Act One: Nef
Inhibits Glucose Uptake by Adipocytes and May Induce Insulin Resistance
in HIV+ Patients
The Human Immunodeficiency virus contributes to the development
of metabolic abnormalities in HIV-infected people, a process independent
of antiretroviral (ART) therapy. For example, gene expression in
primary subcutaneous adipose tissue is altered in ART-naïve
HIV-1-infected patients. Viral accessory proteins like Vpr and Nef
have been implicated in the development of insulin resistance and
changes in cholesterol synthesis and storage.
Insulin resistance occurs when insulin fails to stimulate sufficient
myocyte and adipocyte glucose uptake, resulting in persistent hyperglycemia
with the eventual development of Type II (non-insulin-dependent)
diabetes mellitus. A better understanding of the molecular mechanisms
driving insulin resistance associated with HIV infection could assist
in the control of the disease and its associated morbidities. Nef
is non-structural viral protein that modulates several cell processes.
It downregulates cell surface expression of several proteins, alters
signal transduction pathways, and interacts directly with the cytoskeleton
and proteins involved in reorganizing cytoskeletal structures. Many
of these cell functions are required for successful insulin signaling
and redistribution of Glucose Transporter 4 (Glut4), the primary
insulin-responsive glucose transporter in adipocytes, from intracellular
storage compartments to the membrane. Nef is detectable and antigenic
in patient sera and can influence cell function even in cells uninfected
with HIV-1. We have sought to determine whether Nef alters adipocyte
function in the context of insulin resistance and glucose homeostasis.
We found that when exposed to recombinant HIV Nef, insulin-stimulated
adipocytes take in approximately 50% less glucose than control cells
without affecting the percentage of viable adipocytes. We observed
a significant inhibition of insulin-induced Glut4 fusion with the
plasma membrane following Nef exposure while Glut4 translocation
was slightly diminished. These observations could not be explained
by a disruption to proximal insulin signaling, as there was no appreciable
difference in the amount of insulin-stimulated Akt phosphorylation
between Nef-treated cells and control cells. Insulin-stimulated
rearrangements of the cortical actin ring are required for Glut4
fusion with the plasma membrane. We observed significant disruption
to the cortical actin rings of Nef-exposed cells that mimicked the
effects of Latrunculin-B treatment. This disruption was observed
before and after insulin stimulation. We conclude that Nef decreases
glucose uptake by insulin-stimulated adipocytes by decreasing Glut4
surface expression. This is likely accomplished by inhibition of
insulin-induced cortical actin ring dynamics and Glut4 trafficking
without interfering with proximal insulin signaling. We have identified
HIV Nef as a novel contributor to the development of insulin resistance
in HIV-infected people.
Act Two: Interactions between HIV and Epstein-Barr Virus: Implications
for Abnormal Immune Activation
Much of the notoriety of HIV is derived from the immune destruction
it causes. However, HIV also invokes prolonged generalized immune
system activation that persists throughout the course of the disease.
This is manifest in particular by production of excess inflammatory
mediators and their downstream effectors. Though antiretroviral
therapy (ART) has substantially improved HIV disease outcome, immune
activation and inflammation remain recalcitrant to antiretroviral
treatment. One potential factor influencing immune activation is
concomitant infection with Epstein-Barr Virus (EBV), a ubiquitous
pathogen that is best known for causing Infectious Mononucleosis.
Research has shown that HIV-infected people have more EBV DNA in
their blood than HIV-negative people, and higher circulating EBV-specific
antibodies. In addition, in HIV-infected people the incidence and
severity of EBV-associated malignancies such as Burkitt’s
Lymphoma is higher. We are examining the potential interactions
between HIV and EBV, and the effects of their relationship on abnormal
immune activation and inflammation.
We have determined that HIV-positive patients have a greater number
of EBV-specific T cells than their HIV-negative counterparts. This
finding may reflect the presence of increased EBV antigen load,
and the quantity of circulating EBV DNA in the patients will be
determined by quantitative PCR. We will also investigate whether
the non-structural HIV protein Tat, a transcription factor, enhances
EBV replication.
Publication:
L.
Cheney, J. Hou, S. Morrison, J. Pessin, R. Steigbigel.
(2009). Nef Inhibits Glucose Uptake in Adipocytes and Can Account
for the Insulin Resistance Caused by Human Immunodeficiency Virus
Type-I. Submitted.
Posters:
Oct. 2006: 44th
Annual Meeting of the Infectious Disease Society of America (IDSA):
L. Chirch, L. Cheney, S. Morrison, R. Steigbigel. Elevated Pro-Inflammatory
Cytokines and Cellular Activation in HIV-infected Patients with
Lipoatrophy.
Sept. 2007:
47th Annual Interscience Conference on Antimicrobial Agents and
Chemotherapy (ICAAC): L. Cheney, J. Hou, S. Morrison, J. Pessin,
R. Steigbigel. Retroviral Nef Inhibits Glut-4 Transport and may
contribute to Insulin Resistance in HIV+ Patients.
Feb. 2008: 15th
Annual Conference on Retroviruses and Opportunistic Infections (CROI):
L. Cheney, J. Hou, S. Morrison, J. Pessin, R. Steigbigel. Nef Inhibits
Glucose Uptake and May Induce Insulin Resistance in HIV+ Patients.
Feb. 2009: 16th
Annual Conference on Retroviruses and Opportunistic Infections (CROI):
L. Cheney, J. Hou, S. Morrison, J. Pessin, R. Steigbigel. Nef Alters
Cortical Actin Ring Dynamics and Inhibits Glucose Uptake by Adipocytes:
A Contribution to Insulin Resistance in HIV-Infected People.
