Increasing the alpha 2, 6 Sialylation of Glycoproteins May Contribute to Metastatic Spread and Therapeutic Resistance in Colorectal Cancer

Abnormal glycosylation due to dysregulated glycosyltransferases and glycosidases is a key phenomenon of many malignancies, including colorectal cancer (CRC). In particular, increased ST6 Gal I (beta-galactoside alpha 2, 6 sialyltransferase) and subsequently elevated levels of cell-surface alpha 2, 6-linked sialic acids have been associated with metastasis and therapeutic failure in CRC. As many CRC patients experience metastasis to the liver or lung and fail to respond to curative therapies, intensive research efforts have sought to identify the molecular changes underlying CRC metastasis. ST6 Gal I has been shown to facilitate CRC metastasis, and we believe that additional investigations into the involvement of ST6 Gal I in CRC could facilitate the development of new diagnostic and therapeutic targets. This review summarizes how ST6 Gal I has been implicated in the altered expression of sialylated glycoproteins, which have been linked to CRC metastasis, radioresistance, and chemoresistance.

Mutational analysis for enzyme activity of mouse Gal1,3GalNAc 2,3-sialyltransferase (mST3Gal I).

To determine which amino acid residues are essential for the catalytic activity of mouse Gal1,3GalNAc 2,3-sialyltransferase (mST3Gal I), chemical modification and site-directed mutagenesis were employed against tryptophan and cysteine residues located in the predicted catalytic domain. This enzyme was strongly inhibited by N-bromosuccinimide, a specific blocking reagent for tryptophan residues, and the enzyme activity was completely lost at 3 mM, suggesting the involvement of tryptophan residues in the catalytic activity of mST3Gal I. The N-ethylmaleimide, an irreversible reagent for sulfhydryl group, significantly inhibited the enzyme activity. Seven tryptophan and six cysteine residues conserved in the cloned Gal1,3GalNAc 2,3-sialyltransferases were separately substituted into phenylalanine and serine, respectively. The enzymatic activity assay for tryptophan mutants produced in COS cells showed a complete abolishment of the activity in all of the mutants, except that W70F and W97F retained about 60% and 40% activities of wild type, respectively. In the case of cysteine mutants, no enzyme activity was observed like tryptophan mutants, except for C139S. These results suggest that tryptophan and cysteine residues conserved in ST3Gal I are critical for its activity.

Glycosphingolipid expression in solid tumours and transformed cell lines.

Glycosphingolipids are assumed to play a crucial role in cell-cell and cell-substrate interactions, including cell adhesion, proliferation, differentiation and apoptosis. Furthermore, cell surface glycolipid profile changes in the so called "social disorders", such as malignant transformation. To better investigate these modifications, the ganglioside composition in different solid tumours and in two transformed cell lines was analyzed. In some of these models we also tried to correlate the pattern of gangliosides to the key enzymes involved in their metabolism. The results we obtained can be summarized as follows:(1), meningiomas with or without chromosome 22 deletion: predominance of ganglioside GD3 in the former and of ganglioside GM3 in the latter. Correlation between GM3/GD3 ratio and SAT-2 activity; (2), mammary carcinomas developed in MMTV/c-neu transgenic mice: accumulation of GM3-derived species. The different ganglioside distribution seems to correlate with the tumour size; (3), Sarcoma Galliera-strain cells SGS/3A and normal syngenic murine fibroblasts FG: transformed cells exhibit a lower activity of sialyltransferases (SAT-1, SAT-2, SAT-4) compared to normal fibroblasts, suggesting a possible correlation with the ganglioside pattern. The neuraminidase activity seems to correlate to the glycoprotein sialic acid content; (4), 3T3 normal murine fibroblasts and SVT2 transformed cells: GM3 is absent in 3T3, while it accounts for the main ganglioside species in SVT2. On the contrary, GM2 present in a large amount in normal fibroblasts, is practically absent in transformed cells. No correlation has been observed between ganglioside profile and glycosyltransferase activities so far examined.

Regulation of sialyltransferase activity in intestinal segments of rats.

A differential distribution of sialyltransferase (ST) in different regions of intestine has been shown. Jejunum and ileum homogenates from rats showed almost exclusive presence of alpha-2-3 ST (to Gal in Gal beta-1-4GlcNAc and/or to Gal in Gal beta-1-3GalNAc). In contrast, colon homogenates showed the presence of both alpha-2-3 ST (as above) and alpha-2-6 ST. Incubation of intestinal slices in presence of heat-inactivated horse serum (HHS) showed a time- and temperature-dependent secretion of soluble ST into the medium. Both jejunum and ileum slices showed high rates of secretion of alpha-2-3 ST. Colon slices, though rich in alpha-2-6 ST, secreted only alpha-2-3 ST. Colchicine, an anti-mitotic drug, injected into rats caused about 10-fold increase of the serum ST level. Jejunum slices from colchicine-treated rats showed an increased secretion of alpha-2-6 ST, suggesting that intestine undergoes a change in the expression of normal secretion of alpha-2-3 ST to a secretion of alpha-2-6 ST. The secretion of ST from incubated intestinal slices was inhibited by heparin. Certain protein factors (anti-proteases) in HHS bind to heparin-sepharose column and these protein factors are responsible for causing the secretion of ST into the medium. It has also been found that a supernatant fraction of the colon homogenate activated ST. Gel chromatography on HPLC produced 3-4 protein fractions from the colon cytosol and one of this fraction bearing high molecular weight proteins produced the maximum activation of ST.(ABSTRACT TRUNCATED AT 250 WORDS)