The human promyelocytic leukemia HL-60 cell line has been used extensively to investigate differentiation along the monocyte/macrophage pathway. Phorbol esters induce macrophage differentiation via protein kinase C and vitamin D, tumor necrosis factor-α and γ-interferon induce monocyte differentiation. Distinct patterns of sphingolipid metabolism have been associated with these events. Macrophage differentiation is accompanied by a selective increase in ganglioside GM3 which may mediate crucial aspects of the differentiation program. Macrophage differentiation also involves synthesis of sphingomyelin, which precedes adherence and may directly be involved in the adherence process. In contrast, sphingomyelin degradation via the action of a neutral sphingomyelinase may be involved in monocytic differentiation. In this paradigm, ceramide may function as second messenger. Recent advances also include the discovery of ceramide 1-phosphate and the associated calcium-dependent kinase, ceramide kinase in these cells. The biologic role of ceramide 1-phosphate and the regulation of the kinase are at present undefined. The notion that a sphingomyelin pathway analogous to the phosphoinositide pathway may exist initiated by the action of a sphingomyelinase is presented. These early investigations suggest that biologically relevant signals may be flowing into and out of sphingomyelin.
In eukaryotic cells glycoproteins can have a myriad of mature oligosaccharide structures attached to one to ten specific asparagine residues in a protein. Biosynthesis of these diverse structures begins with common steps involving a polyisoprenol as a carrier lipid. The common biosynthetic steps that occur prior to transfer of an oligosaccharide from the lipid carrier to the nascent protein are the subject of this minireview. Specific enzymatic reactions and the regulation of the reactions involved in the synthesis of the lipid carrier, dolichyl phosphate, and the assembly of Glc3 Man9GlcNAc2-P-P-dolichol are discussed, as are the mechanisms of regulation of the overall glycosylation process itself. Dolichol is a family of isomers consisting of 17-21 isoprenyl groups, with the α isoprenyl group saturated. All cells synthesize dolichol beginning with acetate, sharing the cholesterol biosynthetic pathway through farnesyl pyrophosphate. Beyond this intermediate, cis-polyprenyl transferase, polyprenyl reductase, and a pyrophosphatase combine to synthesize dolichyl phosphate. Dolichyl phosphate is a pivotal intermediate, serving as the substrate for three enzymes and as the product of at least four reactions in the pathway of oligosaccharide-lipid synthesis. The numerous interconversions among the dolichyl phosphate forms are explained in this review. Finally, differences in the regulation of this pathway between tissues and cultured cells are discussed.