| Programmed cell death (apoptosis) is genetically controlled and essential to animal development and homeostasis. Defects in the regulation of the apoptotic program are implicated in the development of cancer, autoimmune diseases, and neurodegenerative diseases. Interferons (IFN) are antiviral and antitumoral cytokines. IFN stimulated genes (ISGs) mediate the biological responses to IFNs. Retinoic acid (RA) is a vitamin A metabolite that can control differentiation and organogenesis. Binding of RA to specific nuclear retinoic acid receptors induces transcription of a wide array of genes. We have previously reported that IFN-β/RA combination induces apoptosis in several tumor cell lines. However, the molecular basis for programmed cell death induced by IFN-β/RA is unknown. Apoptosis induced by IFN-β/RA combination does not appear to be regulated exclusively by known pathways of apoptosis. To identify genes that participate in IFN-β/RA induced apoptosis, we employed a genetic approach. Rescue of the genes was possible by positive growth selection of cells treated with IFN-β/RA after transfection with antisense cDNA expression libraries. Using this system we have isolated several genes termed Genes associated with Retinoid- Interferon induced Mortality (GRIM). When expressed in an antisense orientation, GRIMs confer protection of several cell lines to the IFN-β/RA induced cell death. In this investigation we have characterized two of the GRIM cDNAs. GRIM-12, the first gene we characterized, is identical to human thioredoxin reductase (TR). Increase in TR protein levels and activity correlate with IFN-β/RA induced cell death. Although overexpression of GRIM-12 causes significant cell death, IFN-β/RA treatment enhances it further. Interference with TR function led to growth promotion in the presence of IFN-β/RA combination. Thus, these studies identify a novel function for TR in cell growth regulation. The second gene, GRIM-1, encodes a novel protein. IFN-β/RA combination induces the expression of two GRIM-1 transcripts in several tumor cell lines. GRIM-1 is also expressed in a tissue and developmental stage specific manner. The GRIM-1 gene appears to encode three proteins (α, β, γ). Overexpression of GRIM-1α and γ causes growth suppression in the absence of IFN-β/RA. Transient expression of GRIM-1γ induces cell death. These results identify a novel mediator of cell death regulated by IFN-β/RA. |
