Identification and partial purification of chalone from human epidermis

Cell cycle kinetics of cultured human epidermal keratinocytes

When stratified epithelia maintained in culture are used for autoradiographic studies of labeling index, the emulsion is usually placed over the uppermost strata of the culture. In many cases the distance from the basal cell nucleus to the emulsion exceeds the average pathlength of beta-particle emissions from 14C or 3H. We describe a technique for inverting the cultures so that the emulsion can be brought into close association with the basal cells. Attempts to label cultured human epidermal keratinocytes using a pulse of [3H]- or [14C]-thymidine produced labeling only at the periphery of the colonies. This was noted when emulsion was laid on top of the colonies but also when the emulsion was in close contact with the "basal cells" adhering to the plastic culture vessel. Continuous labeling of the cultures produced nearly 100% labeling of all the basal layer, i.e., central and peripheral, indicating that the central cells were also in rapid cell cycle. The results are interpreted as indicating the presence of an efficient barrier to free diffusion over the center of the colonies, presumably due to the presence of several layers of corneocytes. Percent labeled mitoses (PLM) studies produced an unusual PLM curve with a well-defined third peak which showed a higher PLM than the second peak. These results may indicate that the cultures contain discrete cell populations with different cell kinetic phase durations.

[Tissue specific inhibition of lymphocyte proliferation by spleen extract (lymphocyte chalone) (author's transl)]

Aqueous spleen extracts were purified using acetone precipitation, membrane filtration, affinity chromatography, and dialysis. These extracts were able to inhibit thymidine incorporation into lymphoid cells (MKT-CH and PHA-stimulated lymphocyte cultures). They did not influence non lymphoid tissue (melanoma cells Mel Ei 78 and Ehrlich ascites cells). The inhibition was reversible and the purified extracts were not cytotoxic. The extracts correspond to a chalone. Their importance for prevention of graft versus host reaction and for treatment of lymphoproliferative diseases is discussed.

Epidermal chalone--past to present concept

How the constantly renewing epidermis maintains its homeostatic state remains a mystery. A "negative feedback" mechanism of control with chalone, a tissue-specific, species-nonspecific inhibitor of mitosis, has been suggested by Bullough and Laurence. Progress in isolating, purifying, and characterizing epidermal chalone has been slow and difficult because of the lack of a fast, assurate, and meaningful assay system. A G2 assay (depending upon mitotic counts) was used to obtain the data which served as the basis for postulating the existence of chalone. Since a control point with the G1 phase of the cell cycle is considered to be the one with physiologic implications, in vivo and in vitro assays with pure primary epidermal cell cultures have been used to investigate a G1 block. The incorporation of tritiated thymidine into DNA is often used as a measure of the rate of prokiferation. However, inhibition of thymidine transport and phosphorylation have been encountered in several systems and negate this method as an accurate measure of cell proliferation.

In vitro analysis of the control of keratinocyte proliferation in human epidermis by physiologic and pharmacologic agents

Human keratinocytes grown in vitro as epithelial outgrowths or as organ cultures maintain many of their normal functions such as proliferation and keratinization. These in vitro systems have been used to analyze the effect of various agents on proliferation. All adenine nucleotides, including dibutyryl cyclic AMP, blocked mitosis in the G2 part of the cell cycle at concentrations of 1 times 10(-4) M. Some nonadenine nucleotides also showed this effect, but only at higher concentrations, an indication that the effect was specific for adenine nucleotides. Dibutyryl cyclic AMP and theophylline both depressed the incorporation of [3H]thymidine into DNA. Catecholamines such as isoproterenol, epinephrine, and norepinephrine were also potent inhibitors of mitosis (G2 block) at concentrations of 1 times 10(-8) to 1 times 10(-10) M. The fact that the effect could be blocked by the beta-blocking agent, propranolol, suggests the existence of specific membrane receptor sites. However, dichloroisoproterenol, another beta blocker, had distinct inhibitory properties in itself and thus indicated that the mechanism of action of catecholamines in human keratinocytes is complex and may involve more than binding to specific receptor sites. Histamine at a concentration of 2 times 10(-6) M was also a strong mitotic inhibitor. This finding is directly opposed to that in rat skin where mitosis is stimulated. Imidazole acetate, a histamine breakdown product, was found to be a striking mitotic stimulator in organ culture. A water-extractable protein (chalone) from human skin also caused a block in G2. Most of the substances tested occur naturally in the cell or organism and their ability to stimulate or depress proliferation in vitro suggests that they play a regulatory role in vivo.

Primary cell culture for biochemical studies of human keratinocytes. A method for production of very large numbers of cells without the necessity of subculturing techniques

Starting with large numbers of small split-thickness explants of human skin, it is possible to grow ample numbers of keratinocytes in vitro as primary cell cultures to permit biochemical studies without the need of subculturing techniques.