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Cell and Molecular Biology Section
The
CMBS studies the molecular mechanisms required to control growth, induce
differentiation and suppress tumorigenicity in pediatric neuroectodermal
tumors. Retinoic acid (RA) induced differentiation of Neuroblastoma tumors
continues to be our paradigm for studies that define signal transduction
pathways that mediate these processes. Delineation of these pathways will
not only enable a more efficacious clinical utilization of retinoids and
their cogeners but has the possibility to identify novel molecules with
therapeutic potential for the treatment of neuroectodermal tumors. Trk Gene Expression and Function in Neuroblastoma: Trks and their
ligands are survival and differentiation factors for neural cells. In
NB, Trks serve as tumor markers: Good prognosis tumors express TrkA while
most poor prognosis tumors express BDNF and TrkB. We first identified
that neuroblastomas expressed TrkB from our finding that although cell
lines express little TrkA or TrkB, retinoids induces TrkB in vitro. We
have studied the functional consequences of activation of Trk signal transduction
paths in a NB cell line from a poor prognosis patient that expresses TrkA
or TrkB by gene transfection or selective stimulation. Activation of the
BDNF-TrkB path stimulates cell survival, induces neurite extension, alters
chemosensitivity and increases cell migration (invasion). In contrast,
activation of the NGF-TrkA path decreases cell growth, invasiveness and
tumorigenicity. Collaborative studies are in progress to determine intracellular
signaling molecules that may account for the different functional consequences
of Trk activation in NB cells. Our studies indicate that Trks not only
serve as tumor markers but also impact on the biology of the tumor cell.
If mechanisms can be identified that increase TrkA and decrease TrkB expression
in NB cells then it may be possible to alter the biologic behavior of
the tumor cells in vivo and this may have clinical applicability. Mechanisms of Trk Gene Regulation:To effectively manipulate the
level of Trk mRNA and ultimately protein in a cell, molecular mechanisms
regulating gene transcription need to be defined. Since there have been
no studies delineating Trk gene regulation even in animal models, we have
cloned the human TrkB and TrkA promoters and are in the process of cloning
the TrkC promoter. The functional and biochemical characterization of
the TrkB gene promoter and its regulation by RA identified a novel RA
inducible transcription unit involving an INR element and a 3' Hormone
responsive element. RA induced growth control: TGFB-TGBR:The finding that 1nM RA induces
TrkB and differentiation of NB cells without altering cell proliferation
functionally separated signals stimulating differentiation from those
controlling cell growth. Continuing studies exploring the mechanism of
RA induced growth arrest have focused on the TGFB signaling path. In NB
cell lines that are G1 growth arrested by RA, we find a 40-fold increase
in TGFB1 and TGFRI, RII and RIII expression. Cell lines resistant to RA
induced growth inhibition constitutively express this signal transduction
path and RA decreases expression of ligand and receptors. Current collaborative
studies are characterizing TGFBR structure and differences in signal transduction.
Most RA resistant cells express IGF2 and the crosstalk between these paths
is also under study. Cell cycle gene analysis has demonstrated that RA
induces a decrease in cell cycle kinase activity that is mediated by specific
upregulation of cell cycle inhibitors.
Carol
J. Thiele Ph.D., Cell & Molecular Biology Section
Last Updated:
July 26, 2006
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