Activation state-dependent interaction between Gαq subunits and the Fhit tumor suppressor.

BACKGROUND

The FHIT tumor suppressor gene is arguably the most commonly altered gene in cancer since it is inactivated in about 60% of human tumors. The Fhit protein is a member of the ubiquitous histidine triad proteins which hydrolyze dinucleoside polyphosphates such as Ap3A. 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, we explored the relationship between Gα subunits and Fhit.

RESULTS

Several members of the Gαq subfamily (Gα16, Gα14, and Gαq) were found to co-immunoprecipitate with Fhit in their GTP-bound active state in HEK293 cells. The binding of activated Gαq members to Fhit appeared to be direct and was detectable in native DLD-1 colon carcinoma cells. The use of Gα16/z chimeras further enabled the mapping of the Fhit-interacting domain to the α2-β4 region of Gα16. However, Gαq/Fhit did not affect either Ap3A binding and hydrolysis by Fhit, or the ability of Gαq/16 to regulate downstream effectors including phospholipase Cβ, Ras, ERK, STAT3, and IKK. Functional mutants of Fhit including the H96D, Y114F, L25W and L25W/I10W showed comparable abilities to associate with Gαq. Despite the lack of functional regulation of Gq signaling by Fhit, stimulation of Gq-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.

CONCLUSIONS

Activated Gαq members interact with Fhit through their α2-β4 region which may result in enhancement of the growth inhibitory effect of Fhit, thus providing a possible avenue for G protein-coupled receptors to modulate tumor suppression.