Thyroid hormone and retinoic acid induce the synthesis of insulin-like growth factor-binding protein-4 in mouse osteoblastic cells

Thyroid hormone (T3) is a known regulator of the transcription rate of specific genes. By subtractive hybridization of T2-treated osteoblastic cells, differentially expressed messenger RNAs (mRNAs) were enriched in the form of double stranded complementary DNA (cDNA) fragments. Sequencing of a differentially expressed cDNA that detects a 2.6-kilobase mRNA in Northern blots revealed to homology in the EMBL-Genebank data bases. A mouse genomic library was screened, and the isolated genomic DNA was identified as part of the insulin-like growth factor-binding protein-4 (IGFBP-4) gene including the 3'-untranslated region to which the cloned cDNA fragment was mapped by sequencing. We observed an up-regulation of the 2.6-kilobase IGFBP-4 mRNA transcript in the presence of T3 or retinoic acid. The induction of the IGFBP-4 transcript persisted up to 48 h. This response was inhibited by cycloheximide as well as actinomycin D. Long term induction studies revealed that the T3 effect is present during the complete culture period, with a constant rise in IGFBP-4 mRNA levels until 14 days. Under these culture conditions, the DNA content of MC3T3-E1 cells were significantly reduced by T3 and retinoic acid, indicating the repressive effect of both hormones on cell growth. Western immunoblots showed that the transcriptional induction is consequently transduced to increased IGFBP-4 levels in the conditioned medium of T3-treated cells. Our data show that thyroid hormone and retinoic acid stimulate transcription of IGFBP-4 mRNA in osteoblasts, resulting in increased IGFBP-4 secretion into the medium. IGFBP-4, a known inhibitor of cellular proliferation, might contribute to the antiproliferative effect of T3 and retinoic acid on osteoblasts.