Autophagy (literally to ‘self-eat’) is an intracellular, catabolic mechanism to
degrade and recycle cytoplasmic contents in response to metabolic, oxidative, and
genotoxic stresses. Autophagy plays an important role in cellular homeostasis, and
dysfunctional autophagic activity has been implicated in an array of human diseases.
Importantly, autophagy has recently been identified to function in host defence against
intracellular pathogens, including viruses. For this reason, many viruses have evolved
strategies to subvert or exploit autophagy and block its antiviral effects. Kaposi’s
sarcoma-associated herpesvirus (KSHV) is the etiological agent of Kaposi’s sarcoma
(KS), an AIDS-related cancer of the endothelium. KSHV gene products have evolved to
support viral replication and evade immune surveillance. Some of these same gene
products impact KS tumourigenesis, but the precise mechanisms have yet to be
elucidated. Furthermore, the impact of autophagy on KSHV replication and KS
tumourigenesis remains unexplored.
The KSHV viral G-protein-coupled receptor (vGPCR) is a constitutively active
signalling molecule that stimulates a number of host regulatory pathways that would be
expected to impact autophagy, including PI3K/Akt/mTOR and JNK. Moreover, vGPCR
is expressed during lytic replication when the antiviral autophagic response may threaten
virion production. Here, vGPCR activity has been definitively shown to trigger
autophagy in endothelial cells using immunoblot analysis, fluorescent reporter proteins,
and transmission electron microscopy. Furthermore, preliminary data suggest that this
stimulatory effect is evoked through JNK activation. Taken together, these findings
indicate that vGPCR likely elicits autophagic responses during KSHV lytic replication.
Recently, autophagy has been recognized as a molecular barrier to
tumourigenesis, influencing cell survival, cell death, or a form of cell cycle arrest called
oncogene-induced senescence (OIS). Remarkably, like many host oncogenes, ectopic
expression of vGPCR triggers OIS in endothelial cells. This response is dependent on
vGPCR signalling activity, as an inactive form of vGPCR (R143A) fails to trigger OIS.
Furthermore, vGPCR OIS is atypical in that it does not involve DNA damage responses
(DDRs). Together, these autophagic and senescence responses to ectopic vGPCR
expression illustrate the potency of its oncogenic potential.
The significance of vGPCR-induced autophagy and senescence during KSHV
replication and KS development is presently unclear. I speculate that autophagy
represents a hurdle that the virus must overcome in vivo. In my working model, potent
vGPCR oncogenic signalling activity sets off the alarm, eliciting autophagic responses. It
seems likely that additional lytic viral gene products may serve to undermine these
autophagic responses and permit viral replication and dissemination in vivo.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:NSHD.ca#10222/36316 |
| Date | 16 June 2011 |
| Creators | Cyr, David P |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
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