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Transcriptional activation of c‐ fos and c‐ jun protooncogenes by serum growth factors in osteoblast‐like MC3T3‐E1 cells
Author(s) -
Okazaki Ryo,
Ikeda Kyoji,
Sakamoto Akemi,
Nakano Toshiaki,
Morimoto Kyoko,
Kikuchi Tomoko,
Urakawa Kazumi,
Ogata Etsuro,
Matsumoto Toshio
Publication year - 1992
Publication title -
journal of bone and mineral research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.882
H-Index - 241
eISSN - 1523-4681
pISSN - 0884-0431
DOI - 10.1002/jbmr.5650071006
Subject(s) - cycloheximide , protein kinase c , osteoblast , c fos , staurosporine , messenger rna , biology , microbiology and biotechnology , growth factor , platelet derived growth factor receptor , c jun , transcription (linguistics) , medicine , endocrinology , gene expression , protein biosynthesis , transcription factor , signal transduction , gene , in vitro , biochemistry , receptor , linguistics , philosophy
The present study was undertaken to clarify the relationship between c‐ fos and c‐ jun protooncogene expression and the differentiation and/or proliferation of osteoblasts, using osteoblast‐like MC3T3‐E1 (E1) cells. c‐ fos mRNA was barely detectable, whereas c‐ jun mRNA was constitutively expressed in E1 cells after serum deprivation for 24–72 h. When serum was added, a rapid and transient induction of c‐ fos and c‐ jun mRNAs was observed. The c‐ fos and c‐ jun mRNAs reached peak levels at 30 minutes, with a rapid disappearance of c‐ fos mRNA within 3 h and a much slower decrease in c‐ jun mRNA. The addition of serum together with cycloheximide, an inhibitor of protein synthesis, resulted in the superinduction of both c‐ fos and c‐ jun mRNAs. Among various growth factors, PDGF, EGF, and bFGF mimicked the serum effect, whereas IGF‐I and TGF‐β failed to induce c‐ fos and c‐ jun mRNA. The effects of PDGF, EGF, and bFGF were completely abolished by pretreatment with actinomycin D, an inhibitor of RNA synthesis, suggesting a transcriptional mechanism. Nuclear runoff experiments showed that the transcription rate of c‐ fos and c‐ jun protooncogenes was increased by serum and growth factors. The effects of PDGF, EGF, and bFGF were inhibited by H‐7 or staurosporine, inhibitors of protein kinase C (PKC), but not by HA1004 with a much weaker inhibitory activity, suggesting the involvement of PKC for the activation of the protooncogenes. PDGF, EGF, and bFGF, which induced the expression of c‐ fos and c‐ jun protooncogenes, stimulated the proliferation of E1 cells, whereas IGF‐I and TGF‐β, which failed to induce the expression of the protooncogenes, had no effect or an inhibitory effect on the proliferation of E1 cells, respectively. In addition, when the protooncogene induction was inhibited by H‐7, but not by HA1004, the proliferative responses to the growth factors were also completely abolished. Although E1 cells are known to develop osteoblastic phenotypes during prolonged culture period, the expression of c‐ fos and c‐ jun mRNAs was not altered during 2–28 days of culture at various stages of differentiation. These results suggest that the expression of c‐ fos and c‐ jun protooncogenes may play a role as an immediate early event in the proliferative responses of osteoblasts to various growth factors.