| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
ARTICLES |
Department of Biomedical Sciences and Human Oncology University of Torino Torino I-10126, Italy
Address all correspondence and requests for reprints to: Gianni Bussolati, University of Torino, Department of Biomedical Sciences & Oncology, Via Santena n.7, Torino I-10126, Italy.
 |
Introduction |
|---|
 Top Introduction References |
|---|
These observations are in line with several studies in recent years indicating that the distribution, biological significance, and regulation of the OT/OTR system are unexpectedly much more complex and varied than previously postulated (4). For a long time, OT and OTR traditionally have been considered mainly involved in the regulation of reproductive functions such as the timing and amplification of labor, ovulation, and lactation among others. Subsequently, it became evident that they also participate in the regulation of different behavioral, neuro-mediated functions, including sexual and maternal behavior, memory, food and drink intake, and modulation of anorexia (5). According to these different roles in reproduction and behavior, OTRs were initially identified in the functionally related tissues (uterus, feto-maternal unit, breast, and central nervous system). Similarly, studies on the pathophysiology of the OT/OTR system, including regulation of OTR expression and intracellular signaling, were first focused on these sites. However, recently the biological implications of the OT/OTR interplay have been widely extended because different target tissues hosting OTR, new factors involved in OTR modulation, and possible alternative signaling pathways have been identified. The availability of the highly selective radioiodinatable ligand 125[I]d(CH2)5[Tyr(Me)2, Thr4, Tyr-NH29]OVT [125I-OTA] (6) has led to the discovery of OTR in tissues other than those known to be target of OT action. Among these, two are especially striking: endothelial cells (7, 8) and osteoblasts (9).
In endothelial cells, OTR activation leads to an increase in intracellular calcium coupled to a stimulating effect on cell growth. The demonstration of functional OTR in these cells suggests a role for OT in the regulation of vascular tone and blood pressure (through the calcium-dependent vasodilatatory response via stimulation of the nitric oxide pathway) and induces researchers to investigate on the possible biological meaning of this trophic action of the OT/OTR interplay (7). Together with previous evidence for OTR expression in the heart (10), the demonstration of specific, widespread OT binding sites in the rat vasculature and of OT synthesis at that site (8) opens a new scenario for this peptide, which points to an unexpected autocrine and paracrine role in cardiovascular function.
If it is feasible to speculate on the role of the OT/OTR system in endothelial cells, its biological role in bone cells, such as osteoblasts, is definitely unknown. However, it has been reported that the sequence of a hexapeptide (HP) cleaved by plasmin from the bone protein osteocalcin (OC) is nearly identical to the E2 region of the OTR (11). Although a direct binding of HP to OT has not been reported, in in vitro studies it has been demonstrated that the two molecules can compete on bone-derived neoplastic cells (11). This interaction could possibly account, at least in part, for a possible role of OT in the bone physiology as well as in some bone pathological processes.
Although the effective importance of the OT/OTR system in both endothelial and bone-derived cells still remain to be ascertained, these data clearly indicated that OTR-expressing cells have a much broader distribution than initially expected. Moreover, such "unorthodox" distribution of OTR in normal tissues is definitely shared by OTR expression in tumors.
The presence of OTR has been described in breast cancer cells, using various techniques (12, 13, 14, 15). In these cells, the initial detection of OT binding sites (12) was followed by the actual demonstration of OTR expression at both protein and messenger RNA (mRNA) levels (13, 14, 15). However, the in vitro effects of OT in these cells were reportedly variable from null (15), to antiproliferative (13, 16) or even mitogenic (12). These discrepancies are probably due to the different cell lines used and to variable cell culture conditions.
Beside these opposite effects on cell proliferation, to date only three groups have investigated the signaling following OTR activation in breast cancer cells. Two groups failed to detect, in several breast cancer cell lines, the intracellular calcium increase usually observed after OTR binding to the natural ligand OT (i.e. in myometrial cells among others) (17, 18). On the contrary, a significant increase of intracellular cAMP was observed to accompany the antiproliferative effect of the peptide (17). Moreover, the addition of a PKA inhibitor to the OT treatment fully abolished the inhibiting effect of OT on cell proliferation, further orienting toward the activation of the cAMP-PKA pathway (17). The evidence of such signaling was surprising, as OTR activation traditionally involves IP and intracellular calcium. However, as far back as 1984, an OT-induced increase in intracellular cAMP had already been observed in breast myoepithelial cells (19). The third group reported that, in the Hs578T breast cancer cell line, the activation of functional OTR was coupled to an intracellular calcium increase (20). However, there was no indication of any biological effect on cell proliferation correlated to this signaling.
