Induction of apoptosis and CPP32 expression by thyroid hormone in a myoblastic cell line derived from tadpole tail

During amphibian metamorphosis, the tail and gills that are useful in aquatic life but inappropriate for terrestrial activity are induced to degenerate completely in several days by endogenous thyroid hormone (TH). The dramatic resorption of the tadpole tail has attracted a good deal of attention as an experimental system of cell death, but the mechanism has not been well characterized. To facilitate in vitro analysis, we have established a myoblast cell line (XLT-15) derived from the Xenopus laevis tadpole tail. This cultured cell line died in response to TH and exhibited positive TUNEL reaction and internucleosomal DNA cleavage. Simultaneously, expression of the Xenopus CPP32/apopain/Yama gene was up-regulated by TH in the cell line as it is in regressing tadpole tail, whereas interleukin-1beta-converting enzyme (ICE) mRNA is around 1 copy/cell in tail and undetectable in XLT-15 cells. A CPP32/apopain/Yama inhibitor (acetyl-Asp-Glu-Val-Asp-aldehyde) prevented TH-induced apoptosis of XLT-15 cells, but an ICE inhibitor (acetyl-Tyr-Val-Ala-Asp-aldehyde) did not. These results suggested that an increase of CPP32/apopain/Yama gene expression is involved in TH-dependent apoptosis of XLT-15 and tadpole tail resorption during metamorphosis.