Relationship between the expression of the gap junction protein and osteoblast phenotype in a human osteoblastic cell line during cell proliferation.

We examined i) the kinds of connexins, component proteins of gap junctions, that are expressed in osteoblasts and ii) the relationship between the expression of gap junctions and osteoblastic phenotype during cell proliferation and after the treatment with 1 alpha,25-dihydroxyvitamin D3. Human osteoblastic cells with (SV-HFO) or without (HFO) transformation by simian virus 40 and mouse osteoblast-like cells (MC3T3-E1) expressed connexin 43 (Cx43), but not Cx26 or Cx32, as revealed by Northern blot analysis and immunocytochemistry. The expression of Cx43 was significantly higher in SV-HFO cells in the confluent phase than in the proliferative phase. Similarly, the expression of alkaline phosphatase (ALP) and osteocalcin in SV-HFO cells in the confluent phase were higher than those in the proliferative phase. On the other hand, treatment of 1 alpha,25-dihydroxy-vitamin D3 did not change the expression of Cx43 in SV-HFO cells, but significantly induced the expression of ALP and osteocalcin. These results showed that the expression of gap junction protein in osteoblastic cells was coupled with cell differentiation in association with the expression of osteoblastic phenotype, but that the connexin expression is regulated in a way different from that of ALP and osteocalcin.