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Liu Lab
(Dr. Kebin Liu)
The immune system constantly detects and eliminates proneoplastic cells long before the development of clinical cancer. Tumor-specific immune responses can also be induced in cancer-bearing hosts via active or adaptive immunotherapy, yet tumors still develop in an immune-competent host, and complete tumor eradication occurs infrequently in vivo. The biologic failure of the immune system to effectively suppress neoplastic disease in immune-competent hosts is not fully understood and has remained a fundamental paradox of tumor immunobiology. Our research interests reflect two fundamental aspects of tumor immunobiology: (1) the dynamic nature of the host T lymphocyte-tumor cell interaction and functional suppression of tumor-specific T lymphocytes in the tumor microenvironment; and (2) the molecular mechanisms underlying immunoselection and tumor escape as a negative consequence of an anti-tumor immune response.
Model of immune cell and tumor cell
interactions: when tumor cells (orange) arise in normal tissue, an
inflammatory response is induced which attracts immune cells (green)
migrating to the tumor site. Interactions between immune cells and tumor
cells involve direct cell-cell physical contact and release of modulator
molecules, primarily cytokines and chemokines. In this life and death
battle, on the one hand, the immune cells may prevail and eradicate the
tumor cells. On the other hand, tumor cells can counterattack the immune
cells by producing inhibitory molecules, manipulating the immune suppressive
cells, or acquiring resistant mechanisms to avoid destruction by the immune
system. Thus, the anti-tumor immune response can be a two-edged sword and
can result in both positive and negative consequences.
Project 1. Immune cell-tumor cell
interactions and suppression of tumor-specific T lymphocytes in the tumor
microenvironment.
CD8+ T
lymphocytes and IFNg play critical
roles in controlling tumor development. However, although tumor-specific CD8+
T lymphocytes are often present inside the tumor, these T cells ignore or
tolerate the presence of tumor cells, and the tumor continues to grow and
metastasize. Among various mechanisms, the dysfunction of T cells in the
tumor microenvironment seems to be a common mechanism for many types of
tumors. Tumor-antigen-specific CD8+ T cells can lyse the tumor in
the periphery, but are often functionally tolerant in situ,
even though the cognate antigens are expressed in situ. Furthermore,
removal of T cells from tumor lesions often can reverse the tolerance and
restore T cell cytotoxicity, thereby suggesting that activation of T cell
effector mechanisms is suppressed in the tumor microenvironment.
A mouse model of experimental metastasis and
CTL adoptive transfer immunotherapy. A tumor-specific CD8+ T cell
line (2/20) was isolated from the tumor-bearing mouse. Metastatic tumor
cells were injected into the mouse iv. The transplanted tumor cells
metastasize to and colonize in the lung (left). Adoptive transfer of
tumor-specific CTL effectively eliminates tumor development in the lung
(right).
The general objective of
this project is to elucidate the molecular mechanisms underlying suppression
of cytotoxicity of tumor-specific CD8+ T lymphocytes by immune
suppressive cells or inhibitory molecules in the tumor microenvironment in
mouse tumor models and clinical tumor samples from cancer patients. Focus is
placed on two aspects: identification of mediator molecules released from
both immune suppressive cells and tumor cells, and molecular mechanisms
underlying the transcriptional repression of the effector mechanisms of
tumor-specific T lymphocytes. Genomic, proteomic and conventional protein
biochemical approaches are being used to identify these protein factors.
Project 2. Epigenetic
repression of apoptosis mediators and tumor escape.
The anti-tumor immune
response is a two-edged sword. The immune system consistently monitors the
host and suppresses tumor development. However, at the same time, the
anti-tumor immune response can also “edit” the tumor cells, and promotes or
selects tumor escape variants with enhanced resistance to the extrinsic
cytotoxicity of the immune system. This phenomenon is conceptually akin to
the generation of antibiotic-resistant bacteria, or chemoresistant
neoplastic clones.
