The crucial role of pituitary
GnRH receptors (
GnRH-R) in the control of reproductive functions is well established. These receptors are the target of
GnRH agonists (through receptor desensitization) and antagonists (through receptor blockade) for the treatment of
steroid-dependent pathologies, including
hormone-dependent
tumors. It has also become increasingly clear that
GnRH-R are expressed in
cancer tissues, either related (i.e. prostate, breast, endometrial, and
ovarian cancers) or unrelated (i.e.
melanoma,
glioblastoma, lung, and
pancreatic cancers) to the reproductive system. In
hormone-related
tumors,
GnRH-R appear to be expressed even when the
tumor has escaped
steroid dependence (such as
castration-resistant
prostate cancer). These receptors are coupled to a G(αi)-mediated intracellular signaling pathway. Activation of
tumor GnRH-R by means of
GnRH agonists elicits a strong antiproliferative, antimetastatic, and antiangiogenic (more recently demonstrated) activity. Interestingly,
GnRH antagonists have also been shown to elicit a direct antitumor effect; thus, these compounds behave as antagonists of
GnRH-R at the pituitary level and as agonists of the same receptors expressed in
tumors. According to the
ligand-induced selective-signaling theory,
GnRH-R might assume various conformations, endowed with different activities for
GnRH analogs and with different intracellular signaling pathways, according to the cell context. Based on these consistent experimental observations,
tumor GnRH-R are now considered a very interesting candidate for novel molecular,
GnRH analog-based, targeted strategies for the treatment of
tumors expressing these receptors. These agents include
GnRH agonists and antagonists,
GnRH analog-based cytotoxic (i.e.
doxorubicin) or nutraceutic (i.e.
curcumin) hybrids, and
GnRH-R-targeted nanoparticles delivering anticancer compounds.