Functional Significance of CD9 Association with β1 Integrins in Human Epidermal Keratinocytes

Abstract

CD9 is a member of the tetraspan (TM4) family of proteins and is abundantly expressed in the epidermis. As CD9 forms complexes with β₁ integrins and the integrins are known to regulate keratinocyte behaviour, we investigated CD9 expression and function in human epidermal keratinocytes. CD9 was present in all the living layers of the epidermis, whereas the β₁ integrins were largely confined to the basal layer; the same relative distribution was found in stratified cultures of keratinocytes. There was extensive co-localisation of CD9 and β₁ integrins on microvilli and at cell-cell borders of basal keratinocytes; however, in contrast to the integrins, CD9 was not found in focal adhesions. CD9 was detected in β₁ integrin immunoprecipitates and also in immunoprecipitates of CD44 and syndecan, but not of cadherins. CD9 was associated with α₃β₁ but not α₅β₁; small amounts of CD9 also co-immunoprecipitated with antibodies to α₂β₁, and α₆β₄. Antibodies to CD9 did not affect the proportion of keratinocytes that adhered to laminin 1, type IV collagen and fibronectin, but did inhibit motility of keratinocytes on tissue culture plastic. Like antibodies to the β₁ integrin subunit, anti-CD9 inhibited suspension-induced terminal differentiation. These results suggest that CD9 may play a role in regulating keratinocyte motility and differentiation.