ROLE OF G PROTEINS IN PROLACTIN SIGNAL TRANSDUCTION

Hyperprolactinemia is an important cause of hypogonadism and
infertility in men and women. Prolactin (PRL) receptors have been
identified in many tissues, but how PRL causes these clinical
effects and others are not well understood because what events
follow PRL receptor binding in most tissues are still
controversial. This grant proposal aims to investigate some of the
biochemical events that may follow PRL binding in a known target
cell (the Nb2 lymphoma cell) which demonstrates a biologic response
specific to lactogenic hormones (cell growth and division).
Previously, it has been shown that bacterial toxins, pertussis
toxin, (islet activating protein: IAP) and cholera toxin (CT), can
modify PRL-stimulated mitogenesis in the Nb2 cell. These studies
suggest that one or both toxins may mediate their predominantly
inhibitory actions on the Nb2 cell through generation of increased
cellular concentrations of cyclic 3'5'- adenosine monophosphate
(cAMP). However, these toxins also affect cellular processes
unrelated to the generation of cAMP, such as, for example, the ADP-
ribosylation of non-adenylate cyclase associated GTP binding
proteins (G proteins). Thus, it is also possible that either
toxin, or both, alter PRL-stimulated mitogenesis in the Nb2 cell
through one of these mechanisms. This proposal will test the
specific hypothesis that either IAP, CT, or both toxins alter PRL-
stimulated mitogenesis in the Nb2 cell because, through ADP
ribosylation, they are able to alter the function of one or more
G proteins involved in PRL signal transduction. In order to prove
the importance of G proteins in PRL action, it will be necessary
to demonstrate two or more of the following: 1) IAP and/or CT
stimulate ADP ribosylation CT in Nb2 membranes; 2) GTP or its
analogs alters binding of lactogenic hormone to its receptors; 3)
PRL alters binding of /gtp or its analog to Nb2 membranes; 4) PRL
enhances GTPase activity; or 5) activated GTP or its analog mimics
the action of PRL in membrane or cells. This last possibility is
not yet demonstrable as no second messenger or other biochemical
activity specific to PRL action has yet been discovered. If the
hypothesis is proven correct, the proposal would then lead to the
isolation and characterization of the G protein(s) involved,
including to what second messenger systems it may be linked.