General transcription factor IIA-gamma increases osteoblast-specific osteocalcin gene expression via activating transcription factor 4 and runt-related transcription factor 2.
ATF4 (activating transcription factor 4) is an osteoblast-enriched transcription factor that regulates terminal osteoblast differentiation and bone formation. ATF4 knock-out mice have reduced bone mass (severe osteoporosis) throughout life. Runx2 (runt-related transcription factor 2) is a runt domain-containing transcription factor that is essential for bone formation during embryogenesis and postnatal life. In this study, we identified general transcription factor IIA gamma (TFIIA gamma) as a Runx2-interacting factor in a yeast two-hybrid screen. Immunoprecipitation assays confirmed that TFIIA gamma interacts with Runx2 in osteoblasts and when coexpressed in COS-7 cells or using purified glutathione S-transferase fusion proteins. Chromatin immunoprecipitation assay of MC3T3-E1 (clone MC-4) preosteoblast cells showed that in intact cells TFIIA gamma is recruited to the region of the osteocalcin promoter previously shown to bind Runx2 and ATF4. A small region of Runx2 (amino acids 258-286) was found to be required for TFIIA gamma binding. Although TFIIA gamma interacts with Runx2, it does not activate Runx2. Instead, TFIIA gamma binds to and activates ATF4. Furthermore, TFIIA gamma together with ATF4 and Runx2 stimulates osteocalcin promoter activity and endogenous mRNA expression. Small interfering RNA silencing of TFIIA gamma markedly reduces levels of endogenous ATF4 protein and Ocn mRNA in osteoblastic cells. Overexpression of TFIIA gamma increases levels of ATF4 protein. Finally, TFIIA gamma significantly prevents ATF4 degradation. This study shows that a general transcription factor, TFIIA gamma, facilitates osteoblast-specific gene expression through interactions with two important bone transcription factors ATF4 and Runx2.
Yu
Shibing
S
Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15240, USA.
Jiang
Yu
Y
Galson
Deborah L
DL
Luo
Min
M
Lai
Yumei
Y
Lu
Yi
Y
Ouyang
Hong-Jiao
HJ
Zhang
Jian
J
Xiao
Guozhi
G
D041721:3T3-L1 Cells; D051701:Activating Transcription Factor 4 / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D019556:COS Cells; D045744:Cell Line, Tumor; D002522:Chlorocebus aethiops; D050659:Core Binding Factor Alpha 1 Subunit / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000502:physiology; D006801:Humans; D051379:Mice; D018345:Mice, Knockout; D010006:Osteoblasts; D015675:Osteocalcin / Q000096:biosynthesis* / Q000235:genetics; D011401:Promoter Regions, Genetic / Q000502:physiology; D011485:Protein Binding / Q000502:physiology; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D051381:Rats; D035561:Transcription Factor TFIIA / Q000235:genetics / Q000378:metabolism*; D020798:Two-Hybrid System Techniques
C497109:ATF4 protein, human; C081452:Atf4 protein, mouse; C497110:Atf4 protein, rat; D050659:Core Binding Factor Alpha 1 Subunit; D012333:RNA, Messenger; C493709:RUNX2 protein, human; C493716:Runx2 protein, mouse; C493717:Runx2 protein, rat; D035561:Transcription Factor TFIIA; D015675:Osteocalcin; D051701:Activating Transcription Factor 4