Activated EGFR (epidermal growth factor receptor) undergoes ESCRT (endosomal sorting complex required for transport)-mediated sorting on to the intraluminal vesicles of MVBs (multivesicular bodies) before degradation in the lysosome. Sorting of endocytosed EGFR on to the intraluminal vesicles of MVBs removes the catalytic domain of the EGFR from the cytoplasm, resulting in termination of receptor signalling. The formation of intraluminal vesicles that contain EGFR is promoted by EGF stimulation in a mechanism that depends on the EGFR substrate, annexin 1. Signalling from endocytosed EGFR is also subject to down-regulation through receptor dephosphorylation by PTPs (protein tyrosine phosphatases), such as PTP1B, an enzyme thought to reside on the ER (endoplasmic reticulum). In the present paper, we review how the phosphorylation state of components of the MVB sorting machinery, as well as the EGFR, may play a critical role in regulating EGFR sorting and signalling.
EGF (epidermal growth factor) binding to the EGFR (EGF receptor) promotes increased RTK (receptor tyrosine kinase) activity, receptor dimerization and receptor trans-autophosphorylation. The latter generates multiple sites for the recruitment of signalling proteins and targets of the EGFR kinase and the consequent activation of signalling pathways that can lead to cell proliferation, differentiation, motility or survival, depending upon the cellular context. The activated EGFRs are endocytosed through clathrin-coated pits, although other internalization mechanisms can also operate . The EGFR kinase remains active after endocytosis, and some, but not all, signalling pathways from the EGFR require endocytosis for their maximal activation [2,3]. Endocytosis of EGFR and/or associated proteins may allow access to downstream targets and specific signals from EGFR may be generated in specific subcompartments of the endosome [4,5]. Given the importance of the spatial regulation of signalling, it is not surprising that EGFR signalling from the endosome is subject to multiple regulatory mechanisms. In the present paper, we review two of those potential mechanisms: EGF-stimulated intraluminal vesicle formation and PTP (protein tyrosine phosphate) 1B-mediated EGFR dephosphorylation (Figure 1).
Down-regulation mechanisms potentially operating at the level of the MVB
EGF-stimulated intraluminal vesicle formation within MVBs (multivesicular bodies)
With time, endocytosed activated EGFRs that are destined for lysosomal degradation accumulate on the intraluminal vesicle of MVBs. Receptors that are to be recycled remain on the perimeter membrane of MVBs from where they are returned to the plasma membrane. When all the recycling proteins have been removed, MVBs containing EGFRs fuse directly with the lysosome and the EGFRs are degraded. The activated EGFR is one of the best characterized and most frequently used cargo markers of MVBs. In order to drive EGFRs into MVBs and on to the intraluminal vesicles of MVBs, the cell must be stimulated with EGF. It has only recently been documented that EGF stimulation has profound effects on MVB biogenesis . EGF stimulation causes an increase in the number of MVBs per cell and the number of intraluminal vesicles per MVB . Furthermore, MVB maturation is slowed in EGF-stimulated cells, as shown by the longer time taken for conversion of Rab5 into Rab7 in EGF-stimulated cells [7,8]. The sequestration of EGFRs on the intraluminal vesicles of MVB removes the catalytic domain of the receptor and any signalling proteins associated with the receptor from the cytoplasm. Thus direct signalling from that receptor is terminated. It is therefore likely that EGF-stimulated intraluminal vesicle formation has evolved as a means of regulating signal transduction from the EGFR. An as yet unanswered question is whether or not sequestration of EGFR on internal vesicles of MVB is a ‘point of no return’ or whether, under certain circumstances, the intraluminal vesicles of MVBs can fuse back with the perimeter membrane and potentially resume signalling. Such backfusion of intraluminal vesicles may be involved in lipid transport, as has been demonstrated for intraluminal vesicles containing MHC class II molecules in dendritic cells , and is a process that can be hijacked by anthrax toxin  and vesicular stomatitis virus , but has not been demonstrated for intraluminal vesicles containing EGFR.
Intraluminal vesicle formation within MVBs is topologically opposite to other better characterized budding events within the cell that are mediated by coat proteins, such as clathrin. However, recent findings point to some of the molecular players in this process. In the present paper, we focus on those mechanisms that have been shown to be involved in EGF-stimulated intraluminal vesicle formation.
