Biology Reference
In-Depth Information
rapamycin in the absence of FKBP12 is modest, but when rapamycin is bound to
FKBP12, the affinity is increased by 2,000-fold (Banaszynski et al. 2005).
FKBP12-rapamycin bound to the FKBP-rapamycin binding domain is believed to
prevent association between mTOR and raptor (Halford et al. 2001; Hara et al.
2002; Kim et al. 2002). The difference in rapamycin sensitivity between mTORC1
and mTORC2 may be due to structural variations that result from the different
binding partners in the complexes. In this model, the mTOR-raptor interaction
leaves the FKBP-rapamycin binding domain available while the mTOR-rictor
interaction occludes it, thus affording rapamycin insensitivity.
These examples of mTORC1 inhibition by stress responses and drugs highlight
the need for HCMV to counteract inhibitory mechanisms at multiple points in the
PI3K-Akt-TSC-mTOR pathway. That HCMV can do this has been demonstrated
in studies showing that HCMV infection can proceed under conditions of hypoxia
(Kudchodkar et al. 2004), under conditions where AMPK is activated (Kudchodkar
et al. 2007; N.J. Moorman et al., personal communication) and can overcome the
inhibitory effects of rapamycin (Kudchodkar et al. 2006). In addition, preliminary
data suggest that HCMV can grow, with reduced kinetics, under conditions of
glucose and amino acid depletion (our preliminary data). Indeed, a growing body
of evidence shows that HCMV infection may affect many aspects of the pathway
downstream of Akt. Known points of interaction between the pathway and
HCMV are shown in red in Fig. 3. In the following sections we will highlight
HCMV's effects on AMPK, TSC, the mTOR complexes and mTOR's down-
stream effectors.
The Effects of HCMV Downstream of Akt
Experiments using the AMP mimetic AICAR to activate AMPK have been utilized to
analyze the effects of activated AMPK during HCMV infection (Kudchodkar et al.
2007; N.J. Moorman et al., personal communication). Addition of AICAR at 4 h
p.i. completely inhibits the viral infection due, at least in part, to the AMPK-mediated
inhibition of mTORC1 and translation. Under these conditions, the HCMV immediate
early proteins are not expressed and the infection cannot be established. However,
if immediate early proteins are expressed before the addition of AICAR (e.g.,
AICAR addition at 12 h p.i.), there is little to no effect on mTORC1 activity, as
indicated by the phosphorylation of 4E-BP and S6K. This observation suggests that
the expression and action of the immediate early proteins establishes conditions
that are resistant to the effects of activated AMPK. This may involve direct effects
on AMPK or downstream at the TSC or Rheb-GTP (Fig. 1). Existing data suggest
that HCMV targets AMPK and the TSC (Kudchodkar et al. 2007; N.J. Moorman
et al., personal communication); no studies have been reported examining
Rheb-GTP.
HCMV infection appears to induce a means to limit the level of phosphorylated
AMPK in the infected cell, possibly by dephosphorylation (Kudchodkar et al.
