The loss of desmosomes after retinoic acid treatment results in an apparent inhibition of HaCaT keratinocyte differentiation.

Epithelial tissue cohesion is based on various types of intercellular adhering junctions of which the desmosomes are particularly abundant in stratified epithelia. The desmogleins (dsg) and desmocollins are their transmembrane components. One or more of the three isoforms of these desmosomal cadherins are co-expressed and specific subtypes prevail at different stages of epidermal differentiation. In HaCaT keratinocytes, desmosomal cadherin expression increased with ongoing differentiation, apart from dsg2. Continuous treatment with retinoic acid (RA) inhibits the differentiation of HaCaT keratinocyte cultures. RA strongly increased the shedding of cells into the culture medium where they quickly underwent cellular death. Electron microscopy showed a marked reduction of desmosomes with nearly complete absence of their structural components, suggesting that RA inhibits their synthesis. RA indeed downregulated the transcript levels of all HaCaT desmosomal cadherins, except dsg2. Immunostaining revealed that desmosomal protein contents corresponded to alterations in transcription rates. Our findings indicate that the RA-induced inhibition of differentiation of keratinocyte cultures results from removal of cells committed to differentiation. In vivo, less adhering but still differentiating cells cannot be removed as easily as they can be in a culture system. The consequence is a sticky and fragile skin.