The human 1-8 interferon inducible gene family consists of at least 3 functional genes; 9-27, 1-8D and 1-8U, which are all linked on an 18-kb fragment of chromosome 11 and are highly homologous. It has recently been shown by us and others that the 1-8D gene is overexpressed in colon carcinoma. Here, we show, by sequence comparison of the 1-8D in pairs of tumor/normal colon tissues, the existence of 6 different alleles, containing single-nucleotide polymorphisms with no mutations. Transformation assays revealed a possible role for the 1-8D gene as a transformation inhibitor. Further, transient expression of the human 1-8D gene in multiple mammalian cell lines showed accumulation of cells in the G1 phase followed by elevation in the subG1 phase. SubG1 elevation was confirmed as apoptosis by Annexin-V binding assays and transferase-mediated dUTP nick end labeling assays. Moreover, knock-down of 1-8D provided partial protection from Etoposide and UV-induced apoptosis. The induction of apoptosis by 1-8D is dependent on caspase activities but not on p53 expression. Although 1-8D induces apoptosis independently of p53, p53 expression downregulates 1-8D protein expression. Our data suggest a role for the 1-8D gene as a novel pro-apoptotic gene that will provide new insights into the regulated cellular pathways to death. ' 2009 UICC

Interferon Stimulated Gene 12a, ISG12a, is a member of a family of small intracellular non-secreted proteins (10-20 kDa), mainly induced by type I IFNs and slightly induced by type II IFN. It has been shown that full length ISG12 (ISG12a) is overexpressed in breast cancer, yet the biological function of ISG12 is largely unknown. Here we show that transient transfection of ISG12a into various mammalian cell lines leads to accumulation of cells initially in the G1 phase of the cell cycle, followed by accumulation of cells in the sub G1 phase, and that cells transfected with ISG12a undergo morphological changes, such as rounding up and detachment from the plate, that are characteristic of apoptosis. Induction of apoptosis by ISG12a was confirmed by Annexin V binding assays and by TUNEL assays. Using general and specific caspase inhibitors, we also showed that ISG12a-induced apoptosis is a caspase dependent, but does not involve p53. Elevation in endogenous ISG12a levels following induction of apoptosis with reagents that induce apoptosis in the intrinsic apoptotic pathway, and reduction in ISG12a-induced apoptosis following co-transfection with Bcl-2, indicated that ISG12a induced apoptosis in the intrinsic apoptotic pathway. Our results suggest a role for the ISG12a gene as a novel pro-apoptotic gene. Novelty and Impact Statement: ISG12a is highly overexpressed in immortalised breast cancer cell lines as compared to normal breast cells. Forced overexpression of ISG12a induces G1 arrest and apoptosis, as confirmed by Annexin V binding and TUNEL assays. We investigated the mechanism and showed that ISG12a-induced apoptosis is caspase dependent, but does not involve p53. ISG12a induced apoptosis in the intrinsic apoptotic pathway.