Regulation of Cytosolic Phospholipase A2 in Hen Granulosa Cells by Transforming Growth Factors at Different Stages of Follicular Development1

Abstract

Phospholipase A2 (PLA2) is a key enzyme involved in the release of arachidonic acid and subsequent production of prostaglandins (PGs), which play important roles in regulating ovarian function, including mitogenic signalling by transforming growth factor (TGF) alpha. Cytosolic PLA2 (cPLA2), a newly identified member of the PLA2 family, is distinct from the well-characterized, secreted 14-kDa group I and II PLA2 in that cPLA2 selectively cleaves arachidonic acid-containing phospholipids at the sn-2 position. Our previous studies have demonstrated that TGF alpha stimulates and TGF beta suppresses PG production in hen granulosa cells during ovarian follicular development. The present work extends these findings. A cPLA2 is present as a 100-kDa protein doublet and has a transcript size of 3.2 kilobases (kb) in hen granulosa cells from the largest (F1) and the fifth and sixth (F5-6) preovulatory follicles. Treatment of these cells with TGF alpha caused a shift from an electrophoretically fast-migrating to slow-migrating protein, a phenomenon sensitive to inhibitors of serine/threonine kinase as well as mitogen-activated protein (MAP) kinase pathways and indicative of phosphorylation and activation of cPLA2. In contrast, TGF beta suppressed cPLA2 expression, as evidenced by a marked decrease of cPLA2 mRNA abundance and protein content, but failed to prevent the mobility shift of cPLA2 induced by TGF alpha. Consistent with the influence of this growth factor on PG production observed previously and in contrast to the action of TGF alpha, the inhibition of cPLA2 by TGF beta is more pronounced in granulosa cells at the early stage of follicular development. Our results demonstrate, for the first time, the regulation of granulosa cell cPLA2 at the transcriptional and posttranscriptional levels at different stages of follicular development. We propose that the activation by TGF alpha and suppression by TGF beta of cPLA2 may be important regulatory mechanisms in the control of granulosa cell PG production and thereby the mitogenic response of the cells to the growth factors during ovarian follicular development.