Gangliosides, a family of glycosphingolipids containing lactosylcerimide (Lac-Cer), glucose, N-acetylgalactosmine, galactose and sialic acid (SA) residues, are ubiquitous membrane components, which act in cell interaction, adhesion, differentiation and cell growth.1-3 Membrane ganglioside expression is cell type specific and changes dynamically during myeloid cell differentiation..4, 5 Monocytes and macrophages play an important role in innate immunity providing a front line of defense against pathogens. Two human promyeoloblast leukemia cell line (HL-60) and human monocytic leukemia cell line (THP-1) are useful models for studying monocyte/macrophage differentiation [5.6]. Stimulated HL-60 cells showed increased expression of GM3 synthase, β1,4N-acetylgalactosaminyltransferase-I (4GalNacT-1) and α-N-acetylgalactosaminide α-2,6-sialytransferase (ST6GalNac-3), resulting in α-series biosynthesis. THP-1 could express the enzyme generating a- and α-series gangliosides. These enzymes convert Lac-Cer into GM3, GD3 and GT3 (Figure 1) and finally generate GT1a (a-series), GQ1b (b-series) and GP1c (c-series). GT3 is also important as a precursor of c-pathway gangliosides (c-series). Enzymes such as 4GalNacT-1, 3GalT-4, ST3Gal-2 and ST8Sia-5 functionally overlap to generate complex gangliosides of the o, a- and b-series.6-8 ST6GalNac-III and -V generate GD1α, GT1aα and GQ1bα of the α-series gangliosides. Exposure to Phorbol 12-myristate 13-acetate (PMA) causes these cells to become adherent to plastic surfaces and induces morphological changes, such as the macrophage-like phenotype with prominent pseudopods seen in PMA-induced HL-60 and THP-1 cells (Figure 2A). PMA treatment resulted in a significant increase of CD11b in HL-60 cells and minor increase in THP-1 cells and CD36 expression in HL-60 cells was slightly increased by 24 h of PMA treatment7, 9 (Table 1, Figure 2B). 5′-ATGGCTCTCAGAGTCCTTCTGTTAA-3′ 5′- CATCAAAGAGAACAAGGTTTTGGAC-3′ Sence Antisence 5′-CTGGCTGTGTTTGGAGGTATTCT-3′ 5′-AGCGTCCTGGGTTACATTTTG-3′ Sence Antisence 5′-AAGCTTAGAAGGCCCAGCTTGTTATTAAAAG-3′ 5′-TCTAGAGGTAAATTCACGATCAATGCCTC-3′ Sence Antisence 5′-ATGGCTGTACTGGCGTGGAAGT-3′ 5′-ATGTCTTTCTGGACCACAGAAGGT-3′ Sence Antisence 5′-ATCGGCTACGGGCTCTCATCACC-3′ 5′-ACCTCGCGCACCTTGTCAGTCC-3′ Sence Antisence 5′-GAGCTATTCGGCAAACTTGG-3′ 5′-GGAGGGGGTACAGGATCAAT-3′ Sence Antisence 5′-TACCTGGACTCAGGGGCCCTGGATG-3′ 5′-GGCACTGTGGGGGTCCGGAA-3′ Sence Antisence 5′-CTGTGATTGCTGTGAGCTTCATAG-3′ 5′-AAGAGGAACGCTGGTATGGGACACA-3′ Sence Antisence 5′-CGGCTCGTGCTCATCATCCTGTGCT-3′ 5′-GTGAGCTGCAGCAGCGTGCGGTAGG-3′ Sence Antisence 5′-ATGAAGACCCTGATGCGCCATGGTC-3′ 5′-CTTGTGGCGAGTAATCATGAAGGCC-3′ Sence Antisence 5′-ATGAGTAGCAACAAAGAGCAGCGGT-3′ 5′-AACACCAGGCCCGCTCGCTGGATCA-3′ Sence Antisence 5′-ACCGGGATTTGTTGGGGAGCCGA-3′ 5′-GATCCACTGGTGTCCACCTCA-3′ Sence Antisence 5′-CCCTATTTCTGGAGGACATTGCAACCTA-3′ 5′-GTTGGAGGATCTGGCTGTATTCTTTG-3′ Sence Antisence 5′-CAAGAGATGGCCACGGCTGC-3′ 5′-TCCTTCTGCATCCTGTCGGC-3′ Sence Antisence Differentiated HL-60 and THP-1 cells have increased levels of GM3 synthase but not GD3 synthase (Figure 2C). Untreated HL-60 cells had a very low basal level of GM3 synthase expression compared to THP-1 cells, but all cell lines an increase in GM3 synthase levels upon PMA treatment. In contrast, GD3 synthase was at an undetectable or very low level (Figure 2C) suggesting that differentiated HL-60 and monocyte/macrophage cells do not generate b-series gangliosides, which originate from GD3. GM3 is converted to GM2 by addition of GalNAc by N-acetylgalactosaminyltransferase I (4GalNAcT-1). 