Tumor Necrosis Factor-α Attenuates Thyroid Hormone-Induced Apoptosis in Vascular Endothelial Cell Line XLgoo Established from Xenopus Tadpole Tails

Amphibian metamorphosis induced by T3 involves programmed cell death and the differentiation of various types of cells in degenerated and reconstructed tissues. However, the signaling pathway that directs the T3-dependent cell-fate determinations remains unclear. TNF-α is a pleiotropic cytokine that affects diverse cellular responses. Engagement of TNF-α with its receptor (TNFR1) causes intracellular apoptotic and/or survival signaling. To investigate TNF signaling functions during anuran metamorphosis, we first identified Xenopus laevis orthologs of TNF (xTNF)-α and its receptor. We found that xTNF-α activated nuclear factor-κB in X. laevis A6 cells through the Fas-associated death domain and receptor-interacting protein 1. Interestingly, xTNF-α mRNA in blood cells showed prominent expression at prometamorphosis during metamorphosis. Next, to elucidate the apoptotic and/or survival signaling induced by xTNF-α in an in vitro model of metamorphosis, we established a vascular endothelial cell line, XLgoo, from X. laevis tadpole tail. XLgoo cells formed actin stress fibers and elongated in response to xTNF-α. T3 induced apoptosis in these cells, but the addition of xTNF-α blocked the T3-induced apoptosis. In addition, treatment of the cells with T3 for 2 d induced the expression of thyroid hormone receptor-β and caspase-3, and this thyroid hormone receptor-β induction was drastically repressed by xTNF-α. Furthermore, in organ culture of the tail, xTNF-α significantly attenuated the tail degeneration induced by T3. These findings suggested that xTNF-α could protect vascular endothelial cells from apoptotic cell death induced by T3 during metamorphosis and thereby participate in the regulation of cell fate.