.
Tumor growth recovered in vivo when the Smad4dn expression was lost, proving that BMP-4Smad4 are involved in
tumor development in vivo. BMP-4 and
estrogens act through overlapping intracellular signaling mechanisms on GH3 cell proliferation and c-myc expression: they had additive effects at low concentrations but not at saturating doses, and their action was inhibited by blocking either pathway with the reciprocal antagonist (i.e., BMP-4 with
ICI 182780 or 17beta-
estradiol with Smad4dn). Furthermore, coimmunoprecipitation studies demonstrate that under BMP-4 stimulation Smad4 and Smad1 physically interact with the
estrogen receptor. This previously undescribed
prolactinoma pathogenesis mechanism may participate in tumorigenicity in other cells where
estrogens and the type beta
transforming growth factor family have important roles.