Tuning up STAT1

The transcription factor signal transducer and activator of transcription-1 (STAT1) is a linchpin of immunity. In the presence of the inflammatory cytokine interferon-γ (IFN-γ), STAT1 becomes phosphorylated, dimerizes, and translocates to the nucleus, where it activates transcription of genes needed for resistance to pathogens. However, STAT1 can promote the transcription of different subsets of genes, depending upon the context in which its activation takes place. One explanation for this behavior could be that other cellular factors—for example, other transcription factors or DNA-binding proteins—can modulate the activity of STAT1 at different target genes. But if such factors exist, what are they? That’s the question Daniel Beiting, Sara Cherry, and colleagues sought to address in their paper published this month in PLOS Biology [1]. The researchers reasoned they might be able to find factors that modulate STAT1 activity by looking for proteins that can overcome STAT1 inhibition. Such inhibition frequently occurs during infection by certain pathogens, which attack STAT1 in order to evade detection and control by the immune system. The methods used by pathogens to frustrate STAT1 activity are as varied as the pathogens themselves; some manufacture proteins that interfere with STAT1 phosphorylation or nuclear translocation, whereas others specifically target STAT1 for degradation. Yet another strategy is the one used by the intracellular parasite Toxoplasma gondii, which allows STAT1 to reach the nucleus but then blocks the protein’s ability to activate its target genes in the nucleus. This pathogen likely attacks STAT1 because IFN-γ activity and STAT1 activity are essential for controlling T. gondii infections during both acute and systemic infections and are also important for restraining T.gondii replication in the brain, where the parasite encysts and persists during chronic infection (Fig 1). Beiting and colleagues reasoned that there may be endogenous genes that, when ectopically expressed, can overcome the T. gondii–induced block on STAT1-mediated gene transcription. They therefore initiated a high-throughput screen, interrogating a library of 18,000 mouse and human cDNAs to identify genes that enhance STAT1 activity in T. gondii–infected cells. The screen turned up seven genes, each encoding transcription factors that specifically enhance STAT1 activity. Two of these, COUPTF2 and TLX, belong to the class of orphan nuclear receptors—transcriptional regulators whose natural ligands have not yet been identified but that often exhibit important functions in activating or repressing gene transcription. Because TLX is prominently expressed in the brain, the site of chronic T. gondii infections, the researchers decided to focus their efforts on understanding how this protein modulates STAT1 activity. First, Beiting et al. sought to characterize the genes whose expression might be modulated by TLX by examining the effects of TLX overexpression in a human osteosarcoma cell line. They found that TLX by itself enhanced expression of more than 100 genes involved in neuron differentiation and tissue morphogenesis. However, in the presence of IFN-γ, overexpressed TLX also promoted the up-regulation of many genes related to immune function, including several that are known targets of STAT1. Conversely, knockdown of TLX in astroglioma cells,