Rapid, simple identification of individual osteoblastic cells and their specific products by cell blotting assay

Changes in receptor activator of nuclear factor-kappaB, and its ligand, osteoprotegerin, bone-type alkaline phosphatase, and tartrate-resistant acid phosphatase in ovariectomized rats

We investigated time-course changes in the expression of receptor activator of nuclear factor-kappaB (RANK), its ligand (RANKL), osteoprotegerin (OPG), bone-type alkaline phosphatase (BAP), and tartrate-resistant acid phosphatase (TRAP) in ovariectomized (OVX) rats. Samples of sera and coccyges were used for analysis of the enzyme activities and expression levels of proteins and mRNAs, and an immunohistochemical analysis was also performed. Serum BAP activity increased to 158.6% of the pre-operation value at 1 week after OVX, and then decreased to 38.7% at 8 weeks after OVX. On the other hand, the serum TRAP activity increased to 130.9% of the pre-operation level at 1 week after OVX, and was maintained at a high level, compared with the pre-operation level. The patterns of BAP and TRAP activity in the coccyges specimens were similar to those seen in the sera. The expression profiles of TRAP, RANK, and RANKL proteins in the coccyx specimens were similar to the pattern of serum TRAP activity, while the profiles of the BAP and OPG proteins were similar to the pattern of serum BAP activity in OVX rats. The changes in the mRNA expression levels of the osteogenic proteins were similar to those for protein expression. These biochemical changes in OVX rats were confirmed by immunohistochemical studies. Our results suggest that not only osteoclastogenesis accelerated but also osteoblastogenesis transiently increased during the early phase of osteoporosis.

PTH-induced osteoblast contraction is mediated by cysteine proteases

E-64d, a membrane-permeable cysteine protease inhibitor, was tested for its ability to inhibit PTH-induced contraction in intact mouse MC-3T3-E1 osteoblastic cells. Incubation of MC-3T3-E1 cells with vehicle (DMSO) or E-64c, a nonpermeant cysteine protease inhibitor, in the presence or in the absence of PTH had no effects on cAMP production or on morphology from 0 to 90 minutes after addition. In contrast, treatment with E-64d markedly attenuated PTH-induced contraction in these cells. These findings suggest that cysteine proteases, such as the calcium-activated neutral proteases (calpains), are involved in PTH-induced osteoblastic contraction. The observation that cysteine protease activity mediates PTH-induced osteoblastic contraction also suggests that endogenous inhibitors, such as calpastatin, may also be present in the osteoblast and play a role in the regulation of stimulus-response coupling in bone. This mechanism may provide another regulatory point at which bone cells may be pharmacologically manipulated in clinical situations characterized by excessive bone resorption.

Expression of helix-loop-helix regulatory genes during differentiation of mouse osteoblastic cells

Although much is known about the hormonal regulation of osteoblastic cell differentiation, much less is known about the nuclear regulatory molecules that affect this process. We analyzed the expression of several regulatory molecules of the helix-loop-helix (H-L-H) group in primary mouse calvarial cells and in MC3T3-E1 mouse osteoblastic cells in situations representing different degrees of cellular differentiation. H-L-H class regulators are known to participate directly in directing cell fate and differentiation decisions in other mesodermal lineages. Two of the molecules that we studied, Id and E12, have well-established roles in this process. The other, mTwi, the murine homolog of the Drosophila twist gene, is a newly cloned mammalian H-L-H gene. Levels of E12 RNA remained unchanged during differentiation. On the other hand, in both primary osteoblastic cells and MC3T3-E1 cells, the abundance of Id and mTwi declined with cell maturation; mTwi less dramatically than Id. That Id expression is causally related to differentiation is suggested by the finding that MC3T3-E1 cells transfected with an Id-expression plasmid fail to undergo differentiation. We conclude that helix-loop-helix regulatory genes are expressed in mouse osteoblastic cells, where they are likely to participate in differentiation. The E12 gene product is likely to function as a positive modulating factor. In contrast, Id inhibits differentiation, probably by sequestering other H-L-H gene regulators, including E12, in inactive complexes. The precise role of mTwi is more speculative at this time, but the observed pattern of expression is consistent with a role in early and midmesodermal specification that is terminated as cells differentiate.