SYK and chronic obstructive pulmonary disease: To further verify whether the PLE attenuated COPD airway inflammation via inhibition of the TLR4/Syk pathway, inhibitors of TLR4 (TAK242, 5 nM), Syk (BAY61-3,606, 10 μM), PKC (Rottlerin, 2.5 μM), and NF-κB p65 (BAY11-7,082, 5 μM) were employed to confirmatory study in vitro.