MMP13 and atrial fibrillation: The mRNA expression levels of MMP-3 in AF cells and those of MMP-13 in both AF and NP cells were significantly further upregulated by stimulation of RANKL with rhIL-1β (1.0 ng/mL) compared to those of rhIL-1β only (relative expression: RANKL 10 ng/mL + IL-1β (vs. RANKL 0 ng/mL + IL-1β), AF: MMP-3 2.75 ± 0.38; MMP-13 6.20 ± 1.82; NP: MMP-13 1.69 ± 0.35; p < 0.01, respectively).