The mammalian Fallopian tube is a dynamic, steroid-responsive tissue (Jansen, 1984). The hormone 17β-estradiol (E₂) is implicated in the regulation of cell proliferation, differentiation and apoptosis in the Fallopian tube (Monroe et al., 2002). The key to the biological effects of E₂ is the activation of intracellular estrogen receptors (ER), which are ligand-inducible transcription factors that regulate the expression of many genes that control physiological processes (Dougherty and Sanders, 2005). Withdrawal of E₂ has been shown to induce cell death and increase levels of caspase-1 mRNA in the hen Fallopian tube (Monroe et al., 2002). Our preliminary results, obtained by western blot and immunofluorescence assay, indicate that caspase-1 is expressed in human and mouse Fallopian tubes (unpublished data). Since human and rodent Fallopian tube cells express both ERα and ERβ (Shao et al., 2007a, b; Horne et al., 2009), it would be predicted that the transcriptional regulation of caspase-1 is ER-dependent in the Fallopian tube in vivo; however, the primary function of caspase-1 is to activate inflammatory cytokines rather than promote cell death in the Fallopian tube. When compared with wild-type mice, female mice with a targeted deletion of caspase-1 (Cheng et al., 2008) were less susceptible to tubal inflammatory pathologies, such as the inflammatory infiltration and hydrosalpinx formation by Chlamydia trachomatis infection, one of the common risk factors for the induction of tEP in humans (Farquhar, 2005; Barnhart, 2009). More recently, caspase-1 inhibitor (Z-YVAD-fmk, Biovision, Mountain View, CA, USA) has been shown to suppress the growth and development of C. trachomatis in infected cervical epithelial cells in vitro (Abdul-Sater et al., 2009). Such an observation implies that the activation of caspase-1 may result in the epithelial cells undergoing an inflammatory process during chlamydial infection in vivo. Therefore, further investigations to determine how the activation of endogenous caspase-1 affects the severity of the pathological tubal phenotype are warranted. We have previously shown that the tubal transport of oocyte–cumulus complex in superovulated immature mice is significantly enhanced or blocked in the presence of E₂ or the ER antagonist ICI 182780, respectively, demonstrating effects on ER-mediated tubal transport (Shao et al., 2009a). In agreement with the known role of estrogen in regulating pro- or anti-inflammatory dynamics depending on the tissue/cell type involved, we hypothesize that abrogated E₂/ER signalling and caspase-1-mediated inflammation in the Fallopian tube possibly contribute to infection-induced tEP. We can also expect that the use of caspase-1 knockout (KO) mice treated with E₂ and/or an inducer of inflammation will help us to elucidate novel genes and pathways that play a functional role in the process of tubal transport.

Endocannabinoids, a group of endogenous neuromodulatory lipids, are known to activate two G protein-coupled cannabinoid receptors CB1 and CB2 (Taylor et al., 2007), which are widely and differentially expressed in many tissues and organs. It has been shown that only CB1 is expressed in the Fallopian tube, and the levels of tubal CB1 in the follicular phase are lower than those in the luteal phase in humans (Horne et al., 2008). Progesterone action, via binding to the progesterone receptor, is involved in cellular function in the Fallopian tube (Shao et al., 2006; Wånggren et al., 2008). Although the steroid hormone regulation of tubal CB1 expression has not yet been reported, elevated progesterone levels in the luteal phase have been proposed as a regulator of Fallopian tube CB1 expression (Horne et al., 2008). The biological importance of CB1 in the tubal transport process is illustrated by the presence of retained embryos in mice that lack either CB1 or CB1/2, or mice that have been treated with synthetic CB1 antagonist (Wang et al., 2004). In contrast to CB1 and CB1/2 KO mice, tubal transport is not affected in CB2 KO mice (Wang et al., 2008): this difference in phenotype may reflect a differential distribution of CB receptors in the mouse Fallopian tube. Moreover, clinical validation of the close association between CB1 expression and human tEP comes from the fact that a decrease in CB1 mRNA expression in the Fallopian tube is observed in women with tEP (Horne et al., 2008). Previously, Paria et al. (1996) reported that the Fallopian tubes of pregnant mice are capable of synthesizing anandamide, a metabolic ligand for CB receptors. While smoking is one of the risk factors for the development of tEP (Farquhar, 2005), pre-clinical studies have shown that nicotine exposure results in changes in endocannabinoid content in the brain (Le Foll et al., 2008). Moreover, it has been speculated that an imbalance in endocannabinoid levels in the circulation and reproductive tissues in humans may be associated with an increased incidence of ectopic pregnancy (Taylor et al., 2007). These observations, although limited, provide compelling evidence that disturbances of endocannabinoid/CB1 signalling during tubal transport could result in the development of tEP in humans. The success of future experiments to identify the significant loci of CB1 that predispose individuals to the common form of human tEP will depend on the biological complexity of this disorder (Horne et al., 2008). Interestingly, induction of inflammation has been shown to inhibit CB1-mediated uterine contraction in rats (Dmitrieva and Berkley, 2002). Therefore, it is tempting to hypothesize that the inhibition of tubal CB1 signalling by the in vivo action of cannabinoids may also contribute to inflammation-induced tubal cell injury, and thus, influence tubal transport.

Dicer (encoded by Dicer1), a ribonuclease III enzyme required for micro-RNA processing, has been demonstrated to play an essential role in embryo implantation in mice and in the human endometrium under pathophysiological conditions, such as endometriosis (Hong et al., 2008). While the complete loss of Dicer1 in mice results in early embryonic lethality (Bernstein et al., 2003), a considerable number of studies in female mice carrying a floxed allele of Dicer1 under the control of antimüllerian hormone receptor 2 promoter-driven Cre recombinase has revealed that these mice exhibit tubal hypotrophy with the formation of prominent tubal cysts, leading to the disruption of tubal transport (Hong et al., 2008; Nagaraja et al., 2008; Gonzalez and Behringer, 2009), which is likely a result of abnormalities in the function of the uterotubal junction (Gonzalez and Behringer, 2009). Moreover, disorganization of epithelial cells and smooth muscle cells (Gonzalez and Behringer, 2009) was found to correlate with alterations in the expression of tubal epithelial and smooth muscle cell markers after the removal of Dicer (Nagaraja et al., 2008); however, impaired ciliogenesis and ciliated function were not observed in tissue-specific Dicer1 KO mouse Fallopian tubes (Gonzalez and Behringer, 2009). To date, there are no published reports regarding the expression and regulation of tubal Dicer1 in humans during tubal transport and in tEP. A more conclusive understanding of the precise mechanisms of how aberrant Dicer1 protein leads to human tEP will require further study; however, the results discussed herein support the notion that the Dicer-mediated integrity of normal tubal structure and function is necessary for tubal transport. Although there is no difference in circulating E₂ levels between Dicer1 KO and wild-type mice (Hong et al., 2008; Gonzalez and Behringer, 2009), it has recently been reported that E₂ is able to rapidly up-regulate Dicer1 gene expression through ERα activation in luminal-type A/ERα-positive breast cancer cells in vitro (Bhat-Nakshatri et al., 2009). The differences between the KO study in vivo and the ER agonist results in vitro may be accounted for by the differences in acute versus chronic effects of E₂ in tissues/cells. Thus, in the Fallopian tube, it cannot be completely ruled out that time-dependent E₂ action regulates the micro-RNA pathway by affecting Dicer expression.