Translation of polysomal messenger RNA during epidermal differentiation

Exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA) induces the synthesis of histidine-rich protein (filaggrin) in monolayer cultures of rat keratinocytes

The effect of 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent tumor promoter, on epidermal differentiation has been studied by investigating the appearance of the epidermal differentiative marker protein, histidine-rich protein (HRP), in monolayer cultures of rat keratinocytes grown in low-calcium (0.1 mM) medium. Monoclonal antibody, 3F6-6, raised against the 60K HRP (HRPII) derived from the epidermis of the newborn rat was utilized to detect HRP in cultured keratinocytes. As a major component of keratohyalin in the cells of the granular layer in epidermis, HRP is not normally found in a monolayer culture of rat keratinocytes containing a high population of proliferating cells. However, when these cultures were exposed to 10 ng/ml of TPA, DNA and protein synthesis were decreased by up to 95% and 60%, respectively, and the cells in the monolayer culture became HRP-positive within 18-24 h. TPA has been shown to cause a withdrawal of the basal cells from the cell cycle and a commitment of these cells to an accelerated rate of terminal differentiation. Exposure to TPA at the same time that the calcium level of the monolayer cultures was raised to 2.0 mM in order to allow stratification resulted in positive immunostaining for HRP much earlier and much more intensively than in the absence of TPA. TPA also enhanced stratification of these cultures; however, the positive expression of HRP preceded the stratification. These observations support the view that the induction of terminal differentiation occurs independently of stratification and that the process of terminal differentiation (maturation) of basal cells precedes and triggers the movement of these cells to a suprabasal position (stratification).

The morphological details of globular keratohyalin granules

In quickly dividing epithelia such as that of the tongue, keratohyalin formation takes place in globular keratohyalin granules (KHG). This is in contrast with the irregular KHG as seen in normal, slowly dividing epidermis. The morphogenesis of the globular KHG is explained in this study. In small KHG, dense aggregates of ribosomes can be seen at the site of blebs. It is suggested that these blebs framed with ribosomes are internalized giving rise to "dense homogeneous deposits" or "single granules". Lipid droplets occur in the upper spinous and horny layer. Globular KHG also contain variable amounts of lipids, and the lipid content seems to be inversely related to the protein content, dependent on the degree of cell differentiation or on the rate of cell turnover. It is suggested that in epithelia with a high cell turnover few rigid keratohyalin components are dispersed in lipids, which maintain a globular shape due to the surface tension.

Synthesis in vitro of keratin polypeptides directed by mRNA isolated from newborn and adult mouse epidermis

A method is described which allows the isolation of undegraded total cellular RNA from living epidermis in a one-step procedure. Epidermal tissue is homogenized in 4 M guanidinium thiocyanate/2% sarkosyl buffer and centrifuged through a 5.7 M CsCl cushion. Under appropriate density conditions in the homogenate, large amounts of RNA are sedimented free of DNA and protein to the bottom of the tubes. A prerequisite for the applicability of this method is the preparation of pure epidermis under conditions which efficiently inactivate endogenous RNases. Oligo(dT) affinity chromatography of total RNA yields 3-4% of polyadenylated RNA. Translation of poly(A)-rich fractions from both newborn and adult mouse epidermis in a cell-free reticulocyte system leads to the synthesis of proteins with electrophoretic mobilities identical to those of native keratin polypeptides of the corresponding tissue. Similar to native keratin polypeptides, these proteins are highly insoluble and can selectively be enriched by high-salt extraction of the translation assays. The CNBr cleavage pattern of these purified proteins is identical to that of native keratins. There is no evidence for the synthesis of precursors of these proteins.

Heterogeneous keratohyalin formation in warts

The heterogenecity of keratohyalin (KH) has been demonstrated in rats, then in human warts where similar morphological features have been found and among them, dense homogeneous ovoïd single granules (SG) and composite granules (CG) apparently made up of single granules "coated" with a matrix. This work compares the ultrastructure of the KH of the tongue of newborn rats with that of common warts with and without post-fixation by OsO4, after oxydation by H2O2 and after pepsin treatment. The osmiophily of the SG is confirmed as well as the absence of osmiophily of the matrix of the CG. In man, however, the SG are resistant to oxydation by H2O2 and the CG matrix is oxydation sensitive whereas the opposite is observed in the case of rats. These differences show that it is impossible to deduce the composition of the KH in human warts from that of the rat despite their close ultrastructural resemblance but that important modification of the keratogenesis and KH formation are induced in the cells parasited by the virus of common warts.

Effect of puromycin on the ultrastructure of keratohyalin granules

Keratohyalin granules (KHG) from control and puromycin-treated hamster cheek pouch epithelium were studied by electron microscopy. The animals were killed 1-4 hours after injection with 75-100 microgram puromycin per gram body weight; controls were injected with saline. The KHG from control animals were round or oval and consisted of a finely granular background matrix containing many small, round, homogeneous electron-dense deposits. The KHG from puromycin-treated animals showed marked variation in morphology and size. They consisted of a coarse granular or speckled matrix and round, club-shaped or comma-shaped electron-dense deposits. Since puromycin is known to inhibit protein synthesis, alterations in KHG structure support previous reports suggesting the protein nature of KHG.

Molecular aspects of control in epidermal differentiation

The mammalian epidermis is organized into layers of structurally different cells--the basal, spinous, granular and cornified layers--which represent steps in the differentiative process that terminates in cornification and desquamation. Investigation of the molecular mechanisms that control this ordered sequence of events provides clues to the etiology of certain epidermal pathologies. DNA synthesis and mitosis are normally restricted to the basal layer. Several substances have been implicated in the mitotic control of epidermal cells, the loss of mitotic activity being the first major step in normal keratinization. Investigations performed in this laboratory indicate that isolated differentiated nuclei can replicate their DNA which they are inhibited from doing in situ. Addition of a high speed supernate from homogenized differentiated cells inhibited this synthetic activity in vitro suggesting the existence of a cytoplasmic inhibitor of DNA synthesis. It is not known whether mitotic inhibition in differentiated epidermal cells is a function of the inhibition of DNA replication. Contrary to previous assumptions, recent experimental evidence clearly indicates that, unlike DNA synthesis, RNA synthesis occurs in differentiated cells. Correlated with this synthetic activity is the observation that a protein rich in histidine is specifically formed in the granular cells. This protein appears to be a component of the keratohyalin granules which fill the cells of the granular layer. Investigations were conducted in this laboratory to determine whether control of the synthesis of this protein occurs at the level of translation or transcription. Translation, in vitro, of mRNA obtained from isolated populations of each epidermal cell type suggested that control of protein synthesis in the differentiating epidermis is transcriptional, i.e. only in the granular cell is there an mRNA for the histidine-rich protein. Transcription, in vitro, of chormatin isolated from the separated cell populations produced RNA with a ratio of cytidine to uracil consistent with the predicted mRNA for this protein thus providing additional support for the hypothesis that epidermal differentiation is controlled at the level of 'gene-readout'.

Mammalian epidermal protein synthesis: initiation factors

There is a deficiency of initiation in protein-synthesizing systems prepared from mammalian epidermis. These systems do not respond to inhibitors of initiation although they remain sensitive to elongation inhibitors and exogenous ribonuclease. In spite of this deficiency of initiation, active protein factors which support initiation reactions are present in the potassium chloride extract of mammalian epidermal ribosomes. A factor corresponding to the reticulocyte factor IF-MP has been isolated. An inhibitor of initiation is also present in the epidermal KCl wash.