Hasem Habelhah, PhD
Research Laboratory
Contact Information
Phone: 319-335-8168
1157 Medical Laboratories
Description of Research
The long-term objectives of our laboratory are to define the
mechanisms by which TRAF2 phosphorylation and RIP1 cleavage modulate
activation of transcription factor NF-κB, and to elucidate the
pathophysiological relevance of TRAF2 phosphorylation and RIP1 cleavage
with respect to the survival of cancer cells and their responses to
anti-cancer drugs. TRAF2 and RIP1 are key adaptor proteins that
transduce signals emanating from many members of the TNF receptor
superfamily, resulting in activation of the MAPK and IKK pathways. IKK
activates NF-κΒ, which in turn induces the expression of genes whose
products are involved in inflammation, the immune response, cell
proliferation, and the suppression of both death receptor- and
stress-induced apoptosis. Abnormal NF-κB activation has been observed in
various inflammatory diseases and many types of cancers. However, the
mechanisms underlying constitutive activation of NF-κB in cancer cells
are still largely elusive. We mapped two TRAF2 phosphorylation sites and
two RIP1 cleavage sites, and found that TRAF2 phosphorylation and RIP1
cleavage regulate NF-κB activation in response to stimulation by certain
inflammatory cytokines and cellular stresses.
Main Research Areas
1. The role of TRAF2 phosphorylation in TNFα-induced and NF-κB-dependent gene expression.
Many serine/threonine kinases, such as PKCζ and TBK1, have been shown
to promote NF-κB-dependent gene expression in response to TNFα
stimulation. However, gene-targeting and biochemical studies have
revealed that these kinases do not directly activate IKK. We mapped two
phosphorylation sites (Ser-11 and Ser-55) on the N-terminal region of
TRAF2, and identified TBK1 and PKCζ as the kinases that directly
phosphorylate TRAF2 at Ser-11 and Ser-55, respectively. Functional
studies revealed that TRAF2 phosphorylation at these sites increases
basal and inducible NF-κB activity, resulting in increased expression of
anti-apoptotic proteins. Importantly, we found that TRAF2 is
constitutively phosphorylated in some cancer cell lines as well as in
some human tumor tissues, but not in normal tissues. This suggests that
TRAF2 phosphorylation is one of the events responsible for the elevation
of basal NF-κB activity in cancer cells. Currently, we are
characterizing the mechanism by which TRAF2 phosphorylation regulates
IKK activation and NF-κB-dependent gene expression.
2. The role of TRAF2 phosphorylation in cancer cell adaptation to chronic stress.
In addition to inflammatory cytokines, growth factors, endoplasmic
reticulum (ER) stress, oxidative stress and DNA-damaging agents also
activate NF-κB. We recently found that ER and oxidative stresses also
strongly induce TRAF2 phosphorylation at both sites, and that such TRAF2
phosphorylation plays an essential role in NF-κB activation and cell
survival in the context of chronic ER and oxidative stresses. The tumor
microenvironment is characterized by hypoxia, low glucose and free
radicals, and known to trigger chronic ER and oxidative stresses.
Cancer-cell adaptation to such chronic stresses has profound
consequences for malignant progression and the response to therapy. Our
findings suggest that TRAF2 phosphorylation may play a critical role in
tumor progression by promoting cancer-cell adaptation to chronic ER and
oxidative stresses. Currently, we are characterizing the molecular
mechanisms underlying the regulation of TRAF2 phosphorylation in
response to ER and oxidative stresses, and are exploring its
physiological and pathophysiological significance in NF-κB activation
versus cell death under conditions of ER and oxidative stresses.
3. The role of RIP1 cleavage in death receptor-induced NF-κB activation.
TRAIL, a ligand for death receptors 4 and 5 (DR4/5), is considered a
potential anti-cancer agent as it shows selective high cytotoxicity
toward tumor cells and little or no toxicity against normal cells.
However, recent studies have demonstrated that TRAIL also activates
NF-κB and increases metastasis in DR-resistant cancer cells.
Paradoxically, caspase-8 activity has been show to be essential for both
DR-induced cell death and NF-κB activation. However, the caspase-8
substrates that mediate this form of NF-κB activation have not been
identified. We found that caspase-8 cleaves RIP1 at two sites in
response to TRAIL stimulation, and that caspase-8-mediated partial
cleavage of RIP1 plays a critical role in TRAIL-induced NF-κB
activation. Most importantly, in some human tumor samples, RIP1 is
constitutively cleaved into an active form, suggesting that limited RIP1
cleavage is another culprit that is responsible for the up-regulation
of NF-κB activity in cancer cells. Currently, we are characterizing the
mechanisms by which caspase-mediated cleavage of RIP1 activates NF-κB.
(See complete publications list at PubMed)