Homeodomain proteins directly regulate ATM kinase activity




Johnson, Tanya Ellen

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The ataxia-telangiectasia mutated (ATM) kinase is a master regulator involved in the detection and repair of DNA double-strand breaks (DSBs). The Mre11/Rad50/Nbs1 (MRN) complex recruits and activates ATM at DSBs, causing a signaling cascade to initiate cell-cycle checkpoint arrest, DNA repair, and apoptosis. Alternatively, ATM can be activated by direct oxidation and may act as a redox sensor in the cellular response to oxidative stress. Loss of functional ATM results in ataxia-telangiectasia, a genomic instability disorder characterized by neurodegeneration, DNA repair defects, and predisposition to cancer. Understanding how ATM kinase activity is regulated is critical to understanding its function in the DNA damage and oxidative stress responses. Recently, ATM kinase activity was shown to be stimulated directly by a homeodomain protein, NKX3.1, in prostate cells. NKX3.1 is thought to be the gatekeeper to prostate tumor suppression and, although normally expressed solely in prostate cells, as many as 80% of prostate tumors exhibit loss of NKX3.1 protein. One mechanism of tumor suppression by NKX3.1 may be modulating the DNA damage response via ATM. In in vitro kinase assays, NKX3.1 protein was sufficient to stimulate ATM both in the MRN-mediated and oxidative pathways. Thus, NKX3.1 acts as a tissue-specific modulator of ATM function.
Since the conserved homeodomain was critical for the stimulation of ATM by NKX3.1, it poses the question whether this is a general mechanism of ATM regulation by all homeodomain proteins. Here, I address this question by identifying other tissue-specific regulators of ATM activity within the homeodomain family of proteins. Five homeodomain proteins (TTF1, HOXB7, NKX2.5, NKX2.2, and CDX2) were tested for the ability to regulate ATM activity. In in vitro kinase assays, all of the homeodomain proteins tested stimulated ATM kinase activity to varying degrees, suggesting that the homeodomain itself may act as a conserved regulator of ATM function. I show that CDX2 modulates ATM function in mammalian cell lines with global effects on the DNA damage response. Together, these data support the hypothesis that homeodomain proteins regulate ATM function in a tissue-specific manner.



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