Expression of truncated Int6/eIF3e in mammary alveolar epithelium leads to persistent hyperplasia and tumorigenesis

David L Mack,Robert Callahan,Corinne A Boulanger,Gilbert H Smith

doi:10.1186/bcr1742

David L Mack, Robert Callahan + Show 2 more

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https://doi.org/10.1186/bcr1742

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Abstract

IntroductionInt6 has been shown to be an interactive participant with the protein translation initiation complex eIF3, the COP9 signalosome and the regulatory lid of the 26S proteasome. Insertion of mouse mammary tumor virus into the Int6 locus creates a C-terminally truncated form of the protein. Expression of the truncated form of Int6 (Int6sh) in stably transfected human and mouse mammary epithelial cell lines leads to cellular transformation. In addition, decreased expression of Int6/eIF3e is observed in approximately one third of all human breast carcinomas.MethodsTo validate that Int6sh has transforming activity in vivo, a transgenic mouse model was designed using the whey acidic protein (Wap) promoter to target expression of truncated Int6 to differentiating alveolar epithelial cells in the mammary gland. Microarray analyses were performed on normal, premalignant and malignant WapInt6sh expressing tissues.ResultsMammary tumors developed in 42% of WapInt6sh heterozygous parous females at an average age of 18 months. In WapInt6sh mice, the contralateral mammary glands from both tumorous and non-tumorous tissues contained widespread focal alveolar hyperplasia. Only 4% of WapInt6sh non-breeding females developed tumors by 2 years of age. The Wap promoter is active only during estrus in the mammary tissue of cycling non-pregnant mice. Microarray analyses of mammary tissues demonstrated that Int6sh expression in the alveolar tissue altered the mammary transcriptome in a specific manner that was detectable even in the first pregnancy. This Int6sh-specific transcriptome pattern subsequently persisted in both the Int6sh-expressing alveolar hyperplasia and mammary tumors. These observations are consistent with the conclusion that WapInt6sh-expressing alveolar cells survive involution following the cessation of lactation, and subsequently give rise to the mammary tumors that arise in aging multiparous females.ConclusionThese observations provide direct in vivo evidence that mammary-specific expression of the Int6sh truncation leads to persistence of alveolar hyperplasia with the accompanying increased predisposition to mammary tumorigenesis.

Highlights

Int6 has been shown to be an interactive participant with the protein translation initiation complex eIF3, the COP9 signalosome and the regulatory lid of the 26S proteasome
These observations are consistent with the conclusion that WapInt6sh-expressing alveolar cells survive involution following the cessation of lactation, and subsequently give rise to the mammary tumors that arise in aging multiparous females
These observations provide direct in vivo evidence that mammary-specific expression of the Int6 short (Int6sh) truncation leads to persistence of alveolar hyperplasia with the accompanying increased predisposition to mammary tumorigenesis

Summary

Introduction

Int has been shown to be an interactive participant with the protein translation initiation complex eIF3, the COP9 signalosome and the regulatory lid of the 26S proteasome. Insertion of mouse mammary tumor virus into the Int locus creates a C-terminally truncated form of the protein. Expression of the truncated form of Int (Int6sh) in stably transfected human and mouse mammary epithelial cell lines leads to cellular transformation. The mouse mammary tumor virus (MMTV) integrated in an intron of one allele of Int in the reverse transcriptional orientation to that of Int, generating a chimeric mRNA. Integration events were found in introns 5, 9 and 12 producing different C-terminal truncations of Int. The most extreme truncation (in intron 5) produced an mRNA containing sequences encoding the N-terminal 137 amino acids of Int (out of 445 amino acids), novel sequences from intron 5 upstream of the integration site and reverse sequences from the MMTV 3' LTR upstream of the cryptic stop signal, Int short (Int6sh). No mutations in the remaining Int allele were detected in these MMTV-induced tumors, and the expression

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