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Regulation of cell surface beta 1 integrin levels during keratinocyte terminal differentiation
N A Hotchin1, A Gandarillas, F M Watt
1Keratinocyte Laboratory, Imperial Cancer Research Fund, London, United Kingdom.
Abstract:
Integrins of the beta 1 family play a central role in controlling adhesion and terminal differentiation within the epidermis. When human epidermal keratinocytes undergo terminal differentiation, intracellular transport of newly synthesized integrins is inhibited, and mature receptors are lost from the cell surface. We have examined the mechanisms underlying these processes, using an experimental model in which keratinocytes are placed in suspension to induce terminal differentiation. The block in intracellular transport was keratinocyte- and integrin-specific since it was not observed when fibroblasts were placed in suspension and did not affect E-cadherin synthesis in suspended keratinocytes. Newly synthesized beta 1 integrins associated with an endoplasmic reticulum resident protein, calnexin; the association was prolonged when keratinocytes were placed in suspension, suggesting a role for calnexin in the inhibition of transport. After 24 h, the level of beta 1 integrin mRNA declines in suspended keratinocytes, reflecting inhibition of gene transcription, but in fibroblasts, the level remained constant. Transport of integrins could be blocked in both adherent keratinocytes and fibroblasts by inhibiting total protein synthesis, raising the possibility that transport is coupled to de novo integrin synthesis. 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 experiments reveal novel mechanisms by which integrin levels can be controlled. Together with our earlier evidence for transcriptional regulation and 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.
Insights
Epidermal terminal differentiation halts beta 1 integrin transport and targets cell surface receptors for lysosomal degradation. This study reveals novel mechanisms controlling integrin levels during keratinocyte differentiation.
Area of Science:
- Cell biology
- Dermatology
- Molecular biology
Background:
- Beta 1 integrins are crucial for epidermal adhesion and keratinocyte terminal differentiation.
- During differentiation, keratinocytes inhibit integrin transport and lose cell surface receptors.
Purpose of the Study:
- To investigate the mechanisms behind inhibited intracellular transport and cell surface loss of beta 1 integrins during keratinocyte terminal differentiation.
- To elucidate how integrin levels are regulated in differentiating epidermal cells.
Main Methods:
- Used a keratinocyte suspension model to induce terminal differentiation.
- Employed confocal and immunoelectron microscopy, and biochemical analyses.
- Assessed beta 1 integrin mRNA levels and association with calnexin.
Main Results:
- A keratinocyte- and integrin-specific block in intracellular transport was observed, involving prolonged association with calnexin.
- Beta 1 integrin mRNA levels declined in differentiating keratinocytes but not fibroblasts.
- Endocytosed receptors were routed to lysosomes during differentiation, and transport could be linked to protein synthesis.
Conclusions:
- Novel mechanisms controlling integrin levels during epidermal differentiation include transport inhibition, transcriptional downregulation, and lysosomal degradation.
- These findings contribute to understanding keratinocyte detachment from the basement membrane during terminal differentiation.