Inhibition of p21 Activated Kinase (PAK) Reduces Airway Responsiveness In Vivo and In Vitro in Murine and Human Airways

Abstract

The p21-activated protein kinases (Paks) have been implicated in the regulation of smooth muscle contractility, but the physiologic effects of Pak activation on airway reactivity in vivo are unknown. A mouse model with a genetic deletion of Pak1 (Pak1 −/−) was used to determine the role of Pak in the response of the airways in vivo to challenge with inhaled or intravenous acetylcholine (ACh). Pulmonary resistance was measured in anesthetized mechanically ventilated Pak1 −/− and wild type mice. Pak1 −/− mice exhibited lower airway reactivity to ACh compared with wild type mice. Tracheal segments dissected from Pak1 −/− mice and studied in vitro also exhibited reduced responsiveness to ACh compared with tracheas from wild type mice. Morphometric assessment and pulmonary function analysis revealed no differences in the structure of the airways or lung parenchyma, suggesting that that the reduced airway responsiveness did not result from structural abnormalities in the lungs or airways due to Pak1 deletion. Inhalation of the small molecule synthetic Pak1 inhibitor, IPA3, also significantly reduced in vivo airway responsiveness to ACh and 5-hydroxytryptamine (5-Ht) in wild type mice. IPA3 inhibited the contractility of isolated human bronchial tissues to ACh, confirming that this inhibitor is also effective in human airway smooth muscle tissue. The results demonstrate that Pak is a critical component of the contractile activation process in airway smooth muscle, and suggest that Pak inhibition could provide a novel strategy for reducing airway hyperresponsiveness.

Tracing PAKs from GI inflammation to cancer

“…24 103 – 105 One likely driver of PAK1 activation in IBD is TNFα 88. PAK1 activation triggers signalling pathways which drive proliferation, initiate cell migration or block apoptosis, and may contribute to a loss of intestinal barrier function.…”

Idiopathic gastroparesis is associated with specific transcriptional changes in the gastric muscularis externa

“…Similarly, PAK1 also phosphorylates myosin light chains, in addition to the myosin phosphatase targeting subunit MYPT1, to promote smooth muscle contractility . Studies in mice have shown that decreased expression of Myh11, Mylk1, or PAK1 each individually result in impaired SMC contractility and GI motility . SRF‐myocardin complexes regulate transcription of many smooth muscle contractile proteins including both MYH11 and MYLK1 .…”

“…33 Studies in mice have shown that decreased expression of Myh11, Mylk1, or PAK1 each individually result in impaired SMC contractility and GI motility. 25,32,34 SRFmyocardin complexes regulate transcription of many smooth muscle contractile proteins including both MYH11 and MYLK1. 25,26,35,36 Although we did not observe a significant change in myocardin mRNA in samples from patients with idiopathic gastroparesis, we did see a significant decrease in SRF mRNA (Figure 3).…”

The small GTPase Rac1 is required for smooth muscle contraction

“…Knockout or pharmacological inhibition of Pak reduces tone of airway smooth muscle (Hoover et al . ). Introduction of Pak3 in permeabilized smooth muscle induces a Ca 2+ ‐independent contraction, possibly via phosphorylation of caldesmon (Van Eyk et al .…”