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Comment |
Green C, et al 2001 reported that
p107 is active in the nucleolus in non-dividing human granulosa
lutein cells.
Cells are maintained in a quiescent state by members of the retinoblastoma
protein family, pRb and p130. Both are phosphoproteins and hypophosphorylated
forms of pRb and p130 bind and repress the activity of E2F transcription
factors, thereby preventing entry into the cell cycle. Mitogenic stimulation causes activation of cyclin dependent kinases (cdk) that phosphorylate both
pRb and p130, thereby releasing E2F factors which stimulate the transcription
of a number of genes that are required for DNA synthesis and for regulating
the cell cycle. In non-dividing cells, cdks are maintained in an inactive
state by cdk inhibitor proteins such as p27(Kip1).
The CL is
formed in the ovary after ovulation at the terminal stage of folliculogenesis
after completion of maturation and differentiation of Graafian follicles. As
shown by flow cytometry granulosa luteal cells (GLC) are not dividing, being predominantly in the
G(0)/G(1) phase of the cell cycle and, consistent with this, they contain the
cdk inhibitor protein, p27(Kip1) but not E2F-1 which is normally expressed
only in proliferating cells. The GLC do express E2F-4, hypophosphorylated pRb, and
p130 forms 1 and 2. Bjerregaard B, et al reported the Regulation of Ribosomal RNA Synthesis During the Final Phases of Porcine Oocyte Growth.
In porcine oocytes acquisition of meiotic competence coincides with a decrease of general tran-scriptional activity at the end of the oocyte growth phase and, specifically, of ribosomal RNA (rRNA) synthesis in the nucleolus. The present study investigated the regulation of rRNA synthesis during porcine oocyte growth. Localization and expression of components involved in regulation of the rRNA synthesis, (the RNA polymerase I-associated factor PAF53, upstream binding factor (UBF), and the pocket proteins p130 and pRb) were assessed by immunocytochemistry and semi-quantitative RT-PCR, and correlated with ultrastructural analysis and autoradiography following (3)H-uridine incubation in growing and fully grown porcine oocytes. In addition, meiotic resumption, ultrastructure, and expression of p130, UBF and PAF53 were analyzed in growing and fully grown porcine oocytes cultured with 100 micro M butyrolactone I (BL-I), a potent inhibitor of cyclin dependent kinases (cdk), to gain insight into the regulation of rRNA transcription during meiotic arrest. Immunocytochemical analysis demonstrated that p130 became co-localized with UBF and PAF53, and that the intensity of the PAF53 labeling decreased towards the end of the oocyte growth phase. These data suggest that the decrease in rRNA synthesis is regulated by inhibition of UBF by p130 as well as by decreased availability of PAF53. Moreover, expression of mRNA encoding PAF53 was decreased at the end of the oocyte growth phase. At the morphological level these events coin-cided with inactivation of the nucleolus as visualized by the transformation of the fibrillo-granular nucleolus to an electron-dense fibrillar sphere with remnants of the fibrillar centers at the surface. Meiotic inhibition with 100 micro M BL-I had a detrimental effect on the ability of porcine oocytes to resume meiosis, and on nucleolus morphology resulting in lack of RNA synthetic capability as the fibrillar components, where rRNA transcription and initial processing occur, condensed or even disintegrated.
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