Primary bovine mammary cells express the two IGF receptors (IGF-IR, IGF-IIR),4 insulinreceptor, and four IGFBPs (IGFBP-2, -3, -4, and -5). Examination of the IGF-IR during themammary gland lactation cycle shows that IGF-IR number declines at parturition, a changethat coincides with decreases in the blood level of its ligand, IGF-I. IGF-II and IGF-IIR arelargely unchanged. IGFBP-3 is the predominant mammary IGFBP and its concentration alsodeclines in blood and milk during lactation compared to prepartum and involution periods.Time of lactation and pregnancy were the main determinants of milk but not bloodIGFBP-3 levels. IGFBP-3 binds to membrane proteins of bovine mammary tissue; an IGFBP-3 bindingprotein has been identified as bovine lactoferrin. Lactoferrin has the capacity to compete withIGF binding to IGFBP-3. Appearance of both IGFBP-3 and lactoferrin in conditioned mediaof primary cultures of bovine mammary cells was stimulated by all trans retinoic acid (atRA).Furthermore, atRA was necessary for the entry of exogenously added lactoferrin into themammary cell nucleus, while IGFBP-3 entry into the nuclei of atRA treated cells requiredthe presence of lactoferrin. These findings reveal a novel role for lactoferrin, suggesting thatlactoferrin is critically involved in the regulation of the IGF system during the involution period.

The Madin Darby bovine kidney (MDBK) cell line was used to investigate the mechanisms underlying the cAMP regulation of insulin-like growth factor binding protein-3 (IGFBP-3) gene expression. Treatment of confluent monolayers either with forskolin or cAMP produced a 60- to 75-fold induction of IGFBP-3 mRNA and protein levels. This effect did not require new protein synthesis as inhibition of translation by cycloheximide actually caused a 2-fold increase in the cAMP induction. The rates of IGFBP-3 gene transcription, assessed by nuclear run-on assays, increased approximately 15-fold in cells exposed to cAMP. In addition, the half-life of the IGFBP-3 mRNA transcript was increased approximately 3-fold in the presence of cAMP. Gel mobility shift and competition experiments revealed the specific binding of an approximately 42-kDa cytoplasmic protein factor to the 3'-untranslated region (3'-UTR) of the IGFBP-3 mRNA. A 21-nucleotide uridine-rich segment that contained no AUUUA motif was sufficient for the specific binding. The binding activity of this protein was reduced after cAMP treatment but was increased by phosphatase treatment. In conclusion, the cAMP induction of IGFBP-3 mRNA in MDBK cells occurred at both the transcriptional and posttranscriptional levels. The IGFBP-3 mRNA stabilization in MDBK cells probably involved the phosphorylation of a member of the family of U-rich region mRNA-binding proteins and is the first reported member whose RNA-binding activity is reduced by cAMP.

AbstractThe sections in this article are:InsulinInsulin‐Like Growth FactorsHuman Chorionic Gonadotropin (hCG)Glucagon

We have cloned and characterized the gene for an immunodominant antigen of O. volvulus that is recognized by the sera of 96% of patients with onchocerciasis. Its 1.2-kb mRNA constitutes 0.3% of adult worm poly(A)+ RNA and its cDNA sequence reveals that it is not a highly conserved structural protein such as actin or tubulin. Similar but not identical genes occur in the genomes of related filarie, Brugia malayi and Dirofilaria immitis. The recombinant antigen has both immunodiagnostic and immunoprophylactic significance.