Cytokines are known to signal by interactions with cell-surface receptors coupled to cytoplasmic components that transmit the signal. A growing body of evidence indicates that some cytokines also must act within the cell to exert their complete effect.1,2 Ligand-induced endocytosis of cytokine-receptor complexes traditionally is presumed to function in cellular desensitization and receptor recycling, but several cytokines or their receptors contain nuclear localization signals. Once internalized, these cytokine-receptor complexes may move into the nucleus.
Proteins larger than 40-45 kDa enter the nucleus only by active transport through the nuclear pore complex.2 Protein translocation requires the presence of a nuclear localization signal (NLS) that consists of clusters of cationic residues in one or two closely-spaced groups. A protein binds via its NLS to a complex called importin, which mediates docking to the nuclear pore complex and translocation across the nuclear envelope.
|Figure 1. The IFN-gamma receptor complexes with IFN-gamma and cytoplasmic components including JAK-1, JAK-2, and STAT-1. Following internalization of the complex, it is directed into the nucleus by the NLS of IFN-gamma.|
A number of cytokines or their receptors contain putative NLSs.1,2 Nuclear translocation has been observed for several of these cytokines, including insulin, IFN-gamma, IL-1, IL-5, PDGFs, growth hormone, and members of the FGF family. In some cases the NLS function has been confirmed by mutational analysis or by demonstration of the ability of the NLS to target a heterologous protein to the nucleus.
IFN-gamma is known to be rapidly translocated to the nucleus following receptor binding, and it contains an NLS (RKRKRSR) near its C-terminus that is essential for bioactivity. Receptor-independent delivery of IFN-gamma via microinjection or secretion-defective intracellular expression elicits IFN-gamma bioactivity. Furthermore, human IFN-gamma fails to activate mouse cells when delivered exogenously because of an inability to bind to the mouse receptor, but it does elicit bioactivity when microinjected into mouse macrophages.3
Signaling by IFN-gamma activates JAK-1 and JAK-2 kinases resulting in the phosphorylation of STAT-1.4 Activated STAT-1 forms a homodimer that is translocated to the nucleus and serves to upregulate several genes. The STAT proteins have, however, no nuclear localization signals, and preliminary data suggest that nuclear translocation of STAT-1 occurs while it is complexed with IFN-gamma and the alpha chain of the IFN-gamma R.2 A number of other cytokines also activate STAT proteins; in every case there is a putative NLS on either the cytokine or its receptor.2
Nuclear translocation requires the cytokine-receptor complex, but the role of these complexes within the nucleus is unknown. Cytokine-receptor complexes act as chaperones to accomplish a cytokine-specific nuclear translocation of activated STATs. These complexes might similarly chaperone other signaling molecules, such as kinases or transcription factors, that function within the nucleus. Cytokines and/or receptors in complex with transcription factors might also provide secondary interactions that help discriminate between similar nuclear response elements within multiple promoters. Whatever details emerge in the nuclear signaling mechanisms, cytokine-specific nuclear translocation has the potential to provide an additional degree of signaling specificity.