Involvement of Angiopoietin-Tie System in Bovine Follicular Development and Atresia: Messenger RNA Expression in Theca Interna and Effect on Steroid Secretion1

Abstract

Angiogenesis is involved in the local mechanisms that regulate follicular development and ovulation. Recently, the angiopoietin (ANPT)-Tie system has been shown to be required to regulate angiogenesis and blood vessel regression. Expression of the ANPT-Tie system in the cyclic ovary suggests that the relative changes in the expression of ANPT-1 and ANPT-2 influence the stability of ovarian blood vessels. In this study, we investigated 1) the mRNA expression for ANPT-1, ANPT-2, and endothelial cell-specific receptors Tie1 and Tie2 in the theca interna (TI) of the bovine developing, mature, and atretic follicles by using a semiquantitative reverse transcription polymerase chain reaction assay and 2) the effect of ANPT on the secretion of steroid hormones from bovine preovulatory follicles in vitro using a microdialysis system (MDS) implanted in the thecal layer. Bovine follicles were classified as developing, mature, and atretic according to size, follicular fluid content of estradiol (E2) and progesterone (P4), and characteristics of granulosa cells (GCs). Both ANPT and Tie mRNA were expressed in the TI, whereas GCs expressed ANPT mRNA only. The expression of ANPT-2 mRNA was decreased in the mature follicles. This decrease resulted in a decrease in the ANPT-2:ANPT-1 ratio (an index of instability of blood vessels), indicating that the blood vessels became more stable or mature. The early atretic follicles showed a higher ANPT-2:ANPT-1 ratio and higher Tie2 mRNA expression than did other follicles at healthy or later atretic stages. This finding may imply that blood vessels become unstable at the initial stage of follicular atresia. In both mid and late atretic follicles, Tie2 mRNA expression dramatically decreased, indicating a disruption of the ANPT-Tie system. In the MDS experiment, an infusion of ANPT-1 or ANPT-2 increased P4 release, whereas both ANPTs inhibited the release of androstenedione. ANPT-1 also increased E2 release. These results showed that the mRNA expression for ANPT-1, ANPT-2, Tie1, and Tie2 changes during follicular development, maturation, and atresia in bovine follicles and that ANPTs affect steroidogenesis in the preovulatory follicle. The results suggest that the ANPT-Tie system is involved the structural (angiogenesis) and secretory changes that occur during follicular development and atresia.

Progesterone exposure of seasonally anoestrous ewes alters the expression of angiogenic growth factors in preovulatory follicles

“…ANG-1, ANG-2, and TIE-2 mRNA was mainly expressed by the granulosa layer, in agreement to previous reports on the ANGs mRNA expression in the follicular layers [22,23]. This difference in protein and mRNA expression by the different follicular layers could possibly be explained by the diffusion of the protein from the granulosa to the theca layer [39] or to the follicular fluid.…”

“…Although in small follicles both progesterone priming and the LH surge do not seem to affect ANG-1 protein and ANG-1 mRNA expression levels, in large follicles progesterone priming significantly upregulated ANG-1 protein expression by the theca interna cells after the LH surge, possibly contributing to the early luteinization process that takes place in the cells of the ovulatory follicle [23]. The ANG-2 protein levels in progesterone primed animals after the LH surge were also significantly higher than those found in nonprimed animals.…”

“…Most blood vessels of the early CL are derived from the theca layer [45] and further vascular development originates from sprouting from this existent vascular network [16,17,40]. High VEGF and high Ang-2 protein levels are considered to be essential for neovascularization to occur [23]. The significantly lower levels of VEGF observed in the large follicles of nonprimed animals associated with significantly lower levels of ANG-2 protein after the LH surge is potentially detrimental to angiogenesis and new blood vessel formation.…”

“…When the ANG-2:ANG-1 ratio and VEGF levels are high, new blood vessel networks are formed. However, when the ANG-2:ANG-1 ratio is high but VEGF levels are low, regression of blood vessels happens and a low ANG-2:ANG-1 ratio along with relatively low levels of VEGF result in stabilization of blood vessels [23].…”

Expression and localization of bone morphogenetic protein6 (BMP6) in the corpus luteum during different stages of estrous cycle in the buffalo (Bubalus bubalis)

“…But, it is well established that growth factors produced locally had crucial permissive and modulatory function in growth of follicles, ovulation, dominance and luteal function (Fortune et al, 1988;Hyashi et al, 2003;Berisha & Schams, 2005). These local factors include Transforming growth factor-β (TGF-β),Vascular endothelial growth factor (VEGF), Insulin-like growth factor (IGF), Fibroblast growth factor (FGF), Epidermal Growth Factor (EGF) and angiopoietin etc.…”