RESUMO
Postnatal development of the mammary gland requires interactions between the epithelial and stromal compartments, which regulate actions of hormones and growth factors. IGF-I is expressed in both epithelial and stromal compartments during postnatal development of the mammary gland. However, little is known about how local expression of IGF-I in epithelium or stroma regulates mammary growth and differentiation during puberty and pregnancy-induced alveolar development. The goal of this study was to investigate the mechanisms of IGF-I actions in the postnatal mammary gland and test the hypothesis that IGF-I expressed in stromal and epithelial compartments has distinct functions. We established mouse lines with inactivation of the igf1 gene in mammary epithelium by crossing igf1/loxP mice with mouse lines expressing Cre recombinase under the control of either the mouse mammary tumor virus long-terminal repeat or the whey acidic protein gene promoter. Epithelial-specific loss of IGF-I during pubertal growth resulted in deficits in ductal branching. In contrast, heterozygous reduction of IGF-I throughout the gland decreased expression of cyclins A2 and B1 during pubertal growth and resulted in alterations in proliferation of the alveolar epithelium and milk protein levels during pregnancy-induced differentiation. Reduction in epithelial IGF-I at either of these stages had no effect on these indices. Taken together, our results support distinct roles for IGF-I expressed in epithelial and stromal compartments in mediating growth of the postnatal mammary gland.
Assuntos
Fator de Crescimento Insulin-Like I/fisiologia , Glândulas Mamárias Animais/crescimento & desenvolvimento , Animais , Caseínas/genética , Ciclinas/genética , Epitélio/fisiologia , Feminino , Fator de Crescimento Insulin-Like I/análise , Fator de Crescimento Insulin-Like I/genética , Integrases/fisiologia , Vírus do Tumor Mamário do Camundongo/genética , Camundongos , Camundongos Transgênicos , RNA Mensageiro/análise , Recombinação Genética , Maturidade Sexual/fisiologia , Células Estromais/fisiologiaRESUMO
Induction of cyclin proteins is required for progression of cells through the G(1)-S and G(2)-M cell cycle checkpoints and is a primary mechanism by which mitogens regulate cell cycle progression. IGF-I and the epidermal growth factor (EGF)-related ligands are mitogens for mammary epithelial cells in vitro and are essential for growth of the mammary epithelium during development. We report here that IGF-I in combination with EGF or TGFalpha is synergistic in promoting DNA synthesis in mammary epithelial cells in the intact mammary gland cultured in vitro. We further investigated the role of IGF-I and EGF in cyclin expression and cell cycle progression in the mammary gland and demonstrate that IGF-I and EGF induce expression of early G(1) cyclins. However, we show that IGF-I, but not EGF, induces late G(1) and G(2) cyclins and is required for mammary epithelial cells to overcome the G(1)-S checkpoint. These data demonstrate that IGF-I is essential for cell cycle progression in mammary epithelial cells and that it is required for EGF-mediated progression past the G(1)-S checkpoint in these cells.
Assuntos
Fator de Crescimento Epidérmico/fisiologia , Células Epiteliais/fisiologia , Fator de Crescimento Insulin-Like I/fisiologia , Glândulas Mamárias Animais/citologia , Glândulas Mamárias Animais/fisiologia , Animais , Western Blotting , Ciclo Celular/fisiologia , Divisão Celular/fisiologia , Ciclina G , Ciclina G1 , Ciclina G2 , Ciclinas/biossíntese , Ciclinas/genética , DNA/biossíntese , DNA/genética , Replicação do DNA/fisiologia , Feminino , Regulação da Expressão Gênica , Imuno-Histoquímica , Ligantes , Camundongos , Ensaios de Proteção de Nucleases , Técnicas de Cultura de ÓrgãosRESUMO
Growth factors are among the critical positive and negative regulators of cell proliferation for normal mammary/breast epithelial cells and for breast cancer cells. The mechanisms by which specific growth factors regulate the cell cycle in mammary/breast epithelial cells is beginning to be understood for several growth factor families, including the epidermal growth factor, insulin-like growth factor, and transforming growth factor-beta families. A critical issue for understanding how growth factors regulate the cell cycle in vivo is how individual factors interact with other growth factors or hormones to enhance or inhibit specific molecular targets in the cell cycle machinery. This review addresses what is currently known about how growth factors regulate the cell cycle in mammary/breast epithelial cells both individually and in coordination with other growth regulators.