TLR4 and acute respiratory distress syndrome: The natural compound tetramethylpyrazine (TMP), an active ingredient extracted from ligusticum chuanxiong, alleviates ALI by blocking various signaling pathways including TLR4/TRAF6/NF-κB/NLRP3/caspase-1 and TLR4/caspase-8/caspase-3.