The WD motif-containing protein RACK-1 functions as a scaffold protein within the type I IFN receptor-signaling complex.

The WD repeat-containing protein receptor for activated protein kinase C (RACK)-1 has been linked to a variety of signaling systems including protein kinase C, growth factors, and IFNs. In the IFN system, RACK-1 functions as an adaptor recruiting the transcription factor STAT1 to the receptor complex. However, RACK-1 should play a broader role in type I IFN signaling because mutation of the RACK-1 binding site in the IFN-alpha receptor 2/beta subunit of the type I IFN receptor abrogates not only STAT1, but also STAT2, activation. In this study, we demonstrate that RACK-1 serves as a scaffold protein for a multiprotein complex that includes the IFN-alpha receptor 2/beta-chain of the receptor, STAT1, Janus kinase 1, and tyrosine kinase 2. In vitro data further suggest that within this complex tyrosine kinase 2 is the tyrosine kinase responsible for the phosphorylation of STAT1. Finally, we provide evidence that RACK-1 may also serve as a scaffold protein in other cytokine systems such as IL-2, IL-4, and erythropoietin.

Different requirements for the cytostatic and apoptotic effects of type I interferons. Induction of apoptosis requires ARF but not p53 in osteosarcoma cell lines.

The regulation of cell growth is one of the most important effects of type I interferons (IFNs). This response may involve a cytostatic effect or the induction of apoptosis depending on the cell context. Often the growth-inhibitory response of type I IFNs is studied in tumor cell lines carrying mutations of tumor suppressor genes, and therefore, the growth-inhibitory effect can be influenced by inactivation of these important regulators of cell proliferation. In this report, we explored the role of the ARF-p53 pathway in the growth-inhibitory effect of type I IFNs. We found that p53 is only induced in cells that express p14(ARF) (p19(ARF) in mouse cells). Surprisingly, mouse embryonal fibroblasts that are null for p19(ARF) or P53, even after transformation with oncogenic RAS, respond as well as wild type to the growth-inhibitory effect of type I IFNs. Similarly, human ARF(-/-) U2OS and P53(-/-) SAOS-2 cells show a significant decrease in cell proliferation. However, only SAOS-2 or U2OS reconstituted with inducible p14(ARF) undergo apoptosis in response to IFN beta treatment, and this effect was not inhibited by expression of dominant negative p53. These data suggest that (i) at least in specific cell types, the induction of apoptosis by type I IFNs requires an ARF pathway that is p53-independent and (ii) the cytostatic and pro-apoptotic effects of type I IFNs employ different pathways.