Interferon (IFN)-γ is a key cytokine in the modulation of immune responses
necessary for elimination of viral infections and some cancer cells. IFN-γ signals
through the Janus kinase (JAK)/ signal transducers and activators of transcription
(STAT) signalling pathways. Ataxia telangiectasia mutated (ATM) was identified by
our group as a regulator of IFN-γ responses. Within DNA double-strand break
(DSB) repair, ATM controls chromatin remodelling, which is also indispensable for
transcription. We hypothesised, therefore, that ATM modulated IFN-γ responses at
the transcriptional level by regulating STAT1 chromatin binding and/or chromatin-modifying
proteins. ATM silencing impaired to varying degrees IFN-γ-induced
expression of several IFN-stimulated genes (ISGs) at the mRNA and protein levels
and delayed STAT1 serine 727 phosphorylation within the nucleus. These effects
were independent of ATM enzymatic activity. ATM knockdown impaired IFN-γ-
induced binding of STAT1 to class II transactivator (CIITA) and human leucocyte
antigen (HLA)-DRα promoters, but not to IFN regulatory factor (IRF)-1 and antigen
peptide transporter (TAP) 1 promoters. This could account for HLA Class II
expression being more severely impaired than that of HLA Class I, even though
both are classical IFN-γ target genes. ATM, however, bound to the promoters of all
these ISGs. ATM and STAT1 interacted under basal conditions, with STAT1
becoming tyrosine 701 phosphorylated upon IFN-γ treatment. The siATM
phenotype regarding HLA class II expression was partially or completely
recapitulated by silencing of histone deacetylase (HDAC) 1 and 3, respectively, in
contrast to phenotypes induced by RNAi against HDAC2 and Kruppel-associated
box (KRAB) domain-associated protein 1 (KAP1). KAP1 also acted, directly or
indirectly, as a negative regulator of IFN-γ-induced STAT1 phosphorylation and,
consequently, of HLA class I and II HLA expression. Interestingly, KAP1
interaction with STAT1 decreased after IFN-γ treatment. Overall our data indicates
that ATM plays an important role as a modulator of IFN-γ-mediated transcription
probably by interacting with STAT1, regulating its binding to the promoters of a
specific subset of ISGs and regulating chromatin-modifying proteins such as
KAP1, HDAC1, HDAC2 and HDCA3. Understanding how ATM regulates IFN-γ
signalling may provide new insights into important immune-mediated mechanisms,
such as those involved in tumour suppression and thus have therapeutic value.