Masahiro Ohguchi

Smad7 Is an Activin-inducible Inhibitor of Activin-induced Growth Arrest and Apoptosis in Mouse B Cells

Members of the transforming growth factor-β (TGF-β) family, which includes the activins, relay signals from serine/threonine kinase receptors in membrane to nucleus via intracellular Sma- and Mad-related (Smad) proteins. Inhibitory Smad proteins were found to prevent the interaction between the serine/threonine kinase receptors and pathway-restricted Smad proteins. Smad7 was identified as a TGF-β-inducible antagonist of TGF-β signaling, and it may participate in a negative feedback loop to control TGF-β signaling. Here we demonstrate that the mRNA expression of Smad7 is induced by activin A in mouse B cell hybridoma HS-72 cells, which undergo growth arrest and apoptosis upon exposure to activin A. The ectopic expression of mouse Smad7 in HS-72 cells suppressed the activin A-induced cell cycle arrest in the G1 phase by abolishing the activin A-induced expression of p21CIP1/WAF1 and hypophosphorylation of retinoblastoma protein. Furthermore, Smad7 expression suppressed activin A-induced apoptosis in HS-72 cells. Thus, our data indicate that Smad7 is an activin A-inducible antagonist of activin A-induced growth arrest and apoptosis of B lineage cells.

Inhibition of experimental bone resorption and osteoclast formation and survival by 2-aminoethanesulphonic acid

It is known that bone resorption is mediated by osteoclasts, and lipopolysaccharide (LPS) and inflammatory mediators such as interleukin-1 (IL-1) and prostaglandin E2 (PGE2) induce osteoclast differentiation from haemopoietic cells, 2-aminoethanesulphonic acid, which is known as taurine, is an important nutrient and is added to most synthetic human infant milk formulas. In this study, it was found that 2-aminoethanesulphonic acid inhibits the stimulation of bone resorption mediated by LPS of the periodontopathic microorganism Actinobacillus actinomycetemcomitans Y4 in organ cultures of newborn mouse calvaria. The effect of 2-aminoethanesulphonic acid on the development and survival of osteoclast-like multinucleated cells produced in a mouse bone-marrow culture system was also examined. 2-aminoethanesulphonic acid (100 microg/ml) suppressed the formation of these osteoclast-like cells in the presence of LPS of A. actinomycetemcomitans Y4, IL-1alpha or PGE2 in mouse marrow cultures. On the other hand, 2-aminoethanesulphonic acid did not inhibit 1alpha, 25-dihydroxyvitamin D3-mediated osteoclast differentiation. Although IL-1alpha elongated the survival of the osteoclast-like cells, 2-aminoethanesulphonic acid blocked the supportive effect of IL-1alpha on osteoclast survival. 2-aminoethanesulphonic acid showed no effect on the growth of mouse osteoblasts. Finally, it was found that 2-aminoethanesulphonic acid inhibited alveolar bone resorption in experimental periodontitis in hamsters. These results suggest that 2-aminoethanesulphonic acid is an effective agent in preventing inflammatory bone resorption in periodontal diseases.

The role of activin type I receptors in activin A-induced growth arrest and apoptosis in mouse B-cell hybridoma cells.

Activins transduce their signals by binding to activin type I receptors and activin type II receptors, both of which contain a serine/threonine kinase domain. In this study, we established stable transfectants expressing two types of activin receptors, ActRI and ActRIB, to clarify the role of these receptors in activin signalling for growth inhibition in HS-72 mouse B-cell hybridoma cells. Over-expression of ActRI suppressed activin A-induced cell-cycle arrest in the G1 phase caused by inhibition of retinoblastoma protein phosphorylation through induction of p21CIP1/WAF1, a cyclin-dependent kinase inhibitor, and subsequent apoptosis. In contrast, HS-72 clones that over-expressed ActRIB significantly facilitated activin A-induced apoptosis. These results indicate that ActRI and ActRIB are distinct from each other and that the ActRI/ActRIB expression ratio could regulate cell-cycle arrest in the G1 phase and subsequent apoptosis in HS-72 cells induced by activin A.

Induction of G1 Arrest by Activin A via Cooperative Modulation of Cyclin D2 and p21CIP1/WAF1 in Hybridoma Cells

Activin A, a member of the transforming growth factor- β family, is a homodimer of β A chain and is implicated in the regulation of reproductive endocrine function, erythroid differentiation, mesoderm induction of the Xenopus embryo, and negative cell growth of various cell types including gonadal cells and adrenal cells (1–4). Recently, activin A was independently isolated from cultured media of activated mouse macrophages (5, 6) and mouse bone marrow stromal cells (7) as a factor inhibiting the growth of plasmacytic cells including B-cell hybridoma cells and myeloma cells. The plasmacytic cell growth inhibitory activity of activin A has been shown to be mediated in part by inducing apoptotic cell death (5–8). However, the precise mechanism by which activin A exerts its negative growth effect remains to be elucidated.