Two questions rise from these observations: 1) how might OT increase
intracellular cAMP? And: 2) is it possible that OT may activate
different intracellular mediators and consequently evoke specific,
different effects? With regard to the first question, it has been
reported that, in the rat myometrium, OTR can couple to G
s
(21); because all adenylate cyclases appear to be
activated by this G protein subunit (22), we could imagine
that the coupling of OTR to heterotrimeric G proteins containing Gs
could be responsible for the OT-dependent cAMP increase in breast
cancer cells. Alternatively, OT could stimulate prostaglandin (PG)
production in breast cancer cells, as it does in endometrial and amnion
cells (23). As PG inhibits breast cancer growth by a cAMP
increase, we could postulate an interaction between these two
substances and even an indirect, PG-mediated OT effect. Further studies
are required to confirm and fully understand the meaning of cAMP
OT-signaling in neoplastic cells.
The second question intrinsically suggested that the biological effect mediated by OT could depend on the specific intracellular mediator involved. Interestingly, the OT-dependent cAMP increase previously described in myoepithelial cells was not associated with myosin phosphorylation, contrary to the increase in intracellular calcium also observed in the same cells after OTR activation (19). So, the question remains, what is the biological effect coupled to the OT-induced cAMP increase in myoepithelial cells? This is still to be defined.
Breast cancer cells were the first neoplastic model in which the presence of OTR was described, and is at the moment the only one where the biological effects of OT on cell proliferation has also been verified in vivo (24). Thereafter, OTR protein and mRNA were also described in other tumors such as endometrial carcinomas (25), neuroblastomas and glioblastomas (26), and in osteosarcomas (9, 11). In cell lines derived from such lesions, OTR mediated the inhibiting effect of OT on cell proliferation. Moreover, whenever the signaling coupled to this effect was investigated, it resulted in the activation of the cAMP-PKA pathway (17, 25, 26).
Until very recently, results on the OT/OTR role in neoplastic cells seemed to point mainly toward a uniform, reproducible negative modulation of tumor cell growth, independent of the neoplastic cell phenotype. However, some surprising results recently modified this recurrent theme, leading back to the title of this Editorial: expect the unexpected. In the March issue of Endocrinology, functional OTRs have been reported to be present in choriocarcinoma cells (27). In contrast to all previous data on OTRs in tumors, in these cells the OTR activation led to a significant increase in intracellular calcium, together with a significant mitogenic effect on cell proliferation. This ability of OT to stimulate cell proliferation in normal and neoplastic trophoblast-derived cells is analogous to that reported in normal endothelial cells where, as noted above, OTR activation induced cell growth.
To date, one single OTR type has been cloned (4). This raises the question, how could the OT effect on cell proliferation be so variable? Although there is no answer to this question at present, we can conclude from all the data provided on OT/OTR and cancer cells that the peptide effect is consistent with the intracellular mediator activated: growth inhibition with cAMP increase, growth stimulation with calcium increase. We have to stress that until the last decade, although many biological OT effects were clear-cut (i.e. its effect in cell contraction and induction of PG synthesis, among others), no indications were available on the role of OT as a growth factor. This was only a recent, unexpected finding.
Adding to the complexity of the OT/OTR system is the increased number of factors involved in the regulation of OTR expression. Since the first identification of a somehow simplified modulation of OTR, which was mostly focused on the effects of steroids, it has become more and more clear that OTR modulators are not only present among hormones but also include neurotransmitters and cytokines. This final observation brings us back to the article by Schmidt et al. (1) on IL-1ß and IL-6 inhibiting effects on OTR expression in myometrial cells and in HeLa transfected cells, which appears in this issue of Endocrinology. Their observation is in agreement with that of Rauk and Friebe-Hoffmann (28), who reported the down-regulation of OTR in cultured uterine myocites. Conversely, other researchers demonstrated a significant increase in OTR mRNA in primary cell cultures from human pregnant myometrium (29), and the same group that describes in this issue the negative effect of IL-1ß and IL-6 on the transcriptional regulation of OTR previously reported an opposite, positive effect of IL-6 in the OTR mRNA concentration in uterine explants from pregnant rats (30). Interestingly, in nonpregnant animals this effect was lacking.
What is the reason of such variable findings? It is likely that the
source and the milieu of the myometrial cells could influence responses
to the cytokine treatment. The existence of an immunological cross-talk
involving OTR and IL-6 has already been suggested in the rat brain
during pregnancy (31), as well as, involving a different
cytokine (IFN-), in human myometrial cells in culture
(32). Independently from the pattern of responses, the
existence of an immunological regulation of OTR, possibly modulated by
other factors (i.e. steroids), is becoming more and more
apparent.
