The specificity of epidermal chalone action: the results of in vivo experimentation with two purified skin extracts

Proliferation-dependent effect of skin extracts (chalone) on mouse epidermal cell flux at the G1-S, S-G2 and G2-M transitions

Epidermal cell proliferation in mice was studied from 4 to 11 h following a single intraperitoneal (IP) injection of a crude skin extract. Cell cycle flux parameters were evaluated by a combination of several methods. The G1-S and S-G2 transit rates were estimated by means of a 3(H)TdR double labelling technique, and the mitotic rate by use of colcemid. The 3(H)TdR labelling index was also measured. To examine the possible influence of the circadian rhythm, all experiments were performed at two different times of the day with high or low rate, respectively, of epidermal cell proliferation. All flux parameters were altered for the whole 7-h period. The relative inhibitory effect of the skin extract was related to the proliferative state of the epidermal cell population. Cell flux at the G1-S transition showed only minor circadian variations, and treatment with skin extract was followed by a relative reduction of cell flux at this transition that was similar at the two times of the day investigated. In contrast, cell flux at the S-G2 transition showed pronounced circadian variations. The effect of the skin extract on cells at this transition was different at the two times in the 24-h period. In general, inhibition expressed as percent of the controls was stronger when the skin extract was given at times when the cell flux was low or decreasing