Effects of growth differentiation factor-6 (GDF-6) on p38 phosphorylation under pro-inflammatory condition. (a) Time-course change in p-p38 protein levels relative to p38. We stimulated human intervertebral disc (IVD) nucleus pulposus (NP) cells with 10-ng/mL interleukin (IL)-1β and measured the protein levels of p-p38 and p38 at different time points, i.e., 0 (set as 1.0), 15, 30, 45, and 60 min. (b) p-p38 protein levels relative to p38 in IVD NP cells treated with phosphate-buffered saline (group C; set as 1.0), GDF-6 (100 µg/mL, group G), IL-1β (10 ng/mL, group I), and both (group G + I) for 15 min. Data are the mean ± standard deviation. Results from six independent experiments were analyzed using one-way repeated-measures analysis of variance with the Tukey–Kramer post-hoc test (n = 6). Significant differences are set as * p < 0.050 and ** p < 0.010.