Posttranscriptional Regulation of Bovine and Human Lactoferrin

Summary

Features in the 5′- and 3′-untranslated regions (UTR) of bovine lactoferrin (bLf) mRNA suggested posttranscriptional regulation of bLf expression via a probable stem-loop secondary structure in its 5′-terminus (bases 5–205) potentially capable of inhibiting translation, and putative mRNA destabilization sequences in the 3′-untranslated region (3′-UTR) potentially able to compromise bLf mRNA stability. Their effects on bLf mRNA level and half-life, and protein expression, were tested in transfected cells by separately modifying the 5′-UTR, the 3′-UTR, and the coding regions of a full-length bLf cDNA in expression vectors. Deletion of the 39-bp 5′-UTR from native bLf cDNA and replacement with random vector sequences of 39 or 84 bp to disrupt 5′-terminal stem-loop structure did not affect bLf protein expression. However, replacement of the 3′-UTR of the stable human growth hormone (hGH) mRNA with the 3′-UTR from bLf mRNA markedly decreased the half-life of hGH mRNA. Substitution of the native 3′-UTR of bLf with the 3′-UTR from hGH markedly reduced, rather than increased, the half-life of bLf mRNA, as did deletion of the bLf distal N-lobe coding region. Surprisingly, the half-lives of hLf and bLf N-lobe/hLf C-lobe chimeric mRNAs were similar and about half that for bLf mRNA.

These studies suggest that

• 1. The 5′-terminus of bLf mRNA does not regulate translation;

• 2. Although the 3′-UTR of bLf mRNA indeed contains destabilization sequences, interaction of the bLf 3′-UTR with the distal N-lobe coding region stabilizes bLf mRNA; and

• 3. Contrary to expectation, the half-life of human lac-toferrin (hLf) mRNA is shorter than that for bLf mRNA.