beta1,6 N-acetylglucosaminyltransferase (core 2 GlcNAc-T) expression in normal rat tissues and different cell lines: evidence for complex mechanisms of regulation

The distribution of the Golgi enzyme beta1, 6-N-acetylglucosaminyltransferase (core 2 GlcNAc-T for short) has beeninvestigated in several tissue and cell systems by combining the potentialsof a polyclonal antibody and a novel, sensitive fluorescent enzyme assay.In normal rat tissues, levels of the protein were found to vary and as ageneral trend did not correlate with enzyme activities. Additionally, weobserved tissue-specific core 2 GlcNAc-T forms of various size: 75 kDa(liver), 70 kDa (spleen), 60 kDA (heart), and 50 kDa (heart and lung).These forms might arise from differential protein modifications;alternatively, the smaller form may be a product of proteolytic cleavage,given the presence of a catalytically inactive 50 kDa species in rat serum.Chinese hamster ovary (CHO), MDAY-D2, PSA- 5E, and PYS-2 cell linesconsistently displayed a 70 kDa enzyme. When induced to retrodifferentiatein the presence of butyrate + cholera toxin, CHO cells exhibited a 21-foldincrease in enzyme activity, while protein levels remained constant. Asimilar trend was observed in the embryonal endoderm cell lines PSA-5E andPYS-2, where an approximately 100-fold difference in core 2 GlcNAc-Tactivity was found notwithstanding unchanged amounts of the protein andidentical mRNA levels, as evidenced by RT-PCR. In contrast, levels of core2 GlcNAc-T activity in MDAY-D2 cells correlated well with proteinexpression. Taken together, these observations demonstrate that core 2GlcNAc-T expression may be subjected to multiple mechanisms of regulationand suggest that in at least some instances (i.e., PSA-5E and PYS-2 cells)expression may be regulated exclusively via posttranslational mechanism(s)of control.