THESIS
2013
xiv, 136 pages : illustrations (some color) ; 30 cm
Abstract
The FHIT tumor suppressor gene is arguably the most commonly altered gene in cancer since
it is inactivated in about 60% of human tumors. Despite the fact that Fhit functions as a tumor
suppressor, the pathway through which Fhit inhibits growth of cancer cells remains largely
unknown. Phosphorylation by Src tyrosine kinases provides a linkage between Fhit and
growth factor signaling. Since many G proteins can regulate cell proliferation through
multiple signaling components including Src, I explored the relationship between Gα subunits and Fhit. Several members of the Gα
q subfamily (Gα
16, Gα
14, and Gα
q) have been
found to co-immunoprecipitate with Fhit in their GTP-bound active state in HEK293 cells.
The binding of activated Gα
q members to Fhit appeared to be direct. The use of G...[
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The FHIT tumor suppressor gene is arguably the most commonly altered gene in cancer since
it is inactivated in about 60% of human tumors. Despite the fact that Fhit functions as a tumor
suppressor, the pathway through which Fhit inhibits growth of cancer cells remains largely
unknown. Phosphorylation by Src tyrosine kinases provides a linkage between Fhit and
growth factor signaling. Since many G proteins can regulate cell proliferation through
multiple signaling components including Src, I explored the relationship between Gα subunits and Fhit. Several members of the Gα
q subfamily (Gα
16, Gα
14, and Gα
q) have been
found to co-immunoprecipitate with Fhit in their GTP-bound active state in HEK293 cells.
The binding of activated Gα
q members to Fhit appeared to be direct. The use of Gα
16/z chimeras further enabled the mapping of the Fhit-interacting domain to the α2-β4 region of
Gα
16. Meanwhile, Fhit was up-regulated specifically by activating Gα subunits of the G
q subfamily but not by those of the other G protein subfamilies. This up-regulation effect was
mediated by a PKC/MEK pathway independent of Src-mediated Fhit Tyr
114 phosphorylation. I further demonstrated that elevated Fhit expression was due to the specific regulation of Fhit protein synthesis in the ribosome by activated Gα
q, where the regulations of cap-dependent
protein synthesis were apparently not required. Stimulation of G
q-coupled receptors in
HEK293 and H1299 cells stably overexpressing Fhit led to reduced cell proliferation, as
opposed to an enhanced cell proliferation typically seen with parental cells. Moreover, I
showed that activated Gα
q could increase cell-cell adhesion through Fhit and this effect did
not involve the regulations of the expressions of E-cadherin and MMP2/9, the localizations of
adhesive proteins and the the binding of p63RhoGEF to activated Gα
q. These findings
provide a possible handle to modulate the level of the Fhit tumor suppressor and tumor
suppression by manipulating the activity of G
q-coupled receptors.
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