REGULATION OF CELL-SURFACE BETA(1) INTEGRIN LEVELS DURING KERATINOCYTE TERMINAL DIFFERENTIATION

Integrins of the beta(1) family play a central role in controlling adh esion and terminal differentiation within the epidermis. When human ep idermal keratinocytes undergo terminal differentiation, intracellular transport of newly synthesized integrins is inhibited, and mature rece ptors are lost from the cell surface. We have examined the mechanisms underlying these processes, using an experimental model in which kerat inocytes are placed in suspension to induce terminal differentiation. The block in intracellular transport was keratinocyte- and integrin-sp ecific since it was not observed when fibroblasts were placed in suspe nsion and did not affect E-cadherin synthesis in suspended keratinocyt es. Newly synthesized beta(1) integrins associated with an endoplasmic reticulum resident protein, calnexin; the association was prolonged w hen keratinocytes were placed in suspension, suggesting a role for cal nexin in the inhibition of transport. After 24 h, the level of beta(1) integrin mRNA declines in suspended keratinocytes, reflecting inhibit ion of gene transcription, but in fibroblasts, the level remained cons tant. Transport of integrins could be blocked in both adherent keratin ocytes and fibroblasts by inhibiting total protein synthesis, raising the possibility that transport is coupled to de novo integrin synthesi s. The fate of receptors on the surface of keratinocytes was followed by confocal immunofluorescence microscopy, immunoelectron microscopy, and biochemical analysis: with the onset of terminal differentiation, endocytosed receptors were transported to the lysosomes. These experim ents reveal novel mechanisms by which integrin levels can be controlle d. Together with our earlier evidence for transcriptional regulation a nd affinity modulation of integrins, they highlight the complexity of the mechanisms which ensure that the onset of terminal differentiation is linked to detachment of keratinocytes from the underlying basement membrane.