MT-MMPs are required for tumor cells to invade 3D collagen matrices. (A) HT1080 cells were induced to invade 3D collagen gels (3.75 mg/ml) as in Fig. 2 in the presence of 1 μM LPA and the synthetic MMP inhibitor GM6001 (5 μM), or TIMP-1, -2, -3, or -4 (5 μg/ml). Arrowheads indicate the position of the tumor cell monolayer at the initiation of invasion. Magnification = 20×. Scale bar = 50 μm. (B) Quantitation of HT1080 invasion from panel A. Data are expressed as mean numbers of invading cells per HPF (20×) (± S.D.) from a minimum of 20 fields.