Figure 9

Schematic diagram illustrating the specific cascade and signalling pathway in LPS-primed macrophages stimulated with nanoparticles. LPS priming induces transcription of pro-IL-1β gene upon activation of the transcription factor NF-κB (1). Nanoparticle uptake (2) leads to the active release of intracellular ATP (3) through pannexin/connexin hemichannels (4). This extracellular ATP (eATP) may activate ATP-gated P2X7 receptor (P2X7) to amplify ATP release in a P2X7-dependent way (5). ATP or its derived catabolism products act through on other P1 or P2 (P2X and P2Y) purinergic receptors. In particular, ATP via P2Y2 and ADP through P2Y1 activate PLC-β, which promotes NLRP3 inflammasome via modulation of cellular Ca²⁺ and K⁺ flux (6). Adenosine (Ado), another hydrolysed product of ATP, activates P1 receptors (A2_A, A2_B and A3) leading to NLRP3 inflammasome activation (7). After NLRP3 receptor activation via signalling through multiple purinergic receptors, NLRP3 inflammasome builds up and matures pro-IL-1β (9) into IL-1β, which is secreted by macrophages (10). In case of extracellular Ado accumulation, equilibrative nucleotide transporters (ENTs) regulate adenosine through its cellular reuptake, which may be inhibited by the ENT inhibitors MBMPR and/or 5-Iodotubercidin (a). Metabolisation of intracellular Ado into ATP by adenosine kinase (AK) renews ATP stock (b) and may be inhibited by the AK inhibitor 5-Iodotubercidin. Increased intracellular ATP contents may lead to ATP release (4), NLRP3 inflammasome activation (9) and IL-1β secretion (10)