Production of Gastrointestinal Tumors in Mice by Modulating Latent Transforming Growth Factor Beta 1 Activation.

Transforming growth factor- β (TGF- β ) and its signaling pathways are important mediators in the suppression of cancers of the gastrointestinal (GI) tract. TGF- β is released from cells in a latent complex consisting of TGF- β , the TGF- β propeptide (LAP) and a latent TGF- β binding protein (LTBP). We previously generated mice in which the LTBP-binding cysteine residues in LAP TGF- β 1 were mutated to serine precluding covalent interactions with LTBP. These Tgfb1 C33S/C33S mice develop multiorgan inflammation and tumors consistent with reduced TGF- β 1 activity. To test whether further reduction in active TGF- β levels would yield additional tumors and a phenotype more similar to Tgfb1 -/- mice, we generated mice that express TGF- β 1 C33S and are deficient in either integrin β 8 or TSP-1, known activators of latent TGF- β 1. In addition we generated mice that have one mutant allele and one null allele at the Tgfb1 locus, reasoning that these mice should synthesize half the total amount of TGF- β 1 as Tgfb1 C33S/C33S mice and the amount of active TGF- β 1 would be correspondingly decreased compared to Tgfb1 C33S/C33S mice. These compound mutant mice displayed more severe inflammation and higher tumor numbers than the parental Tgfb1 C33S/C33S animals. The level of active TGF- β 1 in compound mutant mice appeared to be decreased compared to Tgfb1 C33S/C33S mice as determined from analyses of surrogate markers of active TGF- β , such as P-Smad2, C-Myc, KI-67, and markers of cell cycle traverse. We conclude that these mutant mice provide a useful system for modulating TGF- β levels in a manner that determines tumor number and inflammation within the GI tract. decrease in the amount of active TGF- β 1, but little change in the amount of total (latent plus active) secreted TGF- β 1. These results indicated that LTBP, as part of the LLC, is required for proper latent TGF- β 1 activation. We suggested that these animals were hypomorphs rather than nulls for TGF- β 1 because the inflammation and shortened life span were not as severe as observed in TGF- β 1 null mice. We hypothesized that the Tgfb1 C33S/C33S animals activated some secreted TGF- β 1 SLC, albeit inefficiently. Here, we report a series of experiments designed to test this contention. We reasoned that if we further lowered active TGF- β 1 levels in Tgfb1 C33S/C33S mice by eliminating activators of the SLC or by in the tissues reflect decreased TGF- β 1, and the level of active TGF- β 1 from stomach is decreased when Tgfb1 C33S/C33S mice are compared to wild type mice. However, we were unable to detect statistically significant differences in active TGF- β 1 when tissue extracts from Tgfb1 C33S/C33S animals were compared to either Tgfb1 -/C33S or Tgfb1 C33S/C33S ; Itgb8 -/- tissue extracts. The reason for this is not apparent but might reflect the fact that the amount of active TGF- β 1 in the tissue is close to the limit of sensitivity of the assay. The availability of an assay that directly monitored TGF- β 1 signaling within the tissue would resolve the question of tissue levels of active cytokine.