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The role of CD44 in the interaction of
activated lymphocytes with melanoma: The potential use of CD44hi
LAK cells in the treatment of melanoma.
It is estimated that
approximately 50,000 new case of malignant melanoma are diagnosed each year
in the United States. Due to the widespread growth of the metastatic
lesions, surgical treatment is usually ineffective and many of these tumors
are relatively resistant to current chemotherapeutic agents. Therefore,
there is great need for finding new adjuvant therapies. In preliminary work,
we have observed that following exposure to IL-2 there is a significant
increase in the expression of CD44 and that lymphocytes expressing high
levels of CD44 have potent cytolytic activity. Little is know about the role
of CD44 expressed by activated lymphocytes in the interactions with
melanoma. Therefore, we plan to examine the role of CD44 in the interaction
between IL-2-activated lymphocytes and melanomas and to test the hypothesis
that lymphocytes expressing high levels of CD44 (CD44hi) are
responsible for mediating lysis of melanoma tumor cells and that this
increase in CD44 expression is directly due to increased expression of CD44
isoforms. Furthermore, we plan to examine the potential use of the CD44hi
cells in the treatment of melanoma. Knowledge gained from this study will
lead to better understanding of the role of CD44 in lymphocyte-mediated
lysis of melanoma tumor cells and may ultimately lead to significantly
improved treatments for a number of cancers, including melanoma.
The role of CD44 isoforms in the
regulation immune cell activation and death
Currently, we are involved in
research examining the role of CD44 in activation-induced cell death (AICD).
CD44 has been shown to be involved in a number of processes including
cell-cell and cell-matrix interactions, signal transduction and activation
of T cells, B cells, monocytes, and dendritic cells. More recently, we
demonstrated that CD44 may play an important role in AICD (JI
166(10):5889-97, International Immunology 14(9):1015 and Infection and
Immunity 73(1):50-61). In addition, we have shown that the expression of
CD44v7 plays an important role in mediating IL-2-induced VLS (JBC
278(44):43818). In contrast to our findings and others demonstrating
enhanced induction of apoptosis by signaling through CD44, other reports
have shown that signaling through CD44 can enhance survival by suppressing
the induction of apoptosis. Such controversies may have resulted due to the
fact that, in addition to the standard form of CD44, a number of different
isoforms can be expressed. CD44 is encoded by 20 exons and by alternative
splicing up to 10 invariant exons can be inserted within the extracellular
region, leading to the possible expression of a number of CD44 isoforms.
Therefore, the wide range of functions attributed to CD44 may be due to the
expression of specific isoforms. These contrasting findings stress the need
to further investigate the role played by CD44 variant isoforms in T cell
activation and apoptosis. Some initial goals of this work are: 1) To examine
the expression of CD44 isoforms on activated lymphocytes; and 2) To examine
the effect of signaling through specific CD44 isoforms on the regulation of
genes involved in T cell activation and apoptosis. The completion of these
studies will lead to a better understanding of the many roles CD44 plays in
the regulation of the immune response.
The effect of cannabinoid exposure on
tumor growth and the anti-tumor immune response
Previous studies have shown
that THC can suppress or enhance immune functions. In addition, studies have
shown that treatment with THC can be used to treat brain tumors due to the
high expression of CB1 as well as tumors of the immune system which is
mediated through CB2 (Blood 100:627-634). However, little is known about the
effect of THC and other cannabinoids on the immune response to and growth of
other tumors expressing varying levels of cannabinoid receptors. For
example, some tumors originating from breast and ovarian tissue express
considerably lower levels of cannabinoid receptors than tumors originating
from neural tissue and the immune system. Therefore, it is possible that
exposure to THC or other cannabinoids could on one hand; inhibit the growth
of tumors that express high levels of cannabinoids receptors by directly
inducing apoptosis while on other hand; enhance the growth of tumors with
low or no expression of cannabinoid receptors by suppressing the anti-tumor
immune response. In preliminary work we have shown that treatment of mice
with as low as 25 mg/kg THC significantly enhances 4T1 breast cancer growth
through suppression of the tumor-specific immune response. This suggests
that the growth of tumors deficient in cannabinoid receptors may be
facilitated by the ability of cannabinoids to inhibit the anti-tumor immune
response. In addition, CB2 is thought to be almost exclusively expressed on
cells of the immune system and other cells are believed to primarily express
CB1. Therefore, it may be possible to avoid the immunosuppressive effects of
THC by treating the CB1 positive tumor cells with a CB1 selective agonist.
