Insulin-like growth factor-I inhibits growth regulatory responses engaged by estrogen and progesterone in the mouse mammary gland

IGF-I retards proper development of acinar structures formed by bovine mammary epithelial cells via sustained activation of Akt kinase

Insulin-like growth factor-I is involved in mammary gland development, promoting proliferation and inhibiting apoptosis of mammary epithelial cells (MECs). Mitogenic actions of IGF-I are mainly mediated by the phosphatidylinositol-3 kinase (PI3K)/Akt signaling pathway. We have found that in the presence of IGF-I bovine BME-UV1 MECs cultured on reconstituted basement membrane form large spheroids with disrupted polarity and no cavity in the center. These cells showed enhanced phosphorylation of Akt, decreased level of cleaved caspase-3, and sustained proliferative activity throughout the 16-d period of 3-dimensional culture. Inhibition of the PI3K/Akt pathway by a specific inhibitor of PI3K, LY294002, resulted in the restoration of the normal acinar phenotype. However, this effect was noted only when LY294002 was added in the second week of 3-dimensional culture, which corresponded with the time of cell cycle arrest and polarity formation under control conditions. Normal development of acini was also obtained when BME-UV1 cells were treated simultaneously with IGF-I and 17β-estradiol. The addition of 17β-estradiol regulated Akt activation, enabling the subsequent initiation of polarization processes. 17β-Estradiol also increased the level of IGFBP-3 protein in MECs cultured on Matrigel in the presence of IGF-I. The presented results indicate important interactions between signaling pathways activated by estrogen and IGF-I, which regulate alveologenesis in bovine mammary gland.

Polymerization of insulin-like growth factor-binding protein-1 (IGFBP-1) potentiates IGF-I actions in placenta

Insulin-like growth factor (IGF)-binding protein-1 (IGFBP-1), the main secretory protein of decidua that binds to IGFs and has been shown to inhibit or stimulate IGFs' bioactivities. Polymerization, one of the posttranslational modifications of IGFBP-1, has been shown to lead to loss of inhibiting effect of IGFBP-1 on IGF-I actions. The current studies were undertaken to elucidate the effects of steroid hormones on IGFBP-1 polymerization in trophoblast cell cultures. Placental tissues were obtained during legal, elective procedures of termination of pregnancy performed between 7 and 10 weeks of gestation, and primary trophoblast cells were separated. IGFBP-1 polymerization was analyzed by SDS-PAGE and immunoblotting. IGFBP-1 was polymerized when IGFBP-1 was added to trophoblast cell cultures. Polymerization of IGFBP-1 was inhibited by the addition of anti-tissue transglutaminase antibody into the culture media. There was an increase in the intensity of polymerized IGFBP-1 bands with the addition of medroxyprogesterone acetate (MPA), while no such difference was observed upon treatment with estradiol. MPA also increased the expression of tissue transglutaminase on trophoblast cell membranes. IGF-I stimulated trophoblast cell migration, while IGFBP-1 inhibited this IGF-I-induced trophoblast response. Addition of MPA attenuated the inhibitory effects of IGFBP-1 on IGF-I-induced trophoblast cell migration. IGFBP-1 was polymerized by tissue transglutaminase on the cell surface of trophoblasts, and MPA increased tissue transglutaminase expression on the cell surface and facilitated IGFBP-1 polymerization. These results suggest that progesterone might facilitate polymerization of decidua-secreted IGFBP-1 and increase IGF-I actions at feto-maternal interface, thereby stimulating trophoblast invasion of maternal uterus.

Transcriptional responses to estrogen and progesterone in mammary gland identify networks regulating p53 activity

Estrogen and progestins are essential for mammary growth and differentiation but also enhance the activity of the p53 tumor suppressor protein in the mammary epithelium. However, the pathways by which these hormones regulate p53 activity are unknown. Microarrays were used to profile the transcriptional changes within the mammary gland after administration of either vehicle, 17beta-estradiol (E), or progesterone (P) individually and combined (EP). Treatment with EP yielded 1182 unique genes that were differentially expressed compared to the vehicle-treated group. Although 30% of genes were responsive to either E or P individually, combined treatment with both EP had a synergistic effect accounting for 60% of the differentially regulated genes. Analysis of protein-protein interactions identified p53, RelA, Snw1, and Igfals as common targets of genes regulated by EP. RelA and p53 form hubs within a network connected by genes that are regulated by EP and that may coordinate the competing functions of RelA and p53 in proliferation and survival of cells. Induction of early growth response 1 (Egr1) and Stratifin (Sfn) (also known as 14-3-3sigma) by EP was confirmed by reverse transcription-quantitative PCR and shown to be p53 independent. In luciferase reporter assays, Egr1 was shown to enhance transcriptional activation by p53 and inhibit nuclear factor kappaB activity. These results identify a gene expression network that provides redundant activation of RelA to support proliferation as well as sensitize p53 to ensure proper surveillance and integration of their competing functions through factors such as Egr1, which both enhance p53 and inhibit RelA.