Id proteins: novel targets of activin action, which regulate epidermal homeostasis.

Activin is a member of the transforming growth factor beta (TGF-beta) family, which plays a crucial role in skin morphogenesis and wound healing. To gain insight into the underlying mechanisms of action, we searched for activin-regulated genes in cultured keratinocytes. One of the identified target genes encodes Id1, a negative regulator of helix-loop-helix transcription factors. We show that Id1, Id2, and Id3 are strongly downregulated by activin in keratinocytes in vitro and in vivo. To determine the role of Id1 in keratinocyte biology, we generated stable HaCaT keratinocyte cell lines overexpressing this protein. Our results revealed that enhanced levels of Id1 do not affect proliferation of keratinocytes in monoculture under exponential culture conditions or in response to activin or TGF-beta1. However, in three-dimensional organotypic cultures, Id1-overexpressing HaCaT cells formed a hyperthickened and disorganized epithelium that was characterized by enhanced keratinocyte proliferation, abnormal differentiation, and an increased rate of apoptosis. These results identify an important function of Id1 in the regulation of epidermal homeostasis.

Amyloidogenicity of recombinant human pro-islet amyloid polypeptide (ProIAPP).

BACKGROUND: Pancreatic amyloid has been associated with type II diabetes. The major constituent of pancreatic amyloid is the 37-residue peptide islet amyloid polypeptide (IAPP). IAPP is expressed as a 67-residue pro-peptide called ProIAPP which is processed to IAPP following stimulation. While the molecular events underlying IAPP amyloid formation in vitro have been studied, little is known about the role of ProIAPP in the formation of pancreatic amyloid. This has been due in part to the limited availability of purified ProIAPP for conformational and biochemical studies. RESULTS: We present a method for efficient recombinant expression and purification of ProIAPP and a processing site mutant, mutProIAPP, as thioredoxin (Trx) fusion proteins. Conformation and amyloidogenicity of cleaved ProIAPP and mutProIAPP and the fusion proteins were assessed by circular dichroism, electron microscopy and Congo red staining. We find that ProIAPP and mutProIAPP exhibit strong self-association potentials and are capable of forming amyloid. However, the conformational transitions of ProIAPP and mutProIAPP during aging and amyloidogenesis are distinct from the random coil-to-beta-sheet transition of IAPP. Both proteins are found to be less amyloidogenic than IAPP and besides fibrils a number of non-fibrillar but ordered aggregates form during aging of ProIAPP. ProIAPP aggregates are cytotoxic on pancreatic cells but less cytotoxic than IAPP while mutProIAPP aggregates essentially lack cytotoxicity. The Trx fusion proteins are neither amyloidogenic nor cytotoxic. CONCLUSIONS: Our studies suggest that ProIAPP has typical properties of an amyloidogenic polypeptide but also indicate that the pro-region suppresses the amyloidogenic and cytotoxic potentials of IAPP.