Defective processing of the transforming growth factor-beta1 in azoxymethane-induced mouse colon tumors.
High levels of the cell growth inhibitor transforming growth factor-beta1 (TGF-beta1) are often found in a variety of human cancers. However, the physiological significance of this overexpression depends on the availability of the biologically active form of TGF-beta1 within the extracellular matrix of the tumor microenvironment. To determine the expression and activation status of TGF-beta1 in chemically induced tumors, 6-wk-old A/J mice were injected intraperitoneally with either azoxymethane (AOM) (10 mg/kg body weight, once a week for 6 wk) or normal saline solution, and colon tumors were isolated 24 wk following the last injection. An enzyme-linked immunosorbent assay for TGF-beta1 revealed a significant increase (1.7-fold, P < 0.05) in total TGF-beta1 protein in tumors. Interestingly, while 80% of the total TGF-beta1 in the control colon tissues was in the active form, only 50% was found to be active in tumors. Together with our earlier observations that TGF-beta1 mRNA levels are unchanged in A/J tumors, these data further support a mechanism whereby elevated TGF-beta1 levels result from a defective activation and turnover of this protein. Because plasmin is known to be a major activator of TGF-beta1 in vivo, we hypothesized that reduced plasmin activity may be responsible for the observed dysregulation of TGF-beta1 processing in these behaviorally benign tumors. With a fluorogenic peptide substrate for serine proteases, a deficiency in plasmin activity was found in the tumors. Furthermore, semiquantitative reverse transcription (RT)-polymerase chain reaction (PCR) analysis of a panel of genes involved in the plasminogen activation system, including plasminogen activator inhibitor-1 (PAI-1), urokinase-plasminogen activator (u-PA), and urokinase-receptor (u-PAR-1), demonstrated a significant upregulation (approximately fourfold to sixfold, P < 0.05) in the expression of each of these genes in the tumor tissue. In addition, no significant changes were observed in the expression levels of thrombospondin-1 (TSP-1) and insulin-like growth factor type II receptor (IGF-IIR), which also mediate the activation of latent TGF-beta1. To gain further insight into the functionality of the TGF-beta1 pathway, cDNA microarrays were performed and the expression levels of a panel of 21 TGF-beta1-specific target genes were determined in AOM-induced tumors that overexpress the ligand. A significant dysregulation in the expression of each of these targets was observed, providing evidence of aberrant TGF-beta1 signaling in tumors. Overall, the present study demonstrates a very low plasmin activity in A/J colon tumors, possibly as a result of the potent inhibitory effect of PAI-1 on the plasminogen activation cascade. The observed deficiency in plasmin activity may not be sufficiently compensated for by other mechanisms of latent TGF-beta1 activation, including TSP-1 and IGF-IIR, thereby resulting in a decreased fraction of the biologically active form of TGF-beta1 and subsequent aberration in TGF-beta1-specific gene regulation in A/J tumors.
T10 T11 T12 T13 T14 T15 T16 T17 T18 T19 T1 T20 T21 T22 T23 T24 T25 T26 T27 T28 T29 T2 T30 T31 T32 T33 T34 T35 T36 T37 T38 T39 T3 T40 T41 T42 T43 T44 T45 T4 T5 T6 T7 T8 T9
GENE-Y GENE-N GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y CHEMICAL GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y CHEMICAL GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y CHEMICAL GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y GENE-Y CHEMICAL CHEMICAL GENE-Y GENE-Y GENE-Y GENE-Y
TGF-beta1 serine proteases plasmin plasminogen plasminogen activator inhibitor-1 PAI-1 urokinase-plasminogen activator u-PA urokinase-receptor u-PAR-1 serine thrombospondin-1 TSP-1 insulin-like growth factor type II receptor IGF-IIR TGF-beta1 TGF-beta1 TGF-beta1 TGF-beta1 TGF-beta1 plasmin AOM PAI-1 plasminogen plasmin TGF-beta1 TSP-1 IGF-IIR TGF-beta1 TGF-beta1 TGF-beta1 transforming growth factor-beta1 azoxymethane TGF-beta1 TGF-beta1 TGF-beta1 TGF-beta1 TGF-beta1 transforming growth factor-beta1 AOM azoxymethane TGF-beta1 plasmin TGF-beta1 plasmin