DNA damage signals facilitate osmotic stress adaptation
Dietmar Kültz
Physiological Genomics Group, Department of Animal Science, University of California, Davis, California
Submitted 22 April 2005
; accepted in final form 25 April 2005
OSMOTIC STRESS OCCURS WHEN the concentration of dissolved compounds in water surrounding cells and organisms changes. For example, changes in dissolved salt (inorganic ion) concentration in the environment are common for soil bacteria exposed to drought or rain, intertidal and estuary organisms facing fluctuating salinity, and kidney cells of the mammalian renal medulla. Osmotic changes in the environment lead to transient disequilibrium of ion concentration inside animal cells (and, in many cases, cell volume) that is detrimental to cell structure and function. Such changes are the cause of many diseases. All cells have adaptive mechanisms to cope with osmotic stress and restore intracellular inorganic ion homeostasis (and volume). In mammalian renal inner medullary cells, such adaptive mechanisms are in large part mediated by the transcription factor tonicity response element binding protein (TonEBP; also called OREBP). Therefore, the specific mechanisms and proteins by which TonEBP activity is regulated are of great interest to renal physiologists. In this issue of AJP-Renal Physiology, Zhang and co-workers (26) show that the ataxia telangiectasia-muted (ATM) kinase is an upstream regulator of TonEBP that promotes nuclear translocation of TonEBP.
This interesting finding strengthens the mechanistic link between DNA damage signals and osmotic stress adaptation. Hyperosmolality causes DNA double-strand breaks (13) that are recognized by the cellular genome integrity surveillance machinery. This multiprotein machinery includes the mediator of DNA damage checkpoint 1 (MDC1) protein and the Mre11-Rad50-Nbs1 (MRN) protein complex (1). The MRN complex recruits ATM to broken DNA and activates it by promoting Ser-1981 autophosphorylation and stimulating its kinase activity (7, 17). The work reported by Zhang et al. (26) shows that ATM promotes TonEBP function by supporting its translocation into the nucleus where it can bind to osmoprotective target genes (Fig. 1). The same laboratory had previously shown that TonEBP and ATM physically associate and that TonEBP activity may be increased by phosphorylation on Ser-1197, Ser-1247, or Ser-1367, which represent ATM consensus phosphorylation motifs (8). Therefore, DNA damage signals resulting in activation of ATM contribute to TonEBP activation.

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Fig. 1. The osmotic stress factor tonicity response element binding protein (TonEBP) is regulated by a complex signaling network including DNA damage-sensing protein kinases [DNA-PK; ataxia telangiectasia-muted (ATM) kinase]. When activated by this signaling network, TonEBP induces osmoprotective target genes, including heat shock protein 70 (HSP70), urea transporter (UTA), and genes reponsible for the accumulation of compatible organic osmolytes. MRN, Mre11-Rad50-Nbs1.
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An increase in extracellular osmolality caused by plasma membrane-impermeable solutes such as NaCl (hypertonicity) leads to TonEBP activation and increased transactivation of TonEBP target genes via multiple mechanisms. First, TonEBP is itself upregulated by hypertonicity (23). The increase in TonEBP mRNA abundance is transient at the mRNA level (maximum between 4 and 12 h depending on cell type) and more sustained at the protein level, indicating that multiple mechanisms may be involved (2, 10, 22). At the mRNA level, TonEBP is stabilized rather than transcriptionally induced (2). Second, TonEBP is activated during hypertonicity via increased phosphorylation (3). Third, TonEBP forms dimers and dimerization correlates with increased transactivation (19). Finally, nuclear translocation of TonEBP represents another mechanism of activation. These different mechanisms of TonEBP activation may be closely interrelated, and some are known to be prerequisite for others. For instance, TonEBP dimerization is necessary for its phosphorylation (19). The study by Zhang et al. (26) demonstrates that nuclear translocation of TonEBP is promoted by ATM kinase. As a result of hypertonic activation, TonEBP induces a number of prominent target genes that are critical for cellular adaptation to hypertonicity, including urea transporter (UT-A), heat shock protein 70 (HSP70), and the organic osmolyte response genes betaine/glutamate transporter 1 (BGT1) and aldose reductase (AR) (24) (Fig. 1).
ATM kinase is necessary but not sufficient for full activation of TonEBP (26). This may explain why ATM activation by DNA damage resulting from stresses other than hyperosmolality does not affect TonEBP (8). Other kinases are involved in TonEBP activation, but none of them by themselves is sufficient. They include DNA-PK (18), PKA (6), MAPKs (21), and Fyn (9). Moreover, as yet undiscovered proteins may stimulate or block TonEBP activity (Fig. 1). The relative importance of different regulatory proteins for TonEBP activity may differ depending on cell type (cellular differentiation state) because some of the above kinases do not always regulate TonEBP (14, 16).
Like ATM, DNA-PK is also activated in response to DNA damage signals. Both kinases are upstream of an elaborate signaling network that controls cell cycle checkpoints, DNA repair, and apoptosis. Indeed, hyperosmolality causes DNA damage in the form of double-strand breaks (13) and oxidative base modification (25), growth arrest (15), and (when cellular tolerance limits are exceeded) apoptosis (20). An increase in oxidative damage was recently shown to contribute to hypertonic activation of TonEBP (27). Other critical components of checkpoints that are activated by ATM and DNA-PK in response to DNA damage during hyperosmolality include p53, GADD45 proteins, and GADD153 (4, 11, 15). In addition, hyperosmolality seems to modulate the function of Mre11, which is part of the MRN complex discussed above and an upstream regulator of ATM and DNA-PK (5).
The work by Zhang et al. (26) reinforces the emerging view that complex signaling networks rather than simple linear pathways are responsible for the regulation of osmoprotective genes (12). Such networks seem to have an inherent redundancy and flexibility to account for different fates of cellular differentiation. One important question that stands out concerns the generation of stressor-specific output from TonEBP when signals converging at this transcription factor (such as those conferred by ATM activation) are themselves relatively nonspecific. There are two potential solutions to this conundrum: first, additional signals generated by specific osmosensors may be necessary for activation of TonEBP under physiological conditions. Alternatively, although individual signals may be nonspecific, they may converge at TonEBP in particular combinations that only occur during hyperosmotic stress. Zhang et al. (26) show in their study that ATM kinase is one of the upstream regulators of TonEBP that is required for its activation during hyperosmotic stress. Their work stimulates future research directed at elucidating the interplay of different signals at the level of the transcription factor TonEBP, which is a key element of osmotic stress adaptation in animal cells.
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GRANTS
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This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-59470.
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FOOTNOTES
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Address for reprint requests and other correspondence: D. Kültz, Physiological Genomics Group, Dept. of Animal Science, Univ. of California, One Shields Ave., Meyer Hall, Davis, CA 95616 (e-mail: dkueltz{at}ucdavis.edu)
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