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Our previous studies on annexin 5, a member of the annexin family of proteins, have shown its expression in the anterior pituitary gland, its preferential distribution in gonadotropes, and its increase after ovariectomy. In the present study, we examined (1) whether annexin 5 is synthesized in gonadotropes, (2) whether its expression is under the control of gonadotropin-releasing hormone (GnRH), and (3) the effect of annexin 5 on gonadotropin release. Large cells, also called castration cells, appeared in anterior pituitary tissue 3 weeks after ovariectomy. These cells have been confirmed to be hyperfunctioning gonadotropes and are easily discriminated from other pituitary cells without immunostaining. Using in situ hybridization with a digoxigenin-labeled ribonucleic acid probe, enhanced expression of annexin 5 messenger ribonucleic acid (mRNA) in these gonadotropes was clearly demonstrated. Northern blot analysis showed an increase in the level of annexin 5 mRNA expression 3 weeks after ovariectomy. It was lessened 3 h after the injection of Cetrorelix (GnRH antagonist, 10 µg i.v.). Administration of a GnRH analog [GnRHa; Des-Gly 10 (Pro9) GnRH ethylamide, 0.2 ml of 2.5 µg/ml saline ten times intraperitoneally at 30-min intervals] significantly increased pituitary annexin 5 mRNA. In primary cultures of anterior pituitary cells, recombinant rat annexin 5 stimulated luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in a dose-dependent manner. Concomitant administration of annexin 5 (1 µg/ml) and GnRHa augmented the LH and FSH release induced by GnRHa. After a 1-hour incubation, cycloheximide (10 µg/ml) apparently inhibited the LH response to GnRHa, while annexin 5 (2 µg/ml) moderated this inhibition. Further, the antisense oligodeoxynucleotide to annexin 5 mRNA blunted the LH response to GnRHa. It is thus concluded that annexin 5 is synthesized in the gonadotropes under the effect of GnRH, and it is suggested that annexin 5 synthesis mediates at least partly GnRH receptor signaling to stimulate gonadotropin secretion.

Despite recent improvements in diagnostic ability and treatment strategies, advanced gastric cancer (GC) has a high frequency of recurrence and metastasis, with poor prognosis. To improve the treatment results of GC, the search for new treatment targets from proteins related to epithelial–mesenchymal transition (EMT) and cell–cell adhesion is currently being conducted. EMT plays an important role in cancer metastasis and is initiated by the loss of cell–cell adhesion, such as tight junctions (TJs), adherens junctions, desmosomes, and gap junctions. Among these, claudins (CLDNs) are highly expressed in some cancers, including GC. Abnormal expression of CLDN1, CLDN2, CLDN3, CLDN4, CLDN6, CLDN7, CLDN10, CLDN11, CLDN14, CLDN17, CLDN18, and CLDN23 have been reported. Among these, CLDN18 is of particular interest. In The Cancer Genome Atlas, GC was classified into four new molecular subtypes, and CLDN18–ARHGAP fusion was observed in the genomically stable type. An anti-CLDN18.2 antibody drug was recently developed as a therapeutic drug for GC, and the results of clinical trials are highly predictable. Thus, CLDNs are highly expressed in GC as TJs and are expected targets for new antibody drugs. Herein, we review the literature on CLDNs, focusing on CLDN18 in GC.

We investigated a specific relationship between the expression of annexin 5 and prolactin in the corpus luteum of pseudopregnant rats, with particular interest in GnRH and apoptosis of luteal cells. The expression of ovarian annexin 5 mRNA was significantly decreased at mid-pseudopregnancy and recovered at the end, whereas it remained low on the corresponding day of pregnancy. The dopamine agonist CB-154, administered at mid-pseudopregnancy (d 5), increased ovarian annexin 5 mRNA, whereas prolactin, given daily for 3 d to cycling rats, decreased it. An immunocytochemical study also showed that annexin 5 increased in the corpus luteum on d 6 and 7 of pseudopregnancy after treatment with CB-154 on d 5. The distribution of annexin 5-positive cells was not uniform in the corpus luteum and matched that of terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL)-positive cells. Because GnRH stimulates annexin 5 mRNA expression in the gonadotropes, involvement of the GnRH receptor was examined. Local administration of a GnRH antagonist, Cetrorelix, to hemilateral ovarian bursa of pseudopregnant rats simultaneously receiving CB-154 abrogated both the expression of annexin 5 and the TUNEL reaction. The present results clearly demonstrate that prolactin decreases annexin 5 mRNA in the luteal cells during pseudopregnancy. Prolactin is suggested to suppress the local action of GnRH, which stimulates annexin 5 synthesis and apoptosis of functional luteal cells during pseudopregnancy.

Annexin 5, a unique calcium- and phospholipid-binding protein, has been investigated for its specific distribution in rat endocrine organs by immunocytochemistry with a specific antiserum to recombinant rat annexin 5. Follicular epithelial cells and parafollicular cells of the thyroid gland, adrenocortical cells of the zona fasciculata and zona reticularis, luteal cells, testicular interstitial cells, and Sertoli cells were shown to contain annexin 5. To examine whether the synthesis of annexin 5 would be affected by a change in humoral signal, the distribution of annexin 5 in the anterior pituitary was examined three weeks after ovariectomy. The withdrawal of ovarian hormones induced huge castration cells in the anterior pituitary gland, which contained abundant annexin 5. Annexin 5 was not detected in the pineal gland, the parathyroid gland, the islet of Langerhans, the adrenal medulla, zona glomerulosa cells, and granulosa cells. Since annexin 5 was shown to exist in many of the endocrine tissues examined, to be localized in specific cell types, and to be abundant in castration cells, it is suggested that annexin 5 contributes to secretory cell functions, which may be common to endocrine cells secreting chemically different hormones.

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Progesterone secretion remained significantly higher during diestrus in the 5-day cyclic rat than in the 4-day cyclic animal. Injection of a sufficient amount of antiprogesterone serum (APS) at 2300 h on metestrus in a 5-day cycle advances ovulation and completion of the cycle by 1 day in the majority of animals (75 and 80%, respectively). Progesterone (250 micrograms) administered with APS eliminated the effect of the antiserum. Within 2 h after administration of APS, levels of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) elevated significantly, while a significant elevation of plasma estradiol above the control value followed as late as 36 h after the treatment. None of the 5-day cyclic rats treated with APS showed ovulatory increases of FSH and LH at 1700 h on the second day of diestrus, although 3 of the 4 animals receiving the same treatment ovulated by 1100 h on the following day. The onset of ovulatory release of gonadotropins might have been delayed for several hours in these animals. These results indicate that recurrence of the 5-day cycle is due to an elevated progesterone secretion on the morning of diestrus, and suggest that a prolongation of luteal progesterone secretion in an estrous cycle suppresses gonadotropin secretion. Rather than directly blocking the estrogen triggering of ovulatory LH surge, the prolonged secretion of luteal progesterone may delay the estrogen secretion itself, which decreases the threshold of the neural and/or hypophyseal structures for ovulatory LH release.