Modulation of K562 cells with sodium butyrate. Association of impaired NK susceptibility with sialic acid and analysis of other parameters

Abstract

Neuraminidase treatment of parental and butyrate-induced K562 tumor cells was associated with an increase in natural killer (NK) susceptibility of these target cells. The degree of enhancement with neuraminidase was significantly greater for the NK-resistant (NR^R) butyrate-differentiated K562 cells so that the relative difference between the parental NK-sensitive (NK^S) K562 line and its induced NK^R variants, in terms of NK sensitivity, was no longer five- or six-fold but only two-fold. The predominant reason for the altered NK susceptibilities of the target cells after neuraminidase treatment was an increase in the target-cell-binding ability of these cells as assessed by a direct conjugate-forming cell assay using Percoll-enriched NK cells and cold target competition assays. The enhancement did not appear to be due simply to an increased membrane-membrane attraction caused by a reduction of net negative cell surface charges since protamine sulphate, a positively charged molecule, had no effect on NK activity. Compared with the NK^S parental K562 tumor cells, the NK^R butyrate-induced cells had 3.6-to 4.0-fold higher sialo-transferase activities and were associated with significantly greater amounts of cell surface sialic acid detected both in sialyl glycoproteins (2.2- to 2.9-fold higher) and particularly within ganglioside extracts (6.2- to 13.6-fold higher). In conformity with the marked neuraminidase enhancement of NK-mediated cytolysis of the butyrate-induced targets, these NK^R cells were associated with significantly enhanced levels of neuraminidase-accessible sialic acid compared to the NK^S parental K562 cell line. Other parameters such as sensitivity to superoxide radicals, intrinsic superoxide dismutase levels, altered membrane repair mechanisms and transferrin competition, were not significantly different between the NK^S and NK^R target phenotypes. Sugar inhibition studies demonstrated an enhanced inhibition against the butyrate-induced cells with a variety of neutral sugars. The degree of inhibition with phosphorylated sugars was comparable between the parental and induced K562 tumor target cells and is consistent with our previous findings showing that these hexose phosphates may be inhibiting cytolysis at a step independent of target-cell recognition.