Cornifin/small proline-rich proteins
Cornifins/small proline-rich proteins (SPRs ) belong to a family of proline-rich proteins that function as cornified envelope precursors. They consist of a multigene family (SPR1, SPR2, SPR3 and SPR4). SPR1, SPR2, and SPR3 genes are located with loricrin and involucrin genes within a cluster of 1.5 Mbp on chromosome 1q21 and most likely evolved from a common ancestor. In the normal epidermis, SPRR1 was restricted to appendageal areas, SPR2 exists mainly as a CE component, and SPR3 was completely absent. Although the precise roles of SPRs remain unknown, SPRs are cross-linked to insoluble loricrin possibly contributing to movement of the cross-linked products to the cell periphery. By the use of monospecific polyclonal antibodies and cDNA probes, it was shown that SPRR transcripts and proteins expression was restricted to terminally differentiating squamous cells, preferentially located at the cell periphery. As the immunoreactivity was greatly reduced in cells with a mature cornified cell envelope and since detectable SPRR2 and SPRR3 levels were strongly increased in differentiating keratinocyte cultures after addition of LTB-2 (a specific inhibitor of transglutaminases), it has been suggested that SPRR2 and SPRR3 are precursor proteins of the cornified cell envelope. In normal epidermis, SPRR1 was restricted to appendageal areas, SPRR2 was expressed coherently, and SPRR3 was completely absent. In psoriatic epidermis, SPRR1 and SPRR2 were expressed at much higher levels than in normal epidermis. Addition of 10-7M retinoic acid to cultured differentiating keratinocytes significantly down-regulated the expression of SPRR2 and SPRR3 transcripts and slightly decreased that of SPRR1.
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L’épiderme et la différenciation des kératinocytes
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