The Functions of Signal Transducers and Activators of Transcriptions 1 and 3 as Cytokine-Inducible Proteins

The signal transducers and activators of transcription (STAT)1 and STAT3 genes are specifically activated by phosphorylated STATs 1 and 3, respectively, resulting in large and prolonged increases in the levels of unphosphorylated STATs (U-STATs) in response to interferons (for STAT1) or ligands that activate gp130, such as IL-6 (for STAT3). U-STATs 1 and 3 are transcription factors that drive gene expression by mechanisms distinct from those used by phosphorylated STATs. U-STAT3 drives expression of many proteins not induced by phospho-STAT3, including several that are important in tumorigenesis. U-STAT1 prolongs and increases expression of a subset of proteins induced initially in response to phospho-STAT1, leading to antiviral and immune responses that are long-lived. U-STAT1 levels are also high in some cancers, and the protein products of genes induced by U-STAT1 enhance resistance to DNA damage. Therefore, interferons not only drive short-term expression of proteins that inhibit growth and promote apoptosis and immune surveillance, but also promote long-term expression of proteins that facilitate tumor survival.