Prospects for clinical application of studies on the OT/OTR interplay system have to date focused on devising antagonists of OT, primarily aimed to interfere with myometrial contractions during parturition (4). Indeed, the discovery of several additional targets for OT will require an examination of the specificity of such analogs with a view to determining and limiting possible side effects. The functional role of the OT/OTR interplay in organs outside the myometrium might be influenced by various conditions. Because Schmid et al. (1) demonstrate that the inhibition of OTR expression by IL-1ß and IL-6 is not limited to myometrial cells, being also effective in OTR-transfected HeLa cells, such mechanisms might well be active in all cells with OTR structurally identical to the uterine receptors, such as endothelial cells, osteoblasts, and trophoblast-derived cells, among others. In endothelial cells, where activation of OTR leads to calcium-dependent proliferation and to vascular dilation (7), one might expect that inhibition of OTR expression by interleukin-producing inflammatory cells could account for important vascular modulation in reactive tissues. In addition, since OT has recently been shown to evoke an intracellular calcium increase on OTR-expressing osteoblasts (9) and osteoclasts (33), it is highly likely that interleukin produced by bone marrow cells may play a role in bone metabolism. Similarly, OT-induced proliferation of trophoblast cells (27) is likely to be influenced (inhibited?) by inflammation leading to cytokine release.
Much data have demonstrated that estrogen and progesterone act as modulators of the OT/OTR system, and specifically of the expression of both OT and OTR genes (34). Therefore, it may be suggested that modulation of OTR by steroid hormones or by interleukins released by tumor-infiltrating lymphocytes, macrophages, or even neoplastic cells (35) might take place in the numerous tumors expressing OTR. Also, a possible effect on tumor growth of OT synthesized by endothelial cells (8) should not be dismissed.
We are left with a final question: could OT and analogs have a role in clinical oncology? One might argue against this, on the basis of the nearly ubiquitous distribution of OTR. However, in this regard the use of somatostatin-analogs, which have proved successful in clinical practice for the treatment and detection of somatostatin receptor- expressing tumors, is encouraging (36).
Our group has recently been investigating a similar use of OT and analogs as radioactive carriers, to be employed for radioimaging or even possibly radiotherapy of OTR expressing tumors. Linkage of a strong radioactive source to a complex originated from an OT analog bound to a chelating agent and retaining OTR affinity would possibly find application as both a diagnostic or a therapeutic tool (Bussolati G., M. Chinol, B. Chini, A. Nacca, P. Cassoni, and G. Paganelli, submitted for publication).
It can reasonably be concluded that new fields of investigation for the OT/OTR system, its expression and modulation, will continue to yield surprising findings far beyond the pathophysiology of reproduction and the study of behavior and thus uncover new challenges for the basic scientist and clinician alike. The unexpected is not over.
Received January 25, 2001.
|
References |
|---|
|
TopIntroductionReferences |
|---|
downregulates expression of the oxytocin receptor in cultured human
myometrial cells. Am J Physiol 271:E840–E846
This article has been cited by other articles:
 |
|
 |
|
  P. Cassoni, T. Marrocco, B. Bussolati, E. Allia, L. Munaron, A. Sapino, and G. Bussolati Oxytocin Induces Proliferation and Migration in Immortalized Human Dermal Microvascular Endothelial Cells and Human Breast Tumor-Derived Endothelial Cells Mol. Cancer Res., June 1, 2006; 4(6): 351 - 359. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Breuiller-Fouche, C. Moriniere, E. Dallot, S. Oger, R. Rebourcet, D. Cabrol, and M.-J. Leroy Regulation of the Endothelin/Endothelin Receptor System by Interleukin-1{beta} in Human Myometrial Cells Endocrinology, November 1, 2005; 146(11): 4878 - 4886. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 L. Vignozzi, G. B. Vannelli, A. Morelli, R. Mancina, M. Marini, P. Ferruzzi, C. Crescioli, M. Luconi, S. Donati, A. D. Fisher, et al. Identification, characterization and biological activity of oxytocin receptor in the developing human penis Mol. Hum. Reprod., February 1, 2005; 11(2): 99 - 106. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C Pequeux, B P Keegan, M-T Hagelstein, V Geenen, J-J Legros, and W G North Oxytocin- and vasopressin-induced growth of human small-cell lung cancer is mediated by the mitogen-activated protein kinase pathway Endocr. Relat. Cancer, December 1, 2004; 11(4): 871 - 885. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. Pequeux, C. Breton, J.-C. Hendrick, M.-T. Hagelstein, H. Martens, R. Winkler, V. Geenen, and J.-J. Legros Oxytocin Synthesis and Oxytocin Receptor Expression by Cell Lines of Human Small Cell Carcinoma of the Lung Stimulate Tumor Growth through Autocrine/Paracrine Signaling Cancer Res., August 15, 2002; 62(16): 4623 - 4629. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
