Expression levels of GM3 synthase is transcriptionally regulated in HL60 cells differentiated in monocytoid lineage by phorbol esters

INTRODUCTION: During bi-directional differentiation of human myelogenous leukemia cell line HL-60 into monocytoid and granulocytoid lineages, ganglioside GM3 and neolacto series gangliosides (NeuAc-nLCs) are expressed in differentiation direction-specific manner (1). That is, GM3 markedly increases during monocytic differentiation of HL-60 cells induced by 12-O-tetradecanoyilphorbol-13-acetate (TPA), whereas NeuAc-nLCs noticeably increase in granulocytic differentiation induced by all-trans retinoic acid (RA). These observations suggest that the accumulation of specific gangliosides on the cell membrane plays an important role as a trigger in differentiation induction and as determinant of the differentiation direction in human hematopoietic cell lines (1). It is known that two key upstream glycosyltransferases, Lc3Cer synthase and GM3 synthase, play a critical role regulating the glycosphingolipid biosynthesis in HL-60 cells during bi-directional differentiation (2), but the mechanisms controlling expression and activity levels of these enzymes have not yet been elucidated. In this study our attention is directed to investigate the regulation of GM3 synthase activity. MATERIALS AND METHODS: HL-60 cells were grown in RPMI 1640 complete medium at 37°C in a humidified atmosphere enriched with 5% CO2. Granulocytoid differentiation of HL-60 cells was induced by treatment with 1 mM RA for 48 hours; macrophage-like cell differentiation was produced by 4 nM TPA addition to the culture medium for the same period of time. Acidic and neutral glycolipid profiles of control, RA- and TPA-treated cells were quali-quantitatively analysed by HP-TLC and digital scanning of the plates. GM3 synthase activities were determined in control, RA- and TPA-treated cells by an in vitro radioactive assay using 50 mg and 100 mg of the microsomal enriched protein fraction as enzyme source. mRNA expression levels of GM3 synthase gene was determined by RT-PCR. The house-keeping gene encoding for hypoxantine phospho-ribosyl transferase (HPRT) was used as internal standard for quantitative evaluation of the RT-PCR products. RESULTS: After 48 hours RA treatment, a 30% granulocytic differentiation degree of HL-60 cells was evaluated by conventional cytoplasmic/nuclear histochemical staining of the cells. On the contrary, quite 80% of TPA-treated cells showed evident macrophage-like adherent ability and prominent pseudopods, phenotipic markers of differentiation. Indeed, no modification in the glycosphingolipid profiles, in the enzyme activities and in mRNA expression levels of the crucial glycosyltransferases (Lc3Cer synthase and GM3 synthase) were observed in RA-treated cells. On the other hand, in TPA-treated cells there is a sensible increase in ganglioside GM3 content, accompanied by a consistent up-regulation of GM3 synthase activity with respect to undifferentiated and to RA-treated cells. Through quantitative RT-PCR experiments performed on total RNA from undifferentiated, RA- and TPA-treated HL-60 cells, we demonstrate the strict correlation between GM3 synthase activity and its mRNA level: the GM3 synthase transcript is present in equal amount in either undifferentiated and RA-treated cells, but it is dramatically increased (quite 3 times) in TPA-treated cells. These results first give support to a regulation mechanism at the transcriptional level for this enzyme. REFERENCES (1) H. Nojiri et al., Characteristic expression of glycosphingolipid profiles in the bipotential cell differentiation of human promyelocytic leukemia cell line HL-60. Blood 64, 2:534-541 (1984); (2) M. Nakamura et al., Total metabolic flow of glycosphingolipid biosynthesis is regulated by UDP-GlcNAc:Lactosylceramide beta-1,3-N-Acetylglucosaminyltransferase and CMP-NeuAc:Lactosylceramide alfa-2,3sialyltransferase in human hematopoietic cell line HL-60 during differentiation. J. Biol. Chem. 267, 33: 23507-23541 (1992).