Among the multiple
mechanisms, apoptotic resistance seems to be one of the hallmarks of tumor
escape and progression. As the neoplastic process becomes more progressive,
the resulting subpopulations of tumor cells tend to exhibit a more
apoptotic-resistant phenotype. Fas, a key apoptotic effector, is
constitutively expressed at high levels in normal tissue and cells; however,
the pattern of Fas expression is altered in most tumor cells. For example,
in over one-third of the colon carcinoma tissues examined the level of Fas
expression was diminished, while the complete loss of detectable Fas
expression was much more frequent in metastatic lesions as compared with
primary lesions. We and other have demonstrated that loss of Fas expression
and function is characteristic of enhanced metastatic competence in human
colon carcinomas, mouse breast mammary carcinomas and mouse melanoma.
Therefore, loss of Fas function appears to be a hallmark of immunoediting
and immunoselection.
Diminished responsiveness to inflammatory
cytokine IFNg
is associated with metastatic
phenotype in human colon carcinoma cells.
Primary and metastatic colon carcinoma cell
lines were treated with IFNg for 4 and 24 hrs respectively, and analyzed for the changes in
genome-scale gene expression kinetics. Red squares indicate up-regulation,
and green squares indicate down-regulation.
Our research interest is to
elucidate the molecular mechanisms underlying apoptosis resistance in tumor
escape variants and metastatic tumors. Because the expression of Fas and
other molecules in the Fas-mediated death signaling pathway is regulated by
IFNg in many types of tumors, our
effort is focused on the IFNg
signaling pathway in solid tumor cells. IFNg,
secreted primarily by activated T cells and NK cells after antigen is
encountered, is a pleiotropic proinflammatory cytokine that has been shown
to sensitize many types of tumor cells to Fas-mediated apoptosis. Primary
tumor cells are generally sensitive to IFNg
sensitization, whereas malignant tumor cells often show no responses
to IFNg. Therefore, identification
of molecular defects in the IFNg
signaling pathway in malignant tumor cells will likely reveal the molecular
mechanisms of apoptosis resistance. We have recently identified Interferon
Regulatory Factor 8 [IRF8, also known as Interferon
Consensus Sequence-Binding Protein (ICSBP)] as a
key IFNg-regulated molecule that is
involved in regulation of Fas-mediated apoptosis in solid tumor cells. We
revealed that IRF8 is silenced in metastatic colon carcinoma cells by DNA
hypermethylation in its promoter region. Current focus is placed on
molecular interactions between DNMTs, MBDs, and IFNg-regulated transcription factors and chromatin remodeling in the
IRF8 promoter region in metastatic tumors.
Project 3: Regulation of
telomerase activity and T cell replicative potential in tumor-specific T
lymphocytes.
Telomeres are the terminal
structures of linear chromosomes, and consist of hexanucleotide repeats and
telomere-binding proteins. Telomeres are synthesized by telomerase
holoenzyme, a protein complex composed of two essential components: an RNA
template termed TR and a catalytic reverse transcriptase protein termed TERT. The inability of DNA polymerase to fully replicate the ends of the linear
chromosome during cell division leads to incomplete terminal DNA synthesis
of the lagging strand. Therefore, in the absence of telomerase,
approximately 50-200 bases of terminal DNA are lost with each cell
replication cycle. Most types of human cells lack telomerase activity.
Consequently, telomeres shorten with successive rounds of cell division, and
progressive telomere shortening will ultimately lead to the replicative
senescence of the cell, a state characterized by continued cell viability
without further cell division. It has been demonstrated that telomere length
of adoptively transferred lymphocytes correlates with in vivo persistence of the T lymphocytes and tumor regression in human cancer
patients, suggesting that maintenance of telomere length and proliferative
potential of tumor-reactive T lymphocytes play an important role in the
effector function of tumor-specific T lymphocytes in adoptive
immunotherapy.
The
PI-3 kinase signaling pathway is involved in
regulation of telomerase activation in T lymphocytes. T lymphocytes were
isolated from peripheral blood of normal human donors by negative
immunomagnetic separation procedures. The purified T lymphocytes were
stimulated with anti-CD3 and anti-CD28 mAbs in the presence of different
concentrations of LY294002 and analyzed for telomerase activity using a
modified telomeric repeat amplification protocol.
