Li J, Li M, Tsang B K
Department of Obstetrics & Gynaecology, University of Ottawa, Ottawa Civic Hospital Loeb Research Institute, Ontario, Canada.
Biol Reprod. 1997 Oct;57(4):929-35. doi: 10.1095/biolreprod57.4.929.
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.
磷脂酶A2（PLA2）是一种关键酶，参与花生四烯酸的释放以及随后前列腺素（PGs）的生成，前列腺素在调节卵巢功能中发挥重要作用，包括通过转化生长因子（TGF）α进行促有丝分裂信号传导。胞质型磷脂酶A2（cPLA2）是PLA2家族新发现的成员，与特征明确的分泌型14-kDa I型和II型PLA2不同，cPLA2在sn-2位选择性切割含花生四烯酸的磷脂。我们之前的研究表明，在卵巢卵泡发育过程中，TGFα刺激而TGFβ抑制母鸡颗粒细胞中PG的生成。本研究扩展了这些发现。在来自最大的（F1）以及第五和第六个（F5-6）排卵前卵泡的母鸡颗粒细胞中，cPLA2以100-kDa的蛋白双峰形式存在，转录本大小为3.2千碱基（kb）。用TGFα处理这些细胞导致蛋白从电泳快速迁移形式转变为慢速迁移形式，这种现象对丝氨酸/苏氨酸激酶以及丝裂原活化蛋白（MAP）激酶途径的抑制剂敏感，表明cPLA2发生了磷酸化和激活。相反，TGFβ抑制cPLA2表达，表现为cPLA2 mRNA丰度和蛋白含量显著降低，但未能阻止TGFα诱导的cPLA2迁移率改变。与先前观察到的这种生长因子对PG生成的影响一致，且与TGFα的作用相反，TGFβ对cPLA2的抑制在卵泡发育早期的颗粒细胞中更为明显。我们的结果首次证明了在卵泡发育的不同阶段，颗粒细胞cPLA2在转录和转录后水平受到调控。我们提出，TGFα对cPLA2的激活以及TGFβ对其的抑制可能是控制颗粒细胞PG生成从而控制卵泡发育过程中细胞对生长因子促有丝分裂反应的重要调节机制。