PI3K (phosphoinositide 3-kinase) and effectors
Treatment with the PI3K inhibitor wortmannin and micro-injection with antibody against the PI3K Vps (vacuolar protein sorting) 34 both inhibit the generation of intraluminal vesicles within EGFR-containing MVBs , implying a role for PtdIns3P in intraluminal vesicle formation. The formation of intraluminal vesicles containing MHC class II was also found to be inhibited by wortmannin treatment , indicating that PtdIns3P also has a role in intraluminal vesicle formation in non-EGF-stimulated cells. Although Vps34 is not a PI3K that is regulated directly by activated EGFR, it is an effector of Rab5 , and EGF stimulation promotes Rab5 activation  and so might increase Vps34 recruitment and consequent PtdIns3P generation on the perimeter membrane of MVBs.
Fab1p (PIKfyve in mammalian cells), which contains the PtdIns3P-binding FYVE domain, has been implicated in intraluminal vesicle formation and so could at least partly explain the requirement for PtdIns3P in intraluminal vesicle formation. Fab1p is a PtdIns3P 5-kinase that has been shown to be required for efficient sorting of cargo on to intraluminal vesicles in yeast , although the situation seems more complex in mammalian cells. Inhibition of PIKfyve induces the generation of enlarged endosomal vacuoles [17,18], but Drosophila PIKfyve mutants contain MVBs with many intraluminal vesicles . Furthermore, loss of PIKfyve function in Drosophila did not affect receptor silencing, suggesting that it operates at a stage after sequestration on intraluminal vesicles . Components of ESCRT (endosomal sorting complex required for transport) -I bind PtdIns3P and components of ESCRT-III bind PtdIns(3,5)P2 , and so involvement of these lipids in intraluminal vesicle formation could reflect a requirement for ESCRT proteins (see below).
The roles of ESCRTs 0–III in sorting RTKs on the perimeter membrane of MVBs for their inclusion on intraluminal vesicles has recently been reviewed elsewhere . Hrs (hepatocyte growth factor-regulated tyrosine kinase substrate) overexpression and depletion inhibit the formation of intraluminal vesicles within MVBs [23–25]. This suggests either a direct role for Hrs in the formation of intraluminal vesicles or that Hrs could be required for sorting of components of the intraluminal vesicle machinery. Depletion of components of ESCRT-I also inhibit intraluminal vesicle formation within MVBs, but result in a loss of vacuolar endosomes, and so ESCRT-I appears to play a more structural role in regulating MVBs, being required for stabilization of the vacuolar domains from which MVBs are formed [23,26]. The effects of Hrs and Tsg101 (tumour susceptibility gene 101) depletion are much greater in EGF-stimulated cells than in resting cells, indicating that ESCRT proteins are required for both basal and EGF-stimulated intraluminal vesicle formation. The ESCRT-0 complex of Hrs and STAM (signal-transducing adaptor molecule) becomes phosphorylated following EGF stimulation , but how this might promote intraluminal vesicle formation is currently unclear. ESCRT-III may play a direct role in intraluminal vesicle formation. The ESCRT-III components CHMP (charged multivesicular body protein) 4A and 4B, when overexpressed, are recruited to the plasma membrane where they form curved arrays of filaments . In the presence of dominant-negative Vps4, the ATPase required to release ESCRTs from the membrane, these filaments can promote negative curvature , indicating that ESCRT-III complexes can promote the type of curvature required for intraluminal vesicle formation. However, depletion of one component of ESCRT-III, VPS24/CHMP3, inhibited EGFR degradation, but did not prevent the accumulation of EGFR on intraluminal vesicles within MVBs and did not prevent EGFR silencing . This suggests that either ESCRT-III is dispensable for EGF-stimulated intraluminal vesicle formation or that other ESCRT-III components can compensate for loss of VPS24.
Annexin 1 is a calcium- and phospholipid-binding protein that was found in isolated MVBs where it could be tyrosine-phosphorylated in a manner that was dependent on the presence of activated EGFR . Since annexin 1 was known to be able to mediate vesicle aggregation in vitro , it was proposed at the time that annexin 1 might play a role in intraluminal vesicle formation within MVBs. More recently, using cells derived from an annexin 1-knockout mouse and HeLa cells depleted of annexin 1 with siRNA (small interfering RNA), a role for annexin 1 in EGF-stimulated inward vesiculation has been identified . EGF-stimulated inward vesiculation is abolished in annexin 1-knockout cells, whereas basal inward vesiculation is unaffected. EGF-stimulated inward vesiculation can be restored in knockout cells by expression of wild-type annexin 1, but not by an annexin 1 mutant carrying a mutation in the single EGF-stimulated tyrosine phosphorylation site in the N-terminus of the protein . These findings indicate that EGF-stimulated intraluminal vesicle formation within MVBs requires tyrosine phosphorylation of annexin 1. Unlike the ESCRT machinery, which dissociates from the perimeter membrane of the MVB before intraluminal vesicle formation, annexin 1 is transported on to intraluminal vesicles containing EGFR . This, together with the demonstration that EGFRs are still transported to the lysosome in cells lacking annexin 1, albeit primarily on the perimeter membrane, suggests that annexin 1 has a direct role in inward vesiculation and might operate at a late post-ESCRT stage in the process.
