Abstract
We have examined cultures of neonatal human foreskin keratinocytes (HFKs) to determine the ligands and functions of integrins alpha 2 beta 1, and alpha 3 beta 1 in normal epidermal stratification and adhesion to the basement membrane zone (BMZ) in skin. We used three assay systems, HFK adhesion to purified extracellular matrix (ECM) ligands and endogenous secreted ECM, localization of integrins in focal adhesions (FAs), and inhibition of HFK adhesion with mAbs to conclude: (a) A new anti-alpha 3 beta 1 mAb, P1F2, localized alpha 3 beta 1 in FAs on purified laminin greater than fibronectin/collagen, indicating that laminin was the best exogeneous ligand for alpha 3 beta 1. However, in long term culture, alpha 3 beta 1 preferentially codistributed in and around FAs with secreted laminin-containing ECM, in preference to exogenous laminin. Anti-alpha 3 beta 1, mAb P1B5, detached prolonged cultures of HFKs from culture plates or from partially purified HFK ECM indicating that interaction of alpha 3 beta 1 with the secreted laminin-containing ECM was primarily responsible for HFK adhesion in long term culture. (b) In FA assays, alpha 2 beta 1 localized in FAs conincident with initial HFK adhesion to exogenous collagen, but not laminin or fibronectin. However, in inhibition assays, anti-alpha 2 beta 1 inhibited initial HFK adhesion to both laminin and collagen. Thus, alpha 2 beta 1 contributes to initial HFK adhesion to laminin but alpha 3 beta 1 is primarily responsible for long-term HFK adhesion to secreted laminin-containing ECM. (c) Serum or Ca2(+)-induced aggregation of HFKs resulted in relocation of alpha 2 beta 1 and alpha 3 beta 1 from FAs to cell-cell contacts. Further, cell-cell adhesion was inhibited by anti-alpha 3 beta 1 (P1B5) and a new anti-beta 1 mAb (P4C10). Thus, interaction of alpha 3 beta 1 with either ECM or membrane coreceptors at cell-cell contacts may facilitate Ca2(+)-induced HFK aggregation. (d) It is suggested that interaction of alpha 3 beta 1 with a secreted, laminin-containing ECM in cultured HFKs, duplicates the role of alpha 3 beta 1 in basal cell adhesion to the BMZ in skin. Further, relocation of alpha 2 beta 1 and alpha 3 beta 1 to cell-cell contacts may result in detachment of cells from the BMZ and increased cell-cell adhesion in the suprabasal cells contributing to stratification of the skin.