Hareramadas Batchu, Rai Umesh
Comparative Endocrinology Laboratory, Department of Zoology, University of Delhi, North campus, Delhi 110 007, India.
J Exp Zool A Comp Exp Biol. 2006 May 1;305(5):396-409. doi: 10.1002/jez.a.260.
The present study, for the first time in an ectothermic vertebrate, demonstrates the cellular mechanism of estrogen-induced thymic involution. Ovariectomy in lizards during the preparatory phase of the reproductive cycle resulted in distinct differentiation of cortico-medullary regions and increase in cellularity, especially in the cortical region. The ovariectomy-induced changes were reversed following administration of 17-estradiol (E2), suggesting a primary role of E2 in causing thymic atrophy. To understand the cellular mechanism of E2-induced thymic atrophy, in vitro effect of E2 was investigated on thymocyte proliferation and apoptosis. E2 decreased the uptake of tritiated thymidine (3H-TdR) by thymocytes in a dose-dependent manner, suggesting that estrogen directly inhibits the thymocyte proliferation. Unlike proliferation, E2 did not have any direct effect on thymocyte apoptosis, as evident by DNA gel electrophoretic, flow cytometric or fluorescence microscopic studies. However, in the presence of thymic epithelial cell-rich stromal components (TEC), E2 treatment at low or high concentrations resulted in depolarization of plasma membrane, DNA fragmentation and decrease in DNA content. This suggests that E2 indirectly, through TEC-secreted factors, controls thymocyte apoptosis. Similar result was observed following fluorescence microscopy. The indirect effect of E2 was further ascertained with the findings that E2-pretreated TEC-conditioned medium accelerated the thymocyte apoptosis. Nevertheless, exposure of thymocytes to E2 was seen to be inevitable for the apoptotic action of TEC-secreted paracrine factors. In the presence of TEC, a positive reaction for caspase-3, -7 and -9 and enzyme substrate, poly(ADP-ribose) polymerase (PARP) in response to E2 suggests the caspase-dependent thymocyte apoptosis in the wall lizard Hemidactylus flaviviridis. Further, E2 was shown to act through genomic pathway, since the receptor antagonist tamoxifen and transcription/translation inhibitors blocked its apoptotic action. Interestingly, the apoptotic effect of E2 was effectively decreased by progesterone.
本研究首次在变温脊椎动物中揭示了雌激素诱导胸腺退化的细胞机制。在生殖周期的准备阶段对蜥蜴进行卵巢切除，导致皮质 - 髓质区域明显分化，细胞数量增加，尤其是皮质区域。卵巢切除引起的变化在给予17 - 雌二醇（E2）后逆转，表明E2在导致胸腺萎缩中起主要作用。为了解E2诱导胸腺萎缩的细胞机制，研究了E2对胸腺细胞增殖和凋亡的体外作用。E2以剂量依赖的方式降低胸腺细胞对氚标记胸腺嘧啶核苷（3H - TdR）的摄取，表明雌激素直接抑制胸腺细胞增殖。与增殖不同，DNA凝胶电泳、流式细胞术或荧光显微镜研究表明，E2对胸腺细胞凋亡没有直接影响。然而，在富含胸腺上皮细胞的基质成分（TEC）存在的情况下，低浓度或高浓度的E2处理导致质膜去极化、DNA片段化以及DNA含量降低。这表明E2通过TEC分泌的因子间接控制胸腺细胞凋亡。荧光显微镜检查也观察到了类似结果。E2预处理的TEC条件培养基加速胸腺细胞凋亡的发现进一步证实了E2的间接作用。然而，胸腺细胞暴露于E2对于TEC分泌旁分泌因子的凋亡作用是不可避免的。在TEC存在的情况下，针对E2，半蹼壁虎（Hemidactylus flaviviridis）中caspase - 3、 - 7和 - 9以及酶底物聚（ADP - 核糖）聚合酶（PARP）的阳性反应表明存在caspase依赖性胸腺细胞凋亡。此外，由于受体拮抗剂他莫昔芬和转录/翻译抑制剂阻断了其凋亡作用，表明E2通过基因组途径发挥作用。有趣的是，孕酮有效地降低了E2的凋亡作用。
