Enhanced proliferation and differentiation of human articular chondrocytes when seeded at low cell densities in alginate in vitro.

Dedifferentiated human articular chondrocytes exhibited a wide variation in their capacity to proliferate and redifferentiate in an alginate suspension culture system. The greatest extent of proliferation and redifferentiation was seen to be dependent on the formation of clonal populations of chondrocytes and correlated inversely with the initial cell seeding density. Redifferentiating chondrocytes seeded at low density (1 x 10(4) cells/ml alginate) compared with chondrocytes that were seeded at high density (1 x 10(6) cells/ml alginate) showed a nearly 3-fold higher median increase in cell number. a 19-fold greater level of type-II collagen mRNA expression, a 4-fold greater level of aggrecan mRNA expression, and a 6-fold greater level of sulfated glycosaminoglycan deposition at 4 weeks of culture. Matrix molecules from low-density cultures were assembled into chondrocyte-encapsulated, spherical extracellular matrices that were readily visualized in sections from 12-week cultures stained with antibodies against types I and II collagen and aggrecan. Ultrastructural analysis of 12-week low-density cultures confirmed the presence of thin collagen fibrils throughout the matrix.

Targeted overexpression of transforming growth factor alpha in the epidermis of transgenic mice elicits hyperplasia, hyperkeratosis, and spontaneous, squamous papillomas.

To assess the effects of transforming growth factor alpha (TGF-alpha) on mammalian skin in vivo, we have targeted its expression to the epidermis of transgenic mice using a vector based on the human K1 (HK1) gene. Neonatal mice expressing the HK1.TGF-alpha transgene were often smaller than normal littermates and had precocious eyelid opening and wrinkled, scaly skin with diffuse alopecia. Juvenile transgenic mouse epidermis was uniformly hyperkeratotic, but this pattern was generally less pronounced in adult transgenic mice unless they expressed high levels of the HK1.TGF-alpha transgene. Spontaneous, squamous papillomas occurred at sites of wounding in adult mice expressing high levels of HK1.TGF-alpha; however, most were prone to regression. Immunoreactive TGF-alpha was 2-6 times higher in the epidermis of these HK1.TGF-alpha lines. Immunoreactive epidermal growth factor receptor had a normal pattern of expression in nonphenotypic adult epidermis, but a marked reduction in the receptor population was detected in hyperplastic newborn epidermis and phenotypic adult epidermis. Autoradiographic localization of 125I-epidermal growth factor showed a similar pattern of distribution, suggesting that the sites of increased TGF-alpha expression induced epidermal growth factor receptor down-regulation. These data demonstrate the in vivo effect of deregulated TGF-alpha expression on epidermal proliferation and differentiation and suggest a potential role for TGF-alpha in carcinogenesis and other hyperproliferative epidermal disorders.

Identification of a calcium-inducible, epidermal-specific regulatory element in the 3'-flanking region of the human keratin 1 gene.

Previous studies have shown that the process of epidermal differentiation is profoundly influenced by the level of intracellular calcium within keratinocytes. In this study we have identified a 249-bp region, located 7.9 kb downstream from the promoter of the human keratin 1 (HK1) gene, that is able to activate a SV40 minimal promoter chloramphenicol acetyl transferase (CAT) construct in transfected murine keratinocytes. This activity was potentiated by increased levels of calcium and was independent of the position and orientation of the 249-bp fragment. The 249-bp fragment demonstrated a marked specificity for epidermal keratinocytes and was not active in fibroblasts or in a breast epithelial cell line. Moreover, this fragment could activate CAT expression in a construct driven by the HK1 promoter, which alone had no intrinsic CAT activity. A 102-bp fragment derived from the 249-bp fragment was still responsive to calcium but no longer retained cell-type specificity. An AP-1 site at position +7903 and encoded by both the 249-bp and 102-bp fragments is implicated as the cis-element that mediates the calcium response. Taken collectively, these data identify and characterize a regulatory element that is able to activate both heterologous or homologous promoters in response to increased levels of intracellular calcium in keratinocytes.

Hyperplasia, hyperkeratosis and benign tumor production in transgenic mice by a targeted v-fos oncogene suggest a role for fos in epidermal differentiation and neoplasia.

A vector, derived from the human K1 keratin gene, has been employed to target v-fos expression exclusively in the epidermis of transgenic mice. Adult transgenic mice expressors (3-4 months) displayed hyperplasia and hyperkeratosis, initially in wounded (tagged) ears, which later became bilateral. This phenotype appeared at other epidermal sites, most notably in the axilla and inguinal areas. This indicates that a second promoting event, such as wounding or friction, is required to elicit these pathological changes. Highly keratotic benign ear lesions and benign squamous papillomas appeared after long latency at sites of phenotypic epidermis. These data suggest that v-fos may be interfering with c-fos function in normal keratinocyte differentiation, but by itself is insufficient to elicit overt benign lesions.

Induction of epidermal hyperplasia, hyperkeratosis, and papillomas in transgenic mice by a targeted v-Ha-ras oncogene.

The regulatory elements of the human keratin K1 gene have been used to target expression of the v-Ha-ras oncogene exclusively in the epidermis of transgenic mice. We developed 12 transgenic mouse lines that express the HK1.ras transgene, producing epidermal hyperplasia in neonates and hyperkeratosis in juveniles. Eventually this skin phenotype diminished but with time adult animals developed papillomas that could persist or regress. The rate and frequency of tumorigenesis appeared to be limited, which suggests that v-Ha-ras requires a second or even third event to elicit and maintain a benign phenotype in transgenic mice. Since in certain transgenic lines papillomas appeared at wound sites, it appears that the promotion stimulus from wounding may be a second event. We envision that such transgenic mice that express v-Ha-ras in the epidermis will become a powerful model for assessing how environmental and molecular factors affect the process of multistage skin carcinogenesis in vivo, as well as a model for evaluating novel therapeutic protocols.

Mutations in the rod domains of keratins 1 and 10 in epidermolytic hyperkeratosis.

Epidermolytic hyperkeratosis is a hereditary skin disorder characterized by blistering and a marked thickening of the stratum corneum. In one family, affected individuals exhibited a mutation in the highly conserved carboxyl terminal of the rod domain of keratin 1. In two other families, affected individuals had mutations in the highly conserved amino terminal of the rod domain of keratin 10. Structural analysis of these mutations predicts that heterodimer formation would be unaffected, although filament assembly and elongation would be severely compromised. These data imply that an intact keratin intermediate filament network is required for the maintenance of both cellular and tissue integrity.