Introduction 2005 2006 2006 2006 2006 2002 2005 2004 2006 2005 2004 2006 2004 2003 2004 The ErbB receptor system 2006 2005 2006 1 2005 1 2007 2003 2005 Fig. 1 Structure of EGFR. In the extracellular part of the receptor, EGFR harbours two domains (L1 and L2) that upon folding form the ligand-binding pocket. Between L1 and L2 is another domain (S1) that includes the dimerization arm. Intracellularly, EGFR has a kinase domain and a C-terminal tail with several amino acid residues that can be phosphorylated. The tyrosine residues (Y) that are involved in Cbl binding are shown. The parts of wild-type EGFR that are deleted in EGFRvIII, EGFRvIV, and EGFRvV are indicated  Table 1 ErbB receptors and their ligands Receptor Remarks Ligands EGFR (ErbB1)        ErbB2 Constitutively exposed dimerization arm  ErbB3 Kinase dead  ErbB4        ErbB receptor activation 2002 2002 2003 2003 2002 2005 2003 2005 2003 2003 1997 1996 The signal transduction pathways activated by ErbB receptors include well-known signaling cascades such as the Ras-Erk1/2 pathway, the phospholipase Cγ-protein kinase C pathway, the phosphatidyl inositol 3 (PI 3)-kinase-Akt pathway, and STAT signaling. 2004 Endocytic downregulation of EGFR Regarding ligand-induced endocytic receptor downregulation, EGFR has been the most popular model system for many years, and the mechanisms of EGFR endocytosis and intracellular trafficking are therefore relatively well understood. 2 Fig. 2 green grey arrows white arrows black arrows Upper insert Lower insert  Sorting of EGFR to clathrin coated pits: ubiquitination or not? One of the first steps in clathrin-mediated endocytic downregulation is the transport of cargo into CCPs. Despite the importance of regulating whether EGFR is endocytosed, the molecular machinery controlling this is poorly understood. 1995 1993 1995 1995 1 1996 1994 1999 Text box 1 Ubiquitination  1 1999 2003 1 1999 2002 2004 2003 2005 2005 2003 1998 2003 2007 2002 2002 2002 2006 2004 2004 1999 2007 1998 2006 2000 At present, it remains obscure which mechanisms are responsible for sorting of the activated EGFR to coated pits, although it is clear that Cbl and Grb2 are central, and Eps15 may have important functions. One possible explanation for the apparent contradictory results regarding ubiquitin as an endocytic signal for EGFR is that it is not ubiquitination of EGFR itself but rather ubiquitination of accessory proteins by Cbl that is necessary for EGFR internalization. Endosomal sorting of EGFR: a key role of ubiquitination 2 2 2007 2003 2002 2006 2007 2004 2003 1999 2004 2 2002 2001 2002 2002 2006 2007 1 2006 2007 2007 2007 2006 2006 Differential effects of EGFR ligands on EGFR degradation 1 2001 2005 2008 1990 1991 2002 1998 2007 Clathrin-independent endocytic mechanisms involved in EGFR uptake 2005 1979 1988 2004 1995 2005 2005 2008 2006 2006 2005 2005 2002 2001 2002 2006 2006 3 2006 2006 Fig. 3 upper panel Arrows lower panel arrows Bars  In conclusion, although alternative clathrin-independent mechanisms of EGFR endocytosis have been suggested, clathrin-mediated endocytosis still appear to be by far the most important mechanism for EGFR downregulation. Endocytic downregulation of ErbB2 2004 1996 2004 2005 1993 1999 1996 2004 2006 2005 1993 1999 2004 2003 2003a 2003b 2000 1998 2001 2004 2006 2005 4 2004 Fig. 4 a b c c 2004 Bar  1993 1993 2007 2003 2000 1999 2000 2000 2004 1997 1996 2000 2000 2005 1999 1999 1999 1999 2005 1999 Although it is well established that ErbB2 is severely endocytosis impaired compared to EGFR, there is currently no consensus model of how this is achieved. Taking into account the major focus that ErbB2 receive as an oncogene and as a drug target, a better understanding of the mechanisms hindering ErbB2 endocytosis and lysosomal degradation is warranted. Endocytic downregulation of ErbB3 and ErbB4 1996 1997 1998 1999 2007 2007 2002 2002 2007 2007 2007 2002 2007 2007 2003 2002 Escape of endocytic downregulation in cancer 1 2000 2003 2007 2000 2003 2007 2003 2007 1990 1993 2002 1995 2003 2000 1995 1995 1996 1994 2005a 2005b 2000 2001 1998 1997 1995 1996 1998 2006 2007 2006 2007 2007 2007 Endocytic downregulation of ErbB receptors as a therapeutic target in cancer treatment Since lack of endocytic downregulation is an emerging theme in ErbB cancer biology, it is evident that stimulation of ErbB endocytosis and lysosomal degradation is an attractive means to inhibit tumor growth. Antibody-mediated crosslinking can induce internalization of ErbB receptors 4 2005 2004 2007 2003 2001 2001 2004 2004 2005 2007 2006 1986 2006 1995 1997 1999 2005 2004 2004 2005 2005 Hsp90 inhibition stimulates ErbB2 cleavage and internalization 2003 2008 2006 2005 2006 2006 2004 2004 2002 2003 2003 2002 2003 1996 2002 1999 2004 2001 2000 5 2004 2006 2005 2000 2006 2000 2001 2003 2007 5 2007 Fig. 5 a green red red red red green yellow 2006 b green 2007 Bars  2008 Perspectives In spite of its crucial importance both in health and disease, the ErbB receptor system still has hidden secrets regarding its regulation. It is evident that we need to know much more about the molecular mechanisms underlying endocytic downregulation of the otherwise heavily investigated ErbB receptors. One important area that deserves future attention is further mapping of the mechanisms responsible for recruitment of EGFR to CCPs. Moreover, EGFR endocytosis does not seem to be representative of the other ErbB receptor family members, which all appear to be endocytosis impaired to some extent compared to EGFR. It is especially of interest to decipher how formation of heterodimers between the different ErbB receptors affects their endocytic downregulation. In addition, the large number of ErbB ligands adds further complexity to the system, since they may vary in the capability to stimulate endocytosis and lysosomal degradation of their receptors. Interestingly, the few ligands that have been investigated so far differ significantly in their ability to induce endocytic downregulation of ErbB receptors. More insight into endocytic downregulation of ErbB receptors is indeed relevant for understanding the role of these receptors in cancer, and it could lead to identification of novel targets for cancer therapy directed at downregulating signaling receptors. Although still in the making, the clinical as well as preclinical results show that targeted therapy stimulating endocytic downregulating of ErbB receptors is a promising tool in cancer treatment.