Low molecular weight protein tyrosine phosphatase (LMW-PTP) is upregulated
in primary colorectal cancer and affects cancer signaling pathways.
Elmer Hoekstra1, Liudmila L. Kodach2, James C. Hardwick2, Ernst J. Kuipers1, Marco Bruno1,
Maikel P. Peppelenbosch1, Gwenny M. Fuhler1
Department of Gastroenterology and Hepatology, Erasmus MC-University Medical Center, Rotterdam, The
Netherlands1, Department of Gastroenterology-Hepatology, Leiden University Medical Centre, Leiden, The
Netherlands2
Background and aim: Cancer cell functions are tightly regulated by protein phosphorylation and
dephosphorylation. Enhanced kinase activation and phosphorylation is often observed in colorectal
cancer (CRC). Whereas kinases are seen as potential oncogenes, dephosphorylation of proteins by
phosphatases is commonly assumed to be tumor suppressive. However, some phosphatases may
also stimulate tumor growth and invasion. One of these is low molecular weight protein tyrosine
phosphatase (LMW-PTP), overexpression of which in cell lines is sufficient for cellular transformation.
The aim of this study was to investigate the expression levels of LMW-PTP in primary CRC
specimens, and elucidate its role and signaling targets in CRC cells.
Methods: LMW-PTP expression was analysed on microsections from biopsies of low grade dysplasia
(LGD;
n=9),
high
grade
dysplasia
(HGD;
n=7),
and
adenocarcinoma
(AC;
n=12)
by
immunohistochemistry. Normal colon tissue (n=2), active ulcerative colitis (n=6) served as controls.
This staining was subsequently performed on a tissue microarray (TMA) of 72 patients with colorectal
adenoma (N=47) and/or carcinoma (N=164) and their corresponding normal tissue (N=63). To
investigate downstream targets of LMW-PTP, we manipulated the LMW-PTP expression in vitro by
lentiviral transduction of HCT116 and Caco-2 cells with siRNA against LMW-PTP.
Results: In our initial screen LMW-PTP expression in intestinal epithelial cells (IEC) was limited to 9%
of IEC in non-cancerous tissues. In contrast, expression of LMW-PTP significantly increased with
subsequent levels of dysplasia (42%, 80% and 100% positive IEC in LGD, HGD and AC respectively).
Staining of the TMA confirmed these observations: the mean % positive IEC was 27±3% in normal
tissues, 64±4% in adenoma and 90±3% in carcinoma (p<0.001). There was a significant difference
between different cancer-stages within patients (N=15, paired testing; p<0.001), however, Dukes
stage or patient survival did not correlate with LMW-PTP expression. Knocking down LMW-PTP in
CRC cells reduced phosphorylation of the EGF-receptor and β-catenin in vitro by approximately 50%.
Conclusion: LMW-PTP expression is drastically upregulated in epithelial dysplasia and CRC. During
transformation of IECs, LMW-PTP expression increases, suggesting a role for LMW-PTP in the
pathogenesis of CRC. As LMW-PTP knockdown decreases EGFR and β-catenin phosphorylation,
overexpression of LMW-PTP in CRC samples may contribute to the enhanced EGF and β-catenin
signaling, known to play a role in CRC development. In conclusion, LMW-PTP is overexpressed in
CRC, may function as an oncogene, and represent a compelling target for future therapy.