4GalNacT-1 is also responsible for biosynthesis of the GA2, GD2 and GT2. PMA treated HL-60 and THP-1 cells can produce GM2 from GM3 by 4GalNAcT-1 activity (Figure 2D). The high levels of these enzymes imply that GD1a is likely produced from GM1a in differentiated HL-60 and monocytic cells. α2,8-Sialyltransferase 5(ST8Sia-5) synthesizes GD1c, GT1a, GQ1b and GT3 from GM1b, GD1a, GT1b and GD3, respectively. ST6-N-acetylgalactosaminide α-2,6-sialytransferase (ST6GalNac) consists of four members (ST6GalNac-3, 4, 5 and 6), which differ in their substrate preference. GM1b is utilized by ST6GalNac-3, 5. and 6 to produce the α-series GD1α , and ST8Sia V to produce the o-series GD1c. GD1a is utilized by ST6GalNac-3, 4 and ST8Sia-5 to produce the α-series GT1aα and the a-series GT1a, respectively, at the final step of ganglioside biosynthesis. ST8Sia-5 expression was at an undetectable level in control and PMA treated HL-60 cells (Figure 2E). PMA stimulated THP-1 cells showed increased expression of ST8Sia-5, implying an increase in induction of o-series and a-series gangliosides. HL-60 cells had undetectable expression levels of ST6GalNAc-3 in control cells, which was induced significantly upon PMA treatment (Figure 2E). Interestingly, ST6GalNac-3 expression in PMA stimulated THP-1 cells was decreased compared to controls, more noticeably in THP-1 cells. Therefore, ST6GalNac-3 is the major enzyme that regulates myeloid cell differentiation into monocytes and macrophages. These results suggest a pathway where differentiated HL-60 and monocytic cells can generate α-series gangliosides through the sequential addition of SA, GalNAc, Gal and additional SA residues, and macrophages can convert from expressing α-series to expressing a-series gangliosides. In conclusion, the expression and alteration of gangliosides into a- and α-series are required for differentiation into monocytes and macrophages as well as their functions, such as phagocytosis and cell interaction, claiming for the prospective leukemia research. Hyuju Choi, Hee-Do Kim and Dr. Cheorl-Ho Kim contributed to the preparation and collection of original literature and figures and the writing and editing of manuscript. Dr. XXX and Dr. XXX were responsible for the structural designs, scientific quality and writing. Not applicable. This study has in part been supported by the Jungjum yonguso Program (NRF-2019R1A6A1A1007307921). The authors declare no conflict of interest. The paper was handled by editors and has undergone a rigorous peer-review process. Dr. Young-Choon Lee and Dr. Cheorl-Ho Kim were not involved in the journal's review of/or decisions related to this manuscript. Not applicable. Data sharing is on requests and schematic data is not applicable to this article as no new data were created or analyzed in this study.