MMP1 and atrial fibrillation: The authors demonstrated that AF/NP cells interact with ECs (immortalized human microvascular endothelial cells, HMEC-1) also suggesting a possible time line: AF cells respond to IL-1β early during the DDD course (resulting in an increased expression of IL6, -8, MMP1, 3 and VEGF family members), while NP cells interact with ECs by responding with higher levels than AF to ECs secreted factors (e.g. production of IL-6 and -8, VEGF, and MMP-3 was significantly higher in NP cells than in AF cells, under the presence of ECs conditioned medium).