TNF-a increases transcription of Ga_i-2

Context

Tumour necrosis factor (TNF)-a, a proinflammatory cytokine that is produced in the lung, is found in increased amounts in the airways of symptomatic patients with asthma. TNF-a is thought to play a role in increasing airway smooth muscle tone and reactivity. It has been previously shown that TNF-a upregulates Ga_i-2 in various cell types, although the mechanism of TNF-a-induced increases in Ga_i-2 protein levels is unknown. In this report, the mechanism of TNF-a-mediated upregulation of Ga_i-2 in human airway smooth muscle cells is investigated.

Significant findings

Treatment with TNF-a increased Ga_i-2 protein abundance in human airway smooth muscle cells. The increase was not due to a change in viability, cell number, or alterations in protein half-life. Rather, TNF-a increased the steady-state level of Ga_i-2 mRNA. The authors conclude that upregulation of Ga_i-2 mRNA levels was due to an increase in transcription alone and not due to an increase in mRNA half-life. They did not rule out the possibility that other mechanisms that regulate protein level, such as translational efficiency, mRNA splicing, or other intracellular signalling pathways could be involved.

Comments

The authors now add Ga_i-2 to the growing list of signalling proteins that are regulated by TNF-a. TNF-a increases Ga_i-2 protein abundance secondary to increasing Ga_i-2 mRNA transcription. In airway smooth muscle, TNF-a impairs ?-adrenoceptor-mediated airway smooth muscle relaxation and enhances methacholine airway responses. An increase in Ga_i-2-mediated inhibition of adenylyl cyclase and subsequent decrease in cellular cAMP levels leading to impaired smooth muscle cell relaxation could account for these effects. This study elucidates the mechanism of TNF-a-mediated regulation of a specific G protein and its potential effect on airway smooth muscle cell tone.

Methods

Immunoblot analysis, RNA extraction, ribonuclease protection assay