Expression of mRNA for the angiopoietin-tie system in granulosa cells during follicular development in cows

Recent findings indicate that the changing profile of angiopoietins (ANPT) and their receptor Tie2 are closely associated with development and regression of the vascular network in the cyclic ovary. We previously reported that mRNA expression for the ANPT-Tie system in theca interna changes during bovine follicular development and atresia, and both ANPTs affect steroidogenesis in the preovulatory follicle. The aim of this study was to investigate mRNA expression for ANPT1, ANPT-2 and Tie2 in granulosa cells (GC) during follicular development in the cow. Bovine follicles were classified according to the estradiol-17β(E₂) concentration in follicular fluid (FF) as follows: (1) E₂<0.5, (2) 0.5<E₂<5, (3) 5<E₂<20, (4) 20< E₂<180 and (5) E ₂>180 ng/ml FF. Semiquantitative RT-PCR analysis revealed that the expression of ANPT-1 mRNA was not detected in most of the follicle with E ₂ <5 ng/ml (diameter of 5-10 mm), but clearly detected in all follicles with E₂>5 ng/ml (diameter of >10 mm). The mRNA expression for ANPT-2 was drastically decreased in the follicles with E ₂>5 ng/ml. Tie2 mRNA expression remained unchanged at the different stages of follicular development. The present data show that ANPT-1 becomes predominant in the follicle producing high levels of E₂, indicating the possible switch-over from ANPT-2 (antagonist) to ANPT-1 (agonist). Thus, the result suggests that the ANPT-Tie system in bovine GC may stimulate E₂ secretion rather than angiogenesis in the late stages of follicular development.