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Tristetraprolin mediates radiation-induced TNF-α production in lung macrophages.

Ray D, Shukla S, Allam US, Helman A, Ramanand SG, Tran L, Bassetti M, Krishnamurthy PM, Rumschlag M, Paulsen M, Sun L, Shanley TP, Ljungman M, Nyati MK, Zhang M, Lawrence TS - PLoS ONE (2013)

Bottom Line: To study the in vivo relevance, mouse lungs were irradiated with a single dose (15 Gy) and assessed at varying times for TTP alterations.In conclusion, irradiation of lung macrophages causes TTP inactivation via p38-mediated phosphorylation and proteasome-mediated degradation, leading to TNF-α production.These findings suggest that agents capable of blocking TTP phosphorylation or stabilizing TTP after irradiation could decrease RILT.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, United States of America. dipray@umich.edu

ABSTRACT
The efficacy of radiation therapy for lung cancer is limited by radiation-induced lung toxicity (RILT). Although tumor necrosis factor-alpha (TNF-α) signaling plays a critical role in RILT, the molecular regulators of radiation-induced TNF-α production remain unknown. We investigated the role of a major TNF-α regulator, Tristetraprolin (TTP), in radiation-induced TNF-α production by macrophages. For in vitro studies we irradiated (4 Gy) either a mouse lung macrophage cell line, MH-S or macrophages isolated from TTP knockout mice, and studied the effects of radiation on TTP and TNF-α levels. To study the in vivo relevance, mouse lungs were irradiated with a single dose (15 Gy) and assessed at varying times for TTP alterations. Irradiation of MH-S cells caused TTP to undergo an inhibitory phosphorylation at Ser-178 and proteasome-mediated degradation, which resulted in increased TNF-α mRNA stabilization and secretion. Similarly, MH-S cells treated with TTP siRNA or macrophages isolated from ttp (-/-) mice had higher basal levels of TNF-α, which was increased minimally after irradiation. Conversely, cells overexpressing TTP mutants defective in undergoing phosphorylation released significantly lower levels of TNF-α. Inhibition of p38, a known kinase for TTP, by either siRNA or a small molecule inhibitor abrogated radiation-induced TNF-α release by MH-S cells. Lung irradiation induced TTP(Ser178) phosphorylation and protein degradation and a simultaneous increase in TNF-α production in C57BL/6 mice starting 24 h post-radiation. In conclusion, irradiation of lung macrophages causes TTP inactivation via p38-mediated phosphorylation and proteasome-mediated degradation, leading to TNF-α production. These findings suggest that agents capable of blocking TTP phosphorylation or stabilizing TTP after irradiation could decrease RILT.

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Schematic model explaining the role of post-translational TTP modifications (phosphorylation and degradation) in radiation-induced TTP inactivation as an upstream regulator involved in increased TNF-α secretion by mouse lung macrophages.
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pone-0057290-g007: Schematic model explaining the role of post-translational TTP modifications (phosphorylation and degradation) in radiation-induced TTP inactivation as an upstream regulator involved in increased TNF-α secretion by mouse lung macrophages.

Mentions: Altered production of TNF-α, a pro-inflammatory cytokine has long been associated with various pathological conditions that affect the normal lung function, including radiation pneumonitis and lung fibrosis [35]. We have previously demonstrated that radiation-induced lung injury could be mitigated via inhibition of the TNF-α receptor I (TNFRI) either by genetic ablation of the receptor or by specific antisense oligonucleotide (ASO) treatment [14]. Furthermore, inhibition of TNF-α signaling did not decrease tumor cell killing, thus providing therapeutic selectivity [14]. More importantly, inhibition of TNF-α by Etanercept (Enbrel) improves lung function for patients with idiopathic pulmonary syndrome, which results from lung injury after high dose chemotherapy [36]. Taken together, these studies suggest a pivotal role of TNF-α in lung complications resulting from cancer treatment. In this report we have elucidated the mechanism of radiation-induced TNF-α production described by a schematic model shown in Fig. 7. We propose that radiation activates the p38 pathway, which inactivates Tristetraprolin (TTP), an RNA binding protein that is a major negative regulator of TNF-α mRNA. Inactivation occurs through both an inhibitory phosphorylation at position Ser178 and by subsequent proteasome-mediated TTP degradation, releasing TTP’s negative regulation and allowing increased TNF-α mRNA stability and secretion. These observations point not only to a critical role of TTP in RILT, but they also suggest that blocking TTP inactivation may protect the normal lung from radiation injury.