Our interest is to
elucidate the molecular mechanisms underlying the regulation of telomerase
activity in tumor-specific T lymphocytes by posttranscriptional mechanisms.
We have demonstrated previously that telomerase activity in T lymphocytes is
regulated both at the TERT transcriptional level and through
posttranscriptional modification of TERT. Our current data suggest that
other protein factor(s) are associated with the telomerase complexes to
modulate telomerase activity. Therefore, telomerase-associated proteins are
involved, at least in part, in regulation of telomerase activity and
maintenance of telomere length in T lymphocytes. Current emphasis is being
put on identification of the components of telomerase complex in
tumor-activated T lymphocytes and the roles of these protein factors in
regulation of telomerase activity and T lymphocyte replicative potential.
Selected publications:
1. Dafeng Yang, Trina I. Stewart, Kimberly, K. Smith, David Georgi, Scott I. Abrams, and Kebin Liu. 2008. Downregulation of IFN-γR in Association with Loss of Fas Function is Linked to Tumor Progression. Int. J. Cancer. 122:350-362.
2. Kristy M. Greeneltch, Monika Schneider, Seth M. Steinberg, David J. Liewehr, Trina J. Stewart, Kebin Liu, and Scott Abrams. 2007. Host Immunosurveillance Controls Tumor Growth via IRF-8-Dependent Mechanisms. Cancer Res. 67:10406-10416.
3. Dafeng Yang, Muthusamy Thangaraju, Darren D. Browning, Zheng Dong, Borys Korchin, Dina Chelouche Lev, Vadivel Ganapathy, and Kebin Liu. 2007. Interferon Regulatory Factor 8 Mediates Apoptosis in Non-hemopoietic Tumor Cells via Regulation of Fas Expression. J. Immunol. 179:4775-4782.
4. Dafeng Yang, Najam ud Din, Darren D. Browning, Scott I. Abrams, and Kebin Liu. 2007. Targeting Lymphotoxin β Receptor with Tumor-Specific T Lymphocytes for Tumor Regression. Clin. Cancer Res. 13:5202-5210.
5. Dafeng Yang, Muthusamy Thangaraju, Kristy Greeneltch, Darren D. Browning, Patricia Schoenlein, Tomohiko Tamura, Keiko Ozato, Vadivel Ganapathy, Scott I. Abrams, and Kebin Liu. 2007. Repression of IRF8 by DNA Methylation is a Molecular Determinant of Apoptotic Resistance and Metastatic Phenotype in Metastatic Tumor Cells. Cancer Res. 67:3301-3309.
6. Akio Ohta, Elieser Gorelik, Simon Prasad, Franca Ronchese, Dmitriy Lukashev, Michael Wong, Xiaojun Huang, Sheila Caldwell, Kebin Liu, Patrick Smith, Jiang-Fan Chen, Edwin Jackson, Sergey Apasov, Scott Abrams, Michail Sitkovsky. 2006. A2A Adenosine Receptor Protects Tumors From Anti-tumor T cells. Proc. Natl. Acad. Sci. USA. 103:13132-13137.
7. Kebin Liu*, Sheila A. Caldwell*, Kristy Greeneltch, Dafeng Yang and Scott Abrams. 2006. CTL Adoptive Immunotherapy Concurrently Mediates Tumor Regression and Tumor Escape. J. Immunol. 176:3374-3382.
(* equal contribution).
8. Kebin Liu, Sheila A. Caldwell and Scott I. Abrams. 2005. Immune selection and emergence of aggressive tumor variants as negative consequence of Fas-mediated cytotoxicity and altered IFN-γ regulated gene expression. Cancer Res. 65:4376-4388.
9. Kebin Liu, Sheila Caldwell and Scott I. Abrams. 2005. Cooperative disengagement of Fas and ICAM-1 in neoplastic cells confers enhanced colonization efficiency. Cancer Res. 65:1045-1054.
