β₄ Integrin subunit expression is downregulated in low metastatic carcinoma variants

Organization and expression of the gene coding for β₄ integrin subunit were investigated in murine carcinoma lines endowed with low and high metastatic potential maintained both in vivo and in vitro. Probes corresponding to either the extracellular or the cytoplasmic domains of the protein were generated and employed for Southern and Northern blot hybridization experiments. Southern blot analysis demonstrates a restriction fragment length polymorphism between BALB/c and C57BI/6J DNAs. The different genomic organization of the β₄ gene apparently does not generate variation in mRNA length. Northern blot analysis reveals a 7-kb transcript encoding full-length β₄ protein and shorter transcripts that apparently correspond to nonfunctional mRNAs. In carcinoma cells endowed with low metastatic capacity, the 7-kb mRNA cannot be revealed in spite of the presence of shorter transcripts. In contrast, the 7-kb transcript is always present in tumor cells endowed with high metastatic capacity. Accordingly, the β₄ protein is detectable, either by immunofluorescence or by Western blot analyses, only in highly metastatic carcinoma cells. In conclusion, the data presented show that in these murine carcinoma cell lines (i) the 7-kb mRNA is associated with a more invasive phenotype, and (ii) missing expression of the β₄ integrin subunit in low metastatic carcinoma cells depends, at least in part, on mechanisms acting at a post-transcriptional level. The possibility that β₄ subunit expression is a consequence of specific differentiation stages of lung carcinoma cells is discussed.