Induction of C-Terminal SRC Kinase (CSK) Activity as a Putative Mediator of Chemoprevention by Polyethylene Glycol (PEG)

Abstract

Purpose: Polyethylene glycol (PEG) has shown remarkable efficacy in suppressing colon carcinogenesis in experimental models outperforming conventional agents such as NSAIDS (Corpet and Pierre Cancer Epidemiol Biomarkers Prev 2003). Moreover, its outstanding long term clinical safety record (treatment of chronic constipation) makes it an attractive agent for chemoprevention of colon carcinogenesis. Our group and others have previously shown that PEG decreases epithelial hyperproliferation; however, the molecular mechanisms of PEG remain unclear (Mol Cancer Therap 2006). Emerging data indicates that the tumor suppressor gene, c-terminal Src kinase (CSK), plays a central role in control of proliferation. CSK is involved both in progression and also initiation of colon carcinogenesis (FEBS 2005), making it a plausible target in chemoprevention. We, therefore, wanted to assess the effect of PEG treatment on CSK. Methods: HT-29 cells (human CRC cell line) were treated with vehicle or 10% PEG3350 for 24–72 h and subjected to mRNA and protein analysis by RT-PCR and Western blotting, respectively. Standard techniques were used and immunoblots were analyzed with densitometry. CSK activity was gauged by an immunoprecipitation-kinase assay. Results: PEG treatment did not effect the expression of CSK protein or message (91.9 ± 11.2% and 97.1 ± 3.9%). PEG resulted in a dramatic augmentation of CSK activity (173 ± 15.9% of control, P-value < 0.05). While the regulation of CSK activity remains incompletely understood, emerging evidence underscores the role of the tyrosine phosphatase, PRL (protein of regenerating liver)−3, (Liang et al. J Biol Chem 2007). We, therefore, assessed PRL-3 levels and noted a marked decrease in the HT-29 cells treated with 10% PEG (71.5 ± 7.5% of control, P-value < 0.05). Conclusion: We demonstrate for the first time that PEG treatment caused an increase in CSK enzyme which corresponds to our previously described anti-proliferative activity. While the mechanism of the CSK activation was not completely elucidated, our data suggests that suppression of the tyrosine phosphatase PRL-3 may be important. This is the first report to indicate that the PRL-3/CSK signaling axis may represent an important pathway in chemoprevention. Moreover, this may have relevance to treatment of CRC (chemotherapy) given the established role of PRL-3 in CRC invasion/metastasis (Saha et al., Science 2001). Future studies will focus on the role of the PRL-3/CSK pathway in chemoprevention.