The movement of EGFR from the perimeter membrane of the endosome on to intraluminal vesicles occurs during a gradual maturation process that can take 30 min or more to complete [7,8,32]. Thus at least a proportion of EGFRs may remain on the perimeter membrane for some time, during which they are potentially subject to dephosphorylation by cytoplasmically exposed phosphatases.
The phosphorylation state of RTKs reflects a balance between kinase and phosphatase activity. The importance of phosphatases in determining the phosphorylation/activation state of RTKs is shown by the fact that X-rays and peroxide treatment induce sustained activation of RTKs through inhibition of PTPs . The use of substrate-trapping mutants of PTPs, which are catalytically inactive and bind stably to their substrates, has enabled the identification of substrates of PTPs. One PTP shown by this approach to act on the EGFR is PTP1B , an enzyme thought to reside on the ER (endoplasmic reticulum) . In some cell types, the catalytically active N-terminus of PTP1B is cleaved upon ligand stimulation, releasing a soluble domain of PTP1B that can act on its target substrates . However Haj et al.  have demonstrated FRET (fluorescence energy resonance transfer) between ER-localized substrate-trapping mutant PTP1B and expressed EGFR in a reaction that was inhibited by co-expression of dominant-negative dynamin . This suggests that endocytosed EGFR interact with PTP1B on the ER. However, in the many studies examining the traffic of endocytosed EGFRs to the lysosome, an interaction between endosomes and the ER has never been described. A recent report found that EGFR could traffic to the ER, raising the possibility of a PTP1B–EGFR interaction within the plane of the ER . However, traffic of EGFR to the ER occurred over a much longer time course than the interaction between PTP1B and EGFR identified by Haj et al. , and so it is perhaps more likely that the interaction occurs by direct membrane contacts between endosomes and the ER. The ER is intimately associated with a number of organelles, including plasma membrane, mitochondria, Golgi and peroxisomes . The molecular composition of an equivalent interorganellar contact site, the nucleus–vacuole junction in Saccharomyces cerevisiae, has been characterized and is composed of an integral membrane protein of the outer nuclear envelope and a peripheral protein on the vacuole . However, the molecular composition of ER–organelle contacts in mammalian cells is largely unknown.
Where on the endocytic pathway could EGFRs interact with PTP1B? If the EGFRs that undergo ESCRT-mediated sorting within MVBs before lysosomal delivery interact with PTP1B, they must do so before sequestration on the intraluminal vesicles of MVBs. If MVBs interact with the ER, it is possible that PTP1B interacts with other components of the MVB sorting machinery. More than one component of the ESCRTs become tyrosine-phosphorylated in response to EGF, as does annexin 1. It is possible that these components might be additionally regulated at the time of endosome–ER interaction through dephosphorylation by PTP1B.
EGF-stimulated intraluminal vesicle formation promotes sequestration of the catalytic domain of the EGFR from the cytoplasm and so probably represents a mechanism to down-regulate EGFR signalling. The underlying mechanisms of EGF-stimulation of intraluminal vesicle formation are not fully understood, but components of the MVB sorting machinery, including annexin 1  and some of the ESCRT proteins , become tyrosine-phosphorylated in response to EGF, and a major future challenge is to understand the functional significance of that phosphorylation. Dephosphorylation by PTPs of both the EGFR itself and possibly components of the MVB sorting machinery is also likely to play a role in both down-regulation of signalling and sorting of EGFRs within MVBs. Much of our existing knowledge of mechanisms underlying sorting within MVBs in mammalian cells has come from studies of the EGFR. However, some of the components of the MVB sorting machinery become tyrosine-phosphorylated following stimulation by growth factors other than EGF. Whether their target RTKs also promote intraluminal vesicle formation is currently unknown.
ESCRTs: from Cell Biology to Pathogenesis: Biochemical Society Focused Meeting held at Robinson College, Cambridge, U.K., 26–28 August 2008. Organized and Edited by Katherine Bowers (University College London, U.K.), Juan Martin-Serrano (King's College London, U.K.) and Paul Whitley (Bath, U.K.).
charged multivesicular body protein
epidermal growth factor
endosomal sorting complex required for transport
hepatocyte growth factor-regulated tyrosine kinase substrate
protein tyrosine phosphatase
receptor tyrosine kinase
vacuolar protein sorting
Work is supported by Cancer Research UK [grant number C20675], the Wellcome Trust [grant number 078304] and the Biotechnology and Biological Sciences Research Council [grant number BB/D011841].