Human monocyte-specific esterase (MSE) is one of the few haemopoietic cell enzymes that show absolute lineage restriction. Although the function of MSE has yet to be deduced, its potential role in tumour cell killing has stimulated particular interest. Knowledge of subcellular localization of MSE is fundamental to understanding its function and, in this context, it is widely believed that MSE is expressed as a plasma membrane ectoenzyme; a contention that is largely based upon experiments which examined fixed cells by ultrastructural cytochemistry. However, as recent molecular studies of human MSE indicate, a number of inconsistencies between its structure and a membrane localization, we applied the techniques of phase separation in the non-ionic detergent Triton X-114 and differential centrifugation to further investigate whether this particular esterase species is membrane-bound or associated with an intracellular organelle. These studies provide strong evidence that MSE is in fact a soluble intracellular enzyme that is almost certainly located within the lumen of the endoplasmic reticulum.
Leukaemic promyelocytes from 30 cases of hypergranular and 14 cases of hypogranular acute promyelocytic leukaemia (M3) were analysed for the presence of monocyte-associated characteristics to determine whether there was any evidence of mixed (hybrid) granulocytic-monocytic differentiation. Cytochemically, a high proportion of hypergranular cases showed significant alpha-naphthyl acetate esterase (ANAE) staining and simultaneous chloroacetate esterase, and ANAE expression by single cells was commonly seen. These atypical staining patterns were, however, not a feature of hypogranular cases. Immunophenotypic studies revealed that most hypergranular M3 cases were HLA-DR- and that monocyte-associated membrane CD14 expression was low in all cases tested. In addition, serum lysozyme concentrations (20 cases) were generally within the normal range and thus inconsistent with monocytic involvement in the leukaemic process. The significance of atypical ANAE staining of leukaemic promyelocytes was further examined by analysing ANAE isoenzyme components (defined by isoelectric focusing) in 11 cases. The patterns obtained (G1 and G2) were identical to those found in normal granulocytes and did not show any evidence of monocyte-associated esterase isoenzyme expression. On the basis of these findings, it is considered that the differentiation process in acute promyelocytic leukaemia is relatively well conserved and that the atypical esterase cytochemistry of hypergranular promyelocytes does not reflect their mixed lineage nature but is simply a consequence of increased granulation.
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