10. Kebin Liu, Elwood McDuffie and Scott I. Abrams. 2003. Exposure of human primary colon carcinoma cells to anti-Fas interactions influences the emergence of pre-existing Fas-resistant metastatic subpopulations. J. Immunol. 171:4164-4174.
11. Kebin Liu and Scott I. Abrams. 2003. Coordinate regulation of ICSBP and caspase-1 in the sensitization of human colon carcinoma cells to Fas-mediated apoptosis by IFN- γ. J. Immunol 170:6329-6337.
12. Kebin Liu and Scott I. Abrams. 2003. Alterations in Fas expression are characteristic of, but not solely responsible for, enhanced metastatic competence. J. Immunol. 170:5973-5980.
13. Mala Chakraborty, Scott I. Abrams, Kevin Camphausen, Kebin Liu, Tamalee Scott, C. Norman Coleman, and James W. Hodge. 2003. Irradiation of tumor cells upregulates Fas, enhances CTL lytic activity and CTL adoptive immunotherapy. J. Immunol. 170:6338-6347.
14. Kebin Liu, Marta Catalfamo, Yu Li, Pierre A. Henkart, and Nan-ping Weng. 2002. IL15 mimics T cell receptor crosslinking in the induction of cellular proliferation, gene expression, and cytotoxicity in CD8+ memory T cells. Proc. Natl. Acad. Sci. USA. 99:6192-6197.
15. Nan-ping Weng, Kebin Liu, Marta Catalfamo, Yu Li and Pierre A. Henkart. 2002. IL-15 is a growth factor and an activator of CD8 memory T cells. Ann. N.Y.Acad. Sci. 975:46-56.
16. Kebin Liu, Yu Li, Vinayakumar Prabhu, Lynn Young, Kevin Becker, Peter Munson, and Nan-ping Weng. 2001. Augmentation in expression of activation-induced genes differentiates memory from naïve CD4+T cells and is a molecular mechanism of enhanced cellular response of memory CD4+ T cells. J. Immunol. 166:7335-7344.
17. Kebin Liu, Richard J. Hodes and Nan-Ping Weng. 2001. Cutting Edge:Telomerase activation in human T lymphocytes does not require increase in telomerase reverse transcriptase (hTERT) protein but is associated with hTERT phosphorylation and nuclear translocation. J. Immunol. 166:4826-4830.
18. Kebin Liu, Michelle M. Schoonmaker, Bruce L. Levine, Carl H. June, Richard J. Hodes and Nan-Ping Weng. 1999. Constitutive and regulated expression of telomerase reverse transcriptase (hTERT) in human lymphocytes. Proc. Natl. Acad. Sci. USA. 96:5147-5152.
19. Kebin Liu and Guangpu Li. 1998. Catalytic domain of the p120 Ras GAP binds to Rab5 and stimulates its GTPase activity. J. Biol. Chem. 273:10087-10090.
20. Michelle M. Hanna and Kebin Liu. 1998. Nascent RNA in transcription complexes interacts with CspE, a small protein in E. coli implicated in chromatin condensation. J. Mol. Biol. 282:227-239.
21. Kebin Liu, Yuying Zhang, Konstantin Severinov, Asis Das and Michelle M. Hanna. 1996. Role of Escherichia coli RNA polymerase alpha subunit in mudulation of pausing, termination and anti-termination by transcription elongation factor NusA. The EMBO J. 15:150-161.
22. Kebin Liu and Shauna Somerville. 1996. Cloning and characterization of a highly repeated DNA sequence in Hordeum vulgare L. Genome. 39:1159-1168.
23. Kebin Liu and Michelle M. Hanna. 1995. NusA contacts nascent RNA in Escherichia coli transcription complexes. J. Mol. Biol. 247:547-558.
24. Kebin Liu and Michelle M. Hanna. 1995. NusA interferes with interactions between the nascent RNA and the C-terminal domain of the alpha subunit of RNA polymerase in Escherichia coli transcription complexes. Proc. Natl. Acad. Sci. USA. 92:50