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1986). Previous work also demonstrated that HCMV infection leads to EGFR
downregulation from the cell surface but not through binding and activation of the
receptor early in infection, as UV-inactivated virus does not induce downregulation
(Fairley et al. 2002; Beutler et al. 2003). Last, when the contribution of CD13 to
HCMV entry was being investigated, an EGFR polyclonal antibody was used as a
control to demonstrate that blocking the receptor had no effect on HCMV entry
(Soderberg et al. 1993a).
More recently, two other studies have been conducted, testing the role of EGFR
in HCMV entry and signaling. It was demonstrated that HCMV did not induce
activation of EGFR as measured by receptor autophosphorylation and an EGFR
kinase inhibitor was not able to inhibit HCMV gene expression in fibroblast cells
(Cobbs et al. 2007; Isaacson et al. 2007). Additionally, the same EGFR neutralizing
monoclonal antibody that was previously found by Wang et al. to inhibit HCMV
attachment and entry was used to pretreat fibroblasts cells, but led to no reduction
in virus entry (Isaacson et al. 2007). EGFR-null fibroblasts were also found to be
productive for IE gene expression and breast cancer cell lines differentially express-
ing EGFR were infected with HCMV but surprisingly, IE gene expression could not
be detected in either cell line, regardless of EGFR expression (Cobbs et al. 2007;
Isaacson et al. 2007). Additionally, epithelial and endothelial cells were pretreated
with EGFR-neutralizing antibodies and then challenged with a clinical isolates of
HCMV but again, no decrease in virus gene expression was detected (Isaacson
et al. 2007). Given the disparity between these EGFR studies, it is clear that more
work needs to be done to discover the contribution of EGFR to HCMV entry.
Several integrin heterodimers have proven to be HCMV entry receptors (Feire
et al. 2004; Wang et al. 2005). It has often been observed that following HCMV
infection, cells round up, suggesting cytoskeleton rearrangements are occurring
upon virus penetration. In fact, the actin cytoskeleton is dramatically altered
following HCMV entry and both integrins and focal adhesion kinase become
activated (Feire et al. 2004). Through the use of neutralizing integrin antibodies,
it was determined that the integrin heterodimers α2β1, α6β1, αVβ3 acted as
cellular receptors for HCMV entry (Feire et al. 2004). Beta 1 integrin null fibrob-
last cells demonstrated diminished HCMV entry but restoration of β1 integrin
expression also restored HCMV entry (Feire et al. 2004). A highly conserved
integrin-binding domain, the disintegrin-like domain (DLD), was also discovered
on gB, which was found to mediate binding to β1 integrins (Feire et al. 2004, Feire
et al., unpublished results). Additionally, a protein fragment encompassing the
DLD region and antibodies raised against this protein were able to specifically
inhibit HCMV entry (Feire et al., unpublished results). The role of the αVβ3
integrin heterodimer as an HCMV entry receptor was further supported by its
interaction with gH; however, an integrin-binding domain has yet to be discovered
in this glycoprotein (Wang et al. 2005). Additionally, coordination between αVβ3
and EGFR has been proposed, suggesting that gB and gH independently engage
EGFR and αvβ3, respectively, followed by αVβ3 movement into lipid rafts, where
it interacts with EGFR to coordinate virus entry and signaling (Wang et al. 2005).
This is an interesting model that is supported by HCMV entry at cholesterol-rich
