HEMCAM/CD146 downregulates cell surface expression of beta 1 integrins

HEMCAM/gicerin, an immunoglobulin superfamily protein, is involved in hemop hilic and heterophilic adhesion. It interacts with NOF (neurite outgrowth f actor), a molecule of the laminin family. Alternative splicing leads to mRN As coding for HEMCAM with a short (HEMCAM-s) or a long cytoplasmic tail (HE MCAM-l). To investigate the cellular function of these two variants, we sta bly transfected murine fibroblasts with either form of HEMCAM, Expression o f each isoform of this protein in L cells delayed proliferation and modifie d their adhesion properties to purified extracellular matrix proteins. Expr ession of either HEMCAM-s or HEMCAM-l inhibited integrin-dependent adhesion and spreading of fibroblasts to laminin 1, showing that this phenomenon di d not depend on the cytoplasmic region. By contrast, L-cell adhesion and sp reading to fibronectin depended on the HEMCAM isoform expressed. Flow cytom etry and immunoprecipitation studies revealed that the expression of HEMCAM downregulated expression of the laminin-binding integrins alpha3 beta1, al pha6 beta1 and alpha7 beta1, and fibronectin receptor alpha5 beta1 from the cell surface. Semiquantitative PCR and northern blot experiments showed th at the expression of alpha6 beta1 integrin modified by HEMCAM occurred at a translation or maturation level. Thus, our data demonstrate that HEMCAM re gulates fibroblast adhesion by controlling pi integrin expression.