Figure 1. ST6Gal-I is upregulated in human colon tumors and tumor-associated Fas receptors have elevated α2–6 sialylation.

(A) A commercially purchased membrane for immunoblotting was probed for ST6Gal-I protein expression. Mature ST6Gal-I (upper band) is highly expressed in three separate colon tumors as compared to normal colon and placenta. The lower band is consistent with a cleaved, secreted form of ST6Gal-I (19). Densitometry was performed on the upper band. All samples were normalized to GAPDH and then compared to normal colon expression. (B) Tissue homogenates were prepared from colon tumors and pair-matched uninvolved colon specimens and immunoblotted for ST6Gal-I. ST6Gal-I was upregulated in 4/5 of colon tumors as compared to pair-matched uninvolved colon tissues. Densitometry was performed on the upper band. All samples were normalized to β-actin and then tumor samples were compared to pair-matched uninvolved colon. (C) α2–6 sialylated proteins were isolated using SNA1-agarose and immunoblotted for Fas. Total Fas levels were assessed by immunoblotting initial tissue homogenates (not subjected to SNA) for Fas. In all three patient samples, mature Fas (upper band) was downregulated in the tumor tissue relative to the respective uninvolved tissue. In patients with upregulated ST6Gal-I (patients 4 and 5), the proportion of sialylated Fas to total Fas was 5.07 and 2.55 respectively (densitometry was performed on the upper band). In the patient without upregulation of ST6Gal-I (patient 8), the proportion of sialylated Fas to total Fas was much lower at 1.26. U=uninvolved, T=tumor