The influence of elevated CO2 on NF-kappaB pathway activation

Abstract

Carbon dioxide (CO2) makes up 0.04% of the earth’s atmosphere and plays a vital role in many biological processes, including photosynthesis and respiration. Abnormal elevations in CO2 levels (hypercapnia) within the body can be pathogenic or medically induced (termed permissive hypercapnia). Permissive hypercapnia has been found to improve the morbidity and mortality rates of patients suffering from Acute Respiratory Distress Syndrome (ARDS). This has led to the discovery that hypercapnia acidosis potentially reduces both lung injury and NF-kappaB induced inflammation. CO2 is hydrated into carbonic acid by the enzyme carbonic anhydrase, which spontaneously dissociates into bicarbonate and hydrogen ions. Therefore, when CO2 increases there is also an increase in hydrogen ions, causing an acidosis. There is conflicting evidence which indicates that both the elevations in CO2 and secondary pH reductions are the primary cause for attenuating NF-kappaB activation. Evidence from this thesis would indicate that both elevated CO2 and reductions in pH, via different mechanisms, reduces NF-kappaB activation within the NF-kappaB/293/GFP/LucTM reporter cell line.

CO2 is normally an inert gas but under physiological conditions CO2 can interact with neutral amines to form carbamates. Recent advancements in trapping this highly labile interaction, has led to the discovery of carbamate formation at K48 of ubiquitin. Ubiquitin can form chains which tag proteins, and K48 linked polyubiquitin chains signal for protein degradation via the 26S proteasome. NF-kappaB activation requires the degradation of the inhibitor, IkappaBalpha, via K48 polyubiquitin tagging. It is therefore hypothesised that hypercapnia reduces NF-kappaB activation via carbamylation of K48. This would then prevent K48 polyubiquitination and the breakdown IkappaBalpha. Evidence from this thesis confirms the importance of K48 linked ubiquitin for NF-kappaB activation, and provides compelling evidence that hypercapnia acts via reducing K48 polyubiquitination.