Thioredoxin is downstream of Smad7 in a pathway that promotes growth and suppresses cisplatin-induced apoptosis in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive human malignancy in which Smad7 is commonly overexpressed. Analysis by differential display identified thioredoxin-1 (TRX) as a gene whose basal expression is increased in COLO-357 pancreatic cancer cells engineered to overexpress Smad7. To delineate the biological consequences of TRX overexpression, we assessed TRX mRNA levels in PDAC and studied the effects of increased TRX levels in Smad7-overexpressing cells. By northern blotting, TRX mRNA levels were increased in PDAC samples by comparison with the normal pancreas. Moreover, analysis of laser-captured pancreatic cancer cells revealed parallel increases in Smad7 and TRX mRNA levels. Retroviral infection of an antisense TRX cDNA suppressed TRX protein levels and blunted the increased capacity of Smad7-overexpressing cells to form colonies in soft agar. 1-Methyl-propyl-2-imidazolozyl disulfide, a TRX inhibitor, markedly suppressed the growth of sham-transfected COLO-357 cells and enhanced the growth inhibitory actions of cis-diamminedichloroplatinum(II) (CDDP). CDDP also induced apoptosis, as evidenced by induction of DNA laddering, PARP cleavage, and caspase-3/9 activities. These pro-apoptotic actions were greatly attenuated in Smad7-overexpressing cells, which exhibited a more prolonged association of TRX with the apoptosis inducer apoptosis signal-regulating kinase-1, and enhanced nuclear factor kappaB activation in response to CDDP. These findings suggest that TRX is downstream of Smad7 in a pathway that confers a growth advantage to pancreatic cancer cells and that increases their resistance to CDDP-mediated apoptosis, implying novel regulatory functions for Smad7.