Effects of hexamethylene bisacetamide on induction of monocytic differentiation of human U-937 myeloid leukemia cells

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

Differentiating agents modulate topoisomerase I activity in U-937 promonocytic cells

Phorbol 12-myristate 13-acetate (PMA), N,N'-hexamethylenebisacetamide (HMBA) and retinoic acid induce cell differentiation in U-937 promonocytic cells. This report examines the effects of these agents on DNA topoisomerase I activity. A decrease in enzyme activity could be detected as early as 30 min after treatment with all three differentiating compounds and lasted at least 48 h. No alteration in the levels of DNA topoisomerase I transcript or protein was observed during these treatments. The results might be explained by post-translational events that render DNA topoisomerase type I less active.

Melanocyte differentiation marker gp75, the brown locus protein, can be regulated independently of tyrosinase and pigmentation

Human melanoma arises from epidermal melanocytes and displays remarkable phenotypic heterogeneity. This heterogeneity in part reflects the ability of melanoma cells to undergo differentiation along a pathway parallel to differentiation of normal melanocytes. Tyrosinase, encoded by the albino (c), and the tyrosinase-related protein-1 or gp75, encoded by the brown (b) locus, are two of the best-characterized markers for melanocyte differentiation. Both molecules are glycoproteins expressed in melanosomes, the site of pigment synthesis. We studied the regulation of these proteins in human melanoma cells induced by the polar-planar compound hexamethylene bisacetamide (HMBA). In well-differentiated melanoma cell lines, HMBA induced dendritic morphology and specifically regulated the expression of melanosomal glycoproteins (but not a panel of other molecules expressed by melanoma cells). HMBA specifically down-regulated gp75 expression by rapidly decreasing the steady-state level of gp75 mRNA and gp75 synthesis. HMBA was able to down-regulate gp75 expression even in the presence of cholera toxin, which when added alone induced a two- to threefold increase in gp75 expression. In contrast to uniform down-regulation of gp75 expression, HMBA could either up-regulate or down-regulate tyrosinase expression and pigmentation. Based on the differential regulation of gp75 and tyrosinase, melanoma cells could be classified into two groups. In one group, gp75 expression was coordinately regulated with tyrosinase activity and pigmentation. In the other group, gp75 expression and tyrosinase activity and pigmentation were dissociated (with pigmentation coupling to tyrosinase activity, not to gp75 expression). These results show that in mature melanocytic cells, regulation of gp75 expression follows a pattern that can be independent of regulation of tyrosinase and pigmentation.