Requirement of Estrogen Receptor-α in Insulin-like Growth Factor-1 (IGF-1)-induced Uterine Responses and in Vivo Evidence for IGF-1/Estrogen Receptor Cross-talk

In the uterus insulin-like growth factor-1 (IGF-1) signaling can be initiated by estradiol acting through its nuclear receptor (estrogen receptor (ER)) to stimulate the local synthesis of IGF-1. Conversely, in vitro studies have demonstrated that estradiol-independent ER transcriptional activity can be induced by IGF-1 signaling, providing evidence for a cross-talk mechanism between IGF-1 and ER. To investigate whether ER alpha is required for uterine responses to IGF-1 in vivo, both wild-type (WT) and ER alpha knockout (alpha ERKO) mice were administered IGF-1, and various uterine responses to IGF-1 were compared. In both WT and alpha ERKO mice, IGF-1 treatment resulted in phosphorylation of uterine IGF-1 receptor (IGF-1R) and formation of an IGF-1R/insulin receptor substrate-1/ phosphatidylinositol 3-kinase signaling complex. In addition, IGF-1 stimulated phosphorylation of uterine Akt and MAPK in both WT and alpha ERKO mice. However, IGF-1 treatment stimulated BrdUrd incorporation and proliferating cell nuclear antigen expression in WT uteri only. To determine whether ER alpha can be activated in vivo by IGF-1 signaling, transgenic mice carrying a luciferase gene driven by two estrogen response elements (ERE-luciferase mice) were utilized. Treatment of ovariectomized ERE-luciferase mice with IGF-1 resulted in an increase in uterine luciferase activity that was attenuated in the presence of the ER antagonist ICI 182,780. Together these data demonstrate that 1) functional signaling proximal to IGF-1R is maintained in the alpha ERKO mouse uterus, 2) ER alpha is necessary for IGF-1 induction of uterine nuclear proliferative responses, and 3) cross-talk between IGF-1R and ER signaling pathways exists in vivo

Conditional Transgenic System for Mouse Aurora A Kinase: Degradation by the Ubiquitin Proteasome Pathway Controls the Level of the Transgenic Protein

Aurora A is a mitotic kinase that localizes to centrosomes. Expression of this protein is normally limited to the mitotic stage (G(2)-M) of the cell cycle, whereas human cancer cells frequently exhibit overexpression of Aurora A protein regardless of the cell cycle stage. In the present study, Aurora A transgenic mouse lines were generated with a new conditional expression system (cytomegalovirus immediate early enhancer-chicken beta-actin hybrid promoter-Z-enhanced green fluorescent protein) in order to analyze the function of this protein. Although transcripts for Aurora A were elevated in multiple organs of the transgenic mice, the corresponding protein was not detected in extracts analyzed by immunoblotting. The treatment of transgenic-derived embryonic fibroblasts (MEF) with proteasome inhibitors markedly increased the protein level of transgenic Aurora A, indicating that the transgenic Aurora A protein is readily degraded in normal mouse tissues. Under the exponential growth conditions of MEF cells, transgenic Aurora A was detected within the mitotic stage of the cell cycle and localized to centrosomes. In contrast, the marker of the transgenic promoter (enhanced green fluorescent protein) was continuously expressed throughout the cell cycle, indicating the constitutive transcription of transgenic mRNA. These results indicate that transgenic Aurora A is protected from degradation within G(2)-M but is immediately degraded after translation in the G(1)-S stage of the cell cycle. The findings obtained with this transgenic model and derived cells support that the transition from protection to degradation by the ubiquitin proteasome system at the end of mitosis is an important step in controlling the level of Aurora A protein during the cell cycle

Myelination is altered in insulin-like growth factor-I null mutant mice

Increasing evidence indicates that insulin-like growth factor-I (IGF-I) has an important role in oligodendrocyte development. In this study, we examined myelination during postnatal development in IGF-I knock-out (KO) mice by assessing myelin staining, the expression of myelin basic protein (MBP) and proteolipid protein (PLP), two major myelin-specific proteins, and the number of oligodendrocytes and their precursors. For comparison, we also measured the expression of median subunit of the neuron-specific intermediate filament, M-neurofilament (M-NF), to obtain an index of the effects of IGF-I deficiency on neurons. We found that myelin staining, MBP and PLP expression, and the percentage of oligodendrocytes and their precursors are significantly reduced in all brain regions of developing IGF-I KO mice but are similar to controls in adult IGF-I KO mice. In contrast, the abundance of M-NF was decreased in both th