Fig. 9. RA-induced lumen formation requires metalloproteinase activity. (A,B) Effect of the synthetic MMP inhibitor BB94 on RA-induced lumen formation. J3B1A cells suspended in collagen gels were treated with RA alone (1 µM), co-treated with RA (1 µM) and BB94 (30 nM to 3 µM) or co-treated with RA (1 µM) and the inactive isomer BB1268 (3 µM). In A, cyst formation was quantified after 6 days as described in Materials and Methods and results are expressed as mean percentage of cysts±s.e.m. in 20 randomly selected photographic fields (five fields from each of four separate experiments) per experimental condition. Cyst formation is inhibited in a dose-dependent manner by BB94 but is not affected by the inactive isomer BB1268 (^*P<0.05 and ^**P<0.01 compared to cultures incubated with RA alone). In B, lumen diameter was quantified after 6 days by measuring the minor axis of the central cavity in all cystic colonies. Lumen diameter is reduced in a dose-dependent manner by BB94, but is not significantly decreased by BB1268 (^*P<0.01 compared with cultures incubated with RA alone; ^**not significantly different from cultures incubated with RA alone).