Hypercapnia Induces Cleavage and Nuclear Localization of RelB Protein, Giving Insight into CO2 Sensing and Signaling

Viktig studie om CO2 som betennelsesdempende, og den fremtredende rolle i moderne forskning, og hvordan kroppen og cellene bruker det som signalstoff. Nevner at det demper betennelse ved å dempe pro-inflammatoriske genuttrykk.


Carbon dioxide (CO2) is increasingly being appreciated as an intracellular signaling molecule that affects inflammatory and immune responses.

In patients suffering from this syndrome (COPD), therapeutic hypoventilation strategy designed to reduce mechanical damage to the lungs is accompanied by systemic hypercapnia and associated acidosis, which are associated with improved patient outcome.

Recently, a role for the non-canonical NF-κB pathway has been postulated to be important in signaling the cellular transcriptional response to CO2. (Om NF-kB her: http://en.wikipedia.org/wiki/NF-κB)

Taken together, these data demonstrate that RelB is a CO2-sensitive NF-κB family member that may contribute to the beneficial effects of hypercapnia in inflammatory diseases of the lung.

The physiologic gas nitric oxide (NO) is sensed by cells and profoundly impacts upon intracellular signaling pathways through altering the activity of enzymes, including guanylate cyclase and cytochrome c oxidase (1,2). Furthermore molecular oxygen, another physiologic gas, is also sensed by cells and elicits signaling responses through altering hydroxylase activity, leading to activation of the hypoxia-inducible factor (HIF) (3). Carbon dioxide (CO2), a product of oxidative metabolism, is another physiologic gas with a recently appreciated role in the suppression of proinflammatory transcriptional pathways (4).

Patients in respiratory distress who are placed on ventilators have intentionally lowered tidal and minute volumes to protect the lungs against mechanical damage (810). This leads to an increase in paCO2. This protective ventilation strategy is termed “permissive hypercapnia.” In addition to reducing ventilator-associated lung injury, permissive hypercapnia has been demonstrated to decrease mortality in acute respiratory distress syndrome patients (11,12).

The NF-κB family of transcription factors is responsible for the regulation of innate immune, inflammatory, and anti-apoptotic gene expression. We have previously demonstrated a link between hypercapnia and NF-κB signaling (15). Elevated CO2 leads to a less inflammatory phenotype via the suppression of NF-κB-dependent proinflammatory gene expression (10).

In this study, we demonstrate that under conditions of elevated CO2, RelB is cleaved to a low molecular weight form that translocates to the nucleus, where it impacts upon the expression of proinflammatory genes. We dissected the relative contribution of CO2 and pH to RelB processing and inflammatory gene expression. Furthermore, we investigated the requirement of RelB for the suppression of specific inflammatory gene expression under conditions of elevated CO2. Finally, we provide mechanistic insight into RelB processing in response to CO2.

Taken together, these studies indicate that CO2 is a physiologic regulator of inflammatory gene expression and that non-canonical NF-κB family members are key to mediating the anti-inflammatory effects of CO2.

Arterial levels of CO2 can range from ~25 mm Hg (~3.6%) to >100 mm Hg (~13%) in pathophysiologic states. To determine the range of sensitivity of RelB to CO2, we exposed MEFs to 2% or 10% CO2 for 1 h before re-equilibration to 0.03% CO2 conditions for 5 min in each case. We observed a dose-dependent nuclear accumulation of RelB at 2% CO2, which was significantly more pronounced at 10% CO2 (Fig. 2A). Furthermore, return to ambient CO2 levels resulted in a rapid reversal of nuclear RelB localization (Fig. 2A), confirming that the impact of elevated CO2 on RelB is both rapid and reversible.

Hypercapnia is usually accompanied by acidosis in vivo. This is because CO2 forms carbonic acid in solution, leading to a cellular microenvironment that is both hypercapnic and acidic.

Leukocyte nuclear RelB staining in lungs from LPS-treated rats was significantly increased in the 5% CO2 group compared with the 0% CO2 group (Fig. 3, B and C). This enhanced nuclear RelB staining in the therapeutic hypercapnic acidosis group is associated with better survival, improved lung function, and a significant degree of lung protection as a consequence of reduced inflammatory damage (28). These data provide further supportive evidence for RelB nuclear localization under conditions of hypercapnia both in vivo and in vitro and demonstrate a correlation between nuclear RelB expression and improved disease outcome.

Under both neutral and acidic conditions, elevated CO2 suppressed TNFα to the same degree (Fig. 4A), indicating that the effects of elevated CO2 on inflammatory gene expression are independent of alterations in pHe. Thus, exposure to hypercapnia suppresses TNFα-stimulated inflammatory gene expression. Consistent with our previous studies (15), buffering pHe to a neutral value did not affect the suppressive effects of elevated CO2.

In summary, elevated CO2 suppresses cytokine-stimulated inflammatory gene expression, and this suppression is modestly enhanced in cells in which RelB expression is suppressed. Although the specific mechanism remains to be determined, these data support a role for RelB in the regulation of inflammatory gene expression under conditions of hypercapnia.

Hypercapnia is defined as the situation that arises when blood pCO2 is higher than normal. It is associated with a range of diseases, including chronic obstructive pulmonary disease, and is a clinically tolerated consequence of a low tidal volume ventilation strategy for acute respiratory distress syndrome (6). Low tidal volume ventilation strategies have come to prominence given the significant decrease in patient mortality seen with this approach compared with the traditional ventilation strategy in a large multicenter trial (12).

RelB is an NF-κB family member that, along with p52, forms the characteristic dimer of the non-canonical pathway. Knockdown of RelB has previously been demonstrated to impair cellular immunity and to lead to multi-organ inflammation (20), suggesting an anti-inflammatory role for RelB. In addition, RelB acts downstream of signaling molecules previously shown to be involved in CO2signaling.

In summary, we have identified a novel signaling event in which RelB becomes cleaved and localizes to the nucleus under conditions of hypercapnia and hypercapnic acidosis in vitro and is associated with improved outcome in an in vivo model of LPS-induced lung injury. Hypercapnia can influence ligand-induced NF-κB target gene expression independently of pHe. Hypercapnia-dependent RelB processing and localization are sensitive to MG-132 but do not involve GSK3β or MALT-1, as has been described in other models (23). Taken together, they provide new mechanistic insight into the molecular mechanisms underpinning CO2 signaling, with significant implications for clinical medicine.

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