Inhibition of Double-Stranded RNA- and Tumor Necrosis Factor Alpha-Mediated Apoptosis by Tetratricopeptide Repeat Protein and Cochaperone P58^IPK

Abstract

P58^IPK is a tetratricopeptide repeat-containing cochaperone that is involved in stress-activated cellular pathways and that inhibits the activity of protein kinase PKR, a primary mediator of the antiviral and antiproliferative properties of interferon. To gain better insight into the molecular actions of P58^IPK, we generated NIH 3T3 cell lines expressing either wild-type P58^IPK or a P58^IPK deletion mutant, ΔTPR6, that does not bind to or inhibit PKR. When treated with double-stranded RNA (dsRNA), ΔTPR6-expressing cells exhibited a significant increase in eukaryotic initiation factor 2α phosphorylation and NF-κB activation, indicating a functional PKR. In contrast, both of these PKR-dependent events were blocked by the overexpression of wild-type P58^IPK. In addition, the P58^IPK cell line, but not the ΔTPR6 cell line, was resistant to dsRNA-induced apoptosis. Together, these findings demonstrate that P58^IPK regulates dsRNA signaling pathways by inhibiting multiple PKR-dependent functions. In contrast, both the P58^IPK and ΔTPR6 cell lines were resistant to tumor necrosis factor alpha-induced apoptosis, suggesting that P58^IPK may function as a more general suppressor of programmed cell death independently of its PKR-inhibitory properties. In accordance with this hypothesis, although PKR remained active in ΔTPR6-expressing cells, the ΔTPR6 cell line displayed a transformed phenotype and was tumorigenic in nude mice. Thus, the antiapoptotic function of P58^IPK may be an important factor in its ability to malignantly transform cells.

ACKNOWLEDGMENTS

We thank Marjorie Domenowske for help in figure preparation and Dagma Daniel for administrative support.

This investigation was supported by Public Health Service grants AI 22646, AI 41629, and RR 00166 from the National Institutes of Health to M.G.K. and AI 34039 to B.R.G.W. N.M.T. was supported by a Public Health Service National Research Service Award, T32 GM07270, from the National Institute of General Medical Sciences. M.G. is supported by the Helen Hay Whitney Foundation.