HEMCAM/CD146 downregulates cell surface expression of β1 integrins

HEMCAM/gicerin, an immunoglobulin superfamily protein, is involved in homophilic and heterophilic adhesion. It interacts with NOF (neurite outgrowth factor), a molecule of the laminin family. Alternative splicing leads to mRNAs coding for HEMCAM with a short (HEMCAM-s) or a long cytoplasmic tail (HEMCAM-l). To investigate the cellular function of these two variants, we stably transfected murine fibroblasts with either form of HEMCAM. Expression of each isoform of this protein in L cells delayed proliferation and modified their adhesion properties to purified extracellular matrix proteins. Expression of either HEMCAM-s or HEMCAM-l inhibited integrin-dependent adhesion and spreading of fibroblasts to laminin 1, showing that this phenomenon did not depend on the cytoplasmic region. By contrast, L-cell adhesion and spreading to fibronectin depended on the HEMCAM isoform expressed. Flow cytometry and immunoprecipitation studies revealed that the expression of HEMCAM downregulated expression of the laminin-binding integrins α3β1, α6β1 and α7β1, and fibronectin receptor α5β1 from the cell surface. Semi-quantitative PCR and northern blot experiments showed that the expression of α6β1 integrin modified by HEMCAM occurred at a translation or maturation level. Thus, our data demonstrate that HEMCAM regulates fibroblast adhesion by controlling β1 integrin expression. Movies available on-line: http://www.biologists.com/JCS/movies/jcs1886.html, movie 1A, movie 1B, movie 2A, movie 2B