Neuromedin B (NMB) is a mammalian bombesin-like peptide that regulates exocrine/endocrine secretion, smooth muscle contraction, body temperature, and the proliferation of some cell types. Here, we show that mRNA encoding Nmb and its receptor (Nmbr) are expressed in rat bone tissue. Immunohistochemical analysis demonstrated that NMB and NMBR colocalize in osteoblasts, epiphyseal chondrocytes, and proliferative chondrocytes of growth plates from mouse hind limbs. Then, we investigated the effect of NMB on the proliferation of rat primary cultured osteoblasts. Proliferation assays and 5-bromo-2′-deoxyuridine incorporation assays demonstrated that NMB augments the cell number and enhances DNA synthesis in osteoblasts. Pretreatment with the NMBR antagonist BIM23127 inhibited NMB-induced cell proliferation and DNA synthesis. Western blot analysis showed that NMB activates ERK1/2 MAPK signaling in osteoblasts. Pretreatment with the MAPK/ERK kinase inhibitor U0126 attenuated NMB-induced cell proliferation and DNA synthesis. We also investigated the effects of molecules that contribute to osteoblast proliferation and differentiation on N mb expression in osteoblasts. Real-time PCR analysis demonstrated that 17β-estradiol (E2) and transforming growth factor β1 increase and decrease N mb mRNA expression levels respectively. Finally, proliferation assays revealed that the NMBR antagonist BIM23127 suppresses E2-induced osteoblast proliferation. These results suggest that NMB/NMBR signaling plays an autocrine or paracrine role in osteoblast proliferation and contributes to the regulation of bone formation.
Hiroki Saito, Tomoya Nakamachi, Kazuhiko Inoue, Ryuji Ikeda, Kazuo Kitamura, Naoto Minamino, Seiji Shioda and Atsuro Miyata
Mio Tanaka, Morio Azuma, Yumika Nejigaki, Yumiko Saito, Kanta Mizusawa, Minoru Uchiyama, Akiyoshi Takahashi, Seiji Shioda and Kouhei Matsuda
Melanin-concentrating hormone (MCH)-containing neurons directly innervate the adenohypophysis in the teleost pituitary. We examined immunohistochemically the relationship between MCH-containing nerve fibres or endings and somatolactin (SL)-producing cells in the goldfish pituitary. Nerve fibres or endings with MCH-like immunoreactivity were identified in the neurohypophysis in close proximity to the adenohypophysial cells showing SL-like immunoreactivity. We also examined the effect of MCH on SL release from cultured goldfish pituitary cells and SL synthesis using a cell immunoblot and a real-time PCR method. Treatment of individually dispersed pituitary cells with MCH 10−7 M for 3 h decreased the area of SL-like immunoreactivity on immunoblots, and MCH-induced reductions in SL release were blocked by treatment with the mammalian MCH receptor (MCHR) antagonist, compound-30, at a concentration of 10−5 M. Treatment with 10−7 M MCH for 3 h did not affect sl-α and -β (smtla and -b as given in the Zfin Database) mRNA expression levels. These led us to explore the signal transduction mechanism leading to the inhibition of SL release, for which we examined whether MCH-induced reductions in SL release are mediated by the Gi or Gq protein-coupled signalling pathway. The MCH-induced reductions in SL release were abolished by treatment with the Gi/o protein inhibitors, NF023 (10−5 M) or pertussis toxin (260 ng/ml), but not by the phospholipase C inhibitor, U-73122 (3×10−6 M). These results indicate that MCH can potentially function as a hypothalamic factor suppressing SL release via the MCHR, and subsequently through the Gi protein to inhibit the adenylate cyclase/cAMP/protein kinase A-signalling pathway in goldfish pituitary cells.