Cellular responses mediated by the transcription factor STAT1 in murine inflammatory diseases

Abstract

The intracellular transduction of interferon signals from the plasma membrane to the nucleus is mediated by signal transducer and activator of transcription 1 (STAT1) which acts in tetrameric form as a transcription factor in immune responses. In this project, the protomer exchange between STAT1 dimers was studied under kinetic aspects and this process was identified as a potential rate-limiting step in the STAT1 activation/inactivation cycle. The data support an alternative mechanism for the transition between the parallel and anti-parallel conformation of STAT1 dimers based on the dissociation and subsequent reassociation of protomers which does not require reciprocal interactions within the N-terminal domain dimer to stabilize an intermediate conformational state. However, binding to high-affinity STAT-specific target sequences, termed gamma-activated sites (GAS), critically interfered with the exchange dynamics. A tandem GAS element in the promoter of the human gene coding for the cytoskeletal protein ezrin was identified in silico as a putative STAT1 target sequence, and was confirmed to bind dimeric STAT1 on each motif to moderately induce gene transcription. However, mice carrying either an N-terminal substitution mutation of STAT1 affecting cooperative DNA binding or a complete functional knockout of the Stat1 gene did not present with altered expression of ezrin and moesin in bone marrow cells compared with mice expressing the wildtype molecule. In a model of myocardial infarction performed by ligation of the left anterior descending coronary artery, male mice expressing the IFNγ-irresponsive STAT1 mutant displayed higher survival rates, whereas females were protected from adverse cardiac remodelling in the early remodelling phase. The inflammatory infiltrate showed a mild increase in tyrosine-phosphorylated STAT1, while the pool of total STAT1 was significantly reduced in the infarcted areas of knockin animals as compared to their wildtype littermates. In experimental autoimmune encephalomyelitis used as a model for a T helper cell-induced autoimmune disease, cells obtained from lymphatic organs of STAT1-deficient animals at the time of disease onset were hyperproliferative and secreted high amounts of IFNγ and IL-17A. Injection of these mice with lipopolysaccharide during the induction phase of experimental autoimmune encephalomyelitis completely abolished this proliferative phenotype. In summary, the findings demonstrate the role of cooperative DNA binding and tetramer formation of STAT1 in orchestrating complex immunological processes, even in the absence of infectious agents and, furthermore, highlight the function of tyrosine-phosphorylated STAT1 as a key factor facilitating the cross-talk between innate and adaptive immunity.