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Regulation of non-clathrin endocytosis

Clathrin-dependent endocytosis has been much more extensively studied than the non-clathrin process. Recent research findings are changing this balance. It is becoming clear that non-clathrin pathways are numerous and that they are important not only for fluid and nutrient uptake, but also for recycling and lipid composition of the plasma membrane, cell movement, and toxin and viruses uptake. Non-clathrin endocytosis that occurs through large primary endocytic vesicles, and is often proceeded by formation of membrane ruffles is called macropinocytosis. Most studies define active macropinocytosis in macrophages and dendritic cells but recent evidence indicates that the process is common to many cell types. Macropinocytosis appears to be upregulated by growth factors such as EGF and PDGF. Downstream of growth factor receptors macropinocytosis is regulated by modulators of actin cytoskeleton such as Rac and Cdc42. Dominant negative Rac (T17N) decreases fluid uptake of dextrans in contrast to constitutively active Rac (Q61L) that upregulate the process. Further downstream activity of Rac is likely to be modulated by PAK1 kinase. In dendritic cells in response to growth factor stimulation Cdc42 takes up the role of Rac. Another set of macropinocytosis regulators includes small GTPase Arf6 and enzymes such as PI3 kinase and phospholipase C. These enzymes can be activated by and/or act on phosphoinosites in response to transformation by Ras or to activation of non-receptor tyrosine kinases such as Src and Abl. Action of Abl tyrosine kinase may be modulated by Abi family of proteins. They include spectrin SH3 do main and eps8 SH3 domain binding protein Hssh3bp1/e3b1. Hssh3bp1/e3b1 is a substrate of Abl tyrosine kinase, and it regulates macropinosome fusion. Detailed understanding of Hssh3bp1/e3b1 function is likely to provide a new information regarding regulation of macropinocytosis.