Tristetraprolin mediates radiation-induced TNF-α production in lung macrophages.

Ray D, Shukla S, Allam US, Helman A, Ramanand SG, Tran L, Bassetti M, Krishnamurthy PM, Rumschlag M, Paulsen M, Sun L, Shanley TP, Ljungman M, Nyati MK, Zhang M, Lawrence TS - PLoS ONE (2013)

Schematic model explaining the role of post-translational TTP modifications (phosphorylation and degradation) in radiation-induced TTP inactivation as an upstream regulator involved in increased TNF-α secretion by mouse lung macrophages.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3585360&req=5

pone-0057290-g007: Schematic model explaining the role of post-translational TTP modifications (phosphorylation and degradation) in radiation-induced TTP inactivation as an upstream regulator involved in increased TNF-α secretion by mouse lung macrophages.
Mentions: Altered production of TNF-α, a pro-inflammatory cytokine has long been associated with various pathological conditions that affect the normal lung function, including radiation pneumonitis and lung fibrosis [35]. We have previously demonstrated that radiation-induced lung injury could be mitigated via inhibition of the TNF-α receptor I (TNFRI) either by genetic ablation of the receptor or by specific antisense oligonucleotide (ASO) treatment [14]. Furthermore, inhibition of TNF-α signaling did not decrease tumor cell killing, thus providing therapeutic selectivity [14]. More importantly, inhibition of TNF-α by Etanercept (Enbrel) improves lung function for patients with idiopathic pulmonary syndrome, which results from lung injury after high dose chemotherapy [36]. Taken together, these studies suggest a pivotal role of TNF-α in lung complications resulting from cancer treatment. In this report we have elucidated the mechanism of radiation-induced TNF-α production described by a schematic model shown in Fig. 7. We propose that radiation activates the p38 pathway, which inactivates Tristetraprolin (TTP), an RNA binding protein that is a major negative regulator of TNF-α mRNA. Inactivation occurs through both an inhibitory phosphorylation at position Ser178 and by subsequent proteasome-mediated TTP degradation, releasing TTP’s negative regulation and allowing increased TNF-α mRNA stability and secretion. These observations point not only to a critical role of TTP in RILT, but they also suggest that blocking TTP inactivation may protect the normal lung from radiation injury.

Bottom Line: To study the in vivo relevance, mouse lungs were irradiated with a single dose (15 Gy) and assessed at varying times for TTP alterations.In conclusion, irradiation of lung macrophages causes TTP inactivation via p38-mediated phosphorylation and proteasome-mediated degradation, leading to TNF-α production.These findings suggest that agents capable of blocking TTP phosphorylation or stabilizing TTP after irradiation could decrease RILT.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, United States of America. dipray@umich.edu

ABSTRACT
The efficacy of radiation therapy for lung cancer is limited by radiation-induced lung toxicity (RILT). Although tumor necrosis factor-alpha (TNF-α) signaling plays a critical role in RILT, the molecular regulators of radiation-induced TNF-α production remain unknown. We investigated the role of a major TNF-α regulator, Tristetraprolin (TTP), in radiation-induced TNF-α production by macrophages. For in vitro studies we irradiated (4 Gy) either a mouse lung macrophage cell line, MH-S or macrophages isolated from TTP knockout mice, and studied the effects of radiation on TTP and TNF-α levels. To study the in vivo relevance, mouse lungs were irradiated with a single dose (15 Gy) and assessed at varying times for TTP alterations. Irradiation of MH-S cells caused TTP to undergo an inhibitory phosphorylation at Ser-178 and proteasome-mediated degradation, which resulted in increased TNF-α mRNA stabilization and secretion. Similarly, MH-S cells treated with TTP siRNA or macrophages isolated from ttp (-/-) mice had higher basal levels of TNF-α, which was increased minimally after irradiation. Conversely, cells overexpressing TTP mutants defective in undergoing phosphorylation released significantly lower levels of TNF-α. Inhibition of p38, a known kinase for TTP, by either siRNA or a small molecule inhibitor abrogated radiation-induced TNF-α release by MH-S cells. Lung irradiation induced TTP(Ser178) phosphorylation and protein degradation and a simultaneous increase in TNF-α production in C57BL/6 mice starting 24 h post-radiation. In conclusion, irradiation of lung macrophages causes TTP inactivation via p38-mediated phosphorylation and proteasome-mediated degradation, leading to TNF-α production. These findings suggest that agents capable of blocking TTP phosphorylation or stabilizing TTP after irradiation could decrease RILT.

Show MeSH
Related in: MedlinePlus