Biology Reference
In-Depth Information
Energy Depletion
AMPK activation of the TSC is most often associated with energy depletion
(reviewed in Luo et al. 2005; Kimble 2006), which can occur during the highest
metabolic and synthetic periods of an HCMV infection. Binding of AMP to
AMPK is thought to alter its conformation, permitting subsequent phosphorylation
by upstream protein kinases. In contrast, ATP binding prevents phosphorylation
and maintains AMPK in its inactive state. Thus, AMPK responds not only to
changes in AMP concentration but also to changes in the ratio of AMP to ATP.
This means of activating AMPK can be mimicked using the AMP mimetic
5-amino-4-imidazolecarboxamide ribose (AICAR). Under conditions of an
increased AMP/ATP ratio or in the presence of AICAR, the tumor suppressor
LKB1 phosphorylates AMPK. In addition, the calcium/calmodulin-dependent
protein kinase kinase-β (CaMKKβ) also phosphorylates and activates AMPK in
response to changes in intracellular calcium concentrations (Hardie 2007).
Both LKB1 and CaMKKβ-mediated mechanisms for activating AMPK could
occur during HCMV infection. The increased energy utilization in actively infected
cells would increase the AMP:ATP ratio, thus activating AMPK. In addition, it has
been shown that the HCMV UL37×1 protein mobilizes calcium from the endoplasmic
reticulum into the cytosol (Sharon-Friling et al. 2006); this could potentially activate
CaMKKβ.
Amino Acid Depletion
Amino acid depletion inhibits mTORC1 through a mechanism that interferes with
the binding of Rheb-GTP to mTOR kinase (reviewed in Avruch et al. 2006). Amino
acid depletion, resulting in mTORC1 inhibition, could occur during HCMV infec-
tion at times of the greatest viral protein synthesis.
Rapamycin
Direct inhibition of mTOR kinase activity in mTORC1 can be achieved using the
drug rapamycin, a macrolide antifungal agent isolated from
Streptomyces hygro-
scopicus
(Vezina et al. 1975). It is used clinically as an immunosuppressant in
organ transplantation, an inhibitor of restenosis of arteries after angioplasty, and an
anti-cancer drug (Bjornsti and Houghton 2004). Rapamycin forms a complex with
a 12-kDa immunophilin called the FK506 binding protein 12 (FKBP12) and can
also bind a 100-amino acid domain (E2015 to Q2114) of mTOR kinase known as
the FKBP-rapamycin binding domain (Brown et al. 1994; Sabatini et al. 1994;
Chen et al. 1995). The affinity of mTOR's FKBP-rapamycin binding domain for
