The Regulation of Growth Factor Receptors EGFR and IGF-IR and the Growth Factor VEGF by Thioredoxin-1

by Bair III, Warner B.

Thioredoxin-1 (Trx-1) is a redox protein that is overexpressed in many tumors where it is associated with tumor growth, inhibited apoptosis and decreased patient survival. Through redox reactions, Trx-1 is able to reduce a number of proteins including transcription factors. Sp1 activation has been implicated in the regulation of many genes involved in cellular growth and survival and its overexpression in certain cancer correlates with decreased patient survival. We demonstrate that Trx-1 is able to activate Sp1 in a redox dependent manner. Trx-1 overexpression increases Sp1 transactivation and DNA binding whereas a redox inactive Trx-1 has no effect on Sp1 DNA binding. Sp1 has been implicated in vascular endothelial growth factor regulation and we have shown that Trx-1 expression results in increased hypoxic VEGF expression and increased tumor permeability in vivo. Trx-1 overexpression results in an increase in VEGF expression that is dependent upon Sp1, as inhibition of Sp1 expression with siRNA prevented the induction of VEGF expression by Trx-1. These results suggest that Trx-1 increases VEGF expression under normoxic conditions through a redox dependent increase in the DNA binding of the Sp1 transcription factor. VEGF regulation by Sp1 could increase angiogenesis in relatively perfused areas contributing to the stimulation of tumor growth by Trx-1. We hypothesized that Trx-1 regulation of Sp1 may be part of the mechanism of Trx-1 induction of cellular growth. Sp1 regulates many genes involved in cellular growth including epidermal growth factor receptor (EGFR) and insulin-like growth factor I receptor (IGF-IR). These two growth factor receptors are important for cellular growth and have been shown to be important therapeutic targets for cancer treatment. We report 9 that treatment with the Trx-1 inhibitor PX-12 results in decreased Sp1 DNA binding as well as decreased Sp1 activation and transactivation of VEGF, EGFR, and IGF-IR. These results indicate that Trx-1 promotes cellular growth and survival, in part, through the redox regulation of Sp1 responsive growth genes EGFR and IGF-IR. Inhibition of Trx-1, via PX-12, results in a decrease in EGFR and IGF-IR expression and suggests a new mechanism by which Trx-1 inhibition is clinically effective for treating cancer. 10