Effect of increasing tensile strain on MMP-13 mRNA and protein levels in mouse osteoblastic MC3T3-E1 cells. Cells were seeded at a density of 2 × 105 cells/well on Flex I culture plates and cultured in α-MEM medium supplemented with 10% FBS for 24 h. The cells were then loaded with (6%, 12%, 18%) or without (0%) cyclic tensile strain at the indicated percent elongation at 6 cycles/min using a Flexercell strain unit for 24 h. The levels of MMP-13 mRNA and protein were determined by RT-PCR and immunoblot analysis, respectively, as described in the Materials and methods. (A) Agarose gel electrophoresis of the RT-PCR products using specific primers for MMP-13 or GAPDH. (B) MMP-13 mRNA expression levels normalized to GAPDH. (C) Immunoblot analysis of MMP-13 and GAPDH. (D) MMP-13 protein expression levels normalized to GAPDH. The results shown are means ± SD of five independent experiments. Significant differences from the 0% culture are shown by ANOVA and Student's t test (*P < 0.05, ** P < 0.01).