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Epidermal growth factor receptor and STAT3 signal through KRAS have mutually opposite effects on CTEN

Background: CTEN is a protein located at focal adhesions and has been reported to be an oncogene in the colon, breast, lung, and gastric cancer. In this study, we investigated whether two other proposed mechanisms i.e. EGFR and STAT3 signaling were involved in regulating CTEN expression. 
Methodology: Initially, we manipulated EGFR signaling by (i) stimulation with EGF and (ii) inhibition by the PD153035 in the colorectal cancer cell lines SW620 and C32. In C32, EGF stimulation resulted in the up-regulation of KRAS and CTEN whilst exposure to PD153035 resulted in the down-regulation of both KRAS and CTEN. EGFR activation and inhibition were reflected by, respectively, increased and decreased cell motility although the effect of EGFR activation was lost by CTEN knockdown. In SW620, which harbors a KRAS mutation, modulating EGFR activity in this way did not affect either KRAS or CTEN. STAT3 signaling has also been reported to positively regulate CTEN. We tested this in SW620 by directly knocking down STAT3 and exposing cells to interleukin-6 (an activator of STAT3). STAT3 knockdown resulted in increased CTEN whilst STAT3 activation resulted in the downregulation of CTEN. 
Results: Testing for KRAS expression showed that STAT3 was negatively regulating KRAS and this was reflected in the CTEN expression. Functional analysis however showed that inhibition of STAT3 resulted in a reduction of cell motility in a KRAS and CTEN-independent manner. 
Conclusion: We conclude that both EGFR signals through KRAS to modulate CTEN (and consequently ILK/FAK) and stimulates cell motility. STAT3 however negatively regulates KRAS and consequently CTEN although its net effect is to stimulate motility through an alternative mechanism.