Therefore we are interested in further characterizing the effects of THC and
other cannabinoids on tumor growth and the anti-tumor immune response. Some
specific goals of this work include: 1) Determining the expression of CB1
and CB2 on a variety cancer cell lines and determining whether the level of
expression of these receptors correlates with sensitivity to cannabinoid-induced
apoptosis in vitro and in vivo. 2) Characterizing the effect
of exposure to THC or other cannabinoid receptor agonists in vivo on
the anti-tumor immune response and determining the role of CB1 and CB2 in
cannabinoid-induced alterations of the anti-tumor immune response. If
cannabinoids can be shown to induce apoptosis in immune cells in vivo
and thereby regulate the anti-tumor immune functions, such findings could
have a significant impact in determining the possible effects of the use of
marijuana in cancer patients, suggesting that exposure to cannabinoids may
actually exacerbate tumor growth. Furthermore, if we find that the
susceptibility of tumors to cannabinoid-induced apoptosis is directly
related to the level of cannabinoid receptors, it may be possible to
manipulate the expression of cannabinoid receptors or target specific
cannabinoid receptors so that new treatments can be developed to treat
cancers.
The role of the endocannabinoid system
in the regulation of the immune response
The ability of cannabinoids,
such as THC, to alter the functions of the immune response is becoming
increasing clear. For example, exposure to THC has been shown to suppress NK
cells, macrophage and T cell responses. However, the mechanism of
cannabinoid-induced immunosuppression remains unclear. In preliminary work,
we have been able to show that in vitro or in vivo exposure to
cannabinoids leads to the induction of apoptosis in both splenocytes and
thymocytes and that this correlated with reduced responsiveness to
activation by polyclonal mitogens (JPET, 302:451-465). In addition we have
shown that dendritic cells are particularly sensitive to cannabinoid
exposure (JI, 173(4):2373). The recent characterization of the CB1 and CB2
cannabinoid receptors on cells of the immune systems and the discovery of
the endogenous cannabinoids, anandamide and 2-AG, suggests a possible role
of the endocannabinoid system in the regulation of the immune response. To
date, very little is known about the immunoregulatory properties of
endocannabinoids. Therefore, we are interested in examining the role of the
endocannabinoid system in the modulation of the immune response. The
specific areas of interest are: 1) To determine which cells and what
conditions lead to the production of endocannabinoids. In preliminary work,
I have been able to demonstrate the presence of endocannabinoids in various
tumors, suggesting a possible role of endocannabinoids in regulation of the
anti-tumor immune response. In addition, I am interested in examining the
production of endocannabinoids by cells of the CNS as well as specific cell
populations within the immune system. 2) To determine which cells are
sensitive to the effects of cannabinoids. Preliminary work from our
laboratory suggests the possibility that cannabinoids may induce alterations
in Th1/Th2 cytokine profile resulting in suppression of immunity and that
this may be due to a direct effect on T regulatory cells. 3) To determine
the mechanism of action of endocannabinoids by examining the effects of
signaling through the CB1 and CB2 receptors on regulation of genes involved
in lymphocyte regulation, including genes involved in differentiation,
activation, and apoptosis. Knowledge gained from this work will allow for a
better understanding of the regulation of the immune response and possibly
define a new avenue of communication and control between the CNS and the
immune system. In addition these studies may provide important information
leading to the development of new strategies to treat diseases that are
affected by the immune system. Currently, studies are underway examining the
effect of a number of unique synthetic cannabinoids on the immune response.
The results from these studies may lead to the development of powerful drugs
that specifically target the immune response without any unwanted
psychoactive effects. |
