HUMAN INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-1 (HIGFBP-1) IN TRANSGENIC MICE - CHARACTERIZATION AND INSIGHTS INTO THE REGULATION OF IGFBP-1 EXPRESSION

Three hemizygous transgenic (Tg) mouse lines were generated with a fus ion gene composed of the mouse metallothionein promoter (mMT-I) and a full-length human insulin-like growth factor binding protein-1 (hIGFBP -1) complementary DNA that was truncated in its 3'-untranslated region . Despite high serum hIGFBP-1 levels (120-2570 mu g/liter) before pube rty in two of these lines, no significant alterations were observed in somatic growth, nor were marked alterations noted in fasting or rando m serum glucose or in the response of young adult Tg mice to ip glucos e. The transgene was expressed in a number of tissues from each line, but liver was a significant site of transgene expression in only one l ine. Unexpectedly, liver hIGFBP-1 messenger RNA (mRNA) expression in t his line was regulated in fashion similar to the native liver IGFBP-1 mRNA: 1) its abundance waned with advancing postnatal age and became m inimal in early adult life, despite continuous zinc supplementation to stimulate its transcription; and 2) fasting increased its abundance 3 - to 4.3-fold. The decline in transgene expression with aging was not due to a deletion, rearrangement, or a change in the methylation of li ver transgene DNA. Transcriptional mechanisms also were not likely to account for the observed regulation of the transgene mRNA, because liv er expression of the mMT-I gene, which shares identical genomic 5'-reg ulatory elements with the transgene, was not similarly altered by agin g or fasting. Because cycloheximide (CHX) treatment of cultured rat H4 IIE cells has been shown to prolong IGFBP-1 mRNA half-life while decre asing its transcription, Tg mice were treated with CHX to test the pos sibility that instability of the liver transgene mRNA influenced its a bundance. After CHX and under conditions of chronic zinc supplementati on, liver transgene mRNA abundance increased in parallel with that of the native IGFBP-1 mRNA. Although CHX is known to activate mMT-I trans cription by mechanisms involving the 5'-regulatory regions contained i n the transgene, CHX-induced transcription only in part accounted for the increase in liver transgene mRNA, because CHX induced an earlier a nd greater increase in liver transgene mRNA than in mMT-I mRNA. Taken together, these data indicate that both transgene and native IGFBP-1 l iver mRNA are regulated by factors that alter mRNA stability. The find ing that native liver IGFBP-1 mRNA abundance is influenced by transgen e expression further supports the concept that both mRNAs share some c ommon mechanisms of regulation. Although IGFBP-1 mRNAs contain putativ e destabilizing AU-rich elements in their 3'-untranslated regions, the se sequences were omitted from the transgene. Shared sequences within the coding region, possibly an AU-rich element that resides 17 basepai rs 5' of the termination codon, probably account for the transgene mRN A instability and contribute in part to the instability of the native mRNA. We speculate that the postnatal activation or expression of a la bile protein, which can interact with sequences within the IGFBP-1-cod ing region, regulates the liver expression of this transgene and affec ts the abundance of the native IGFBP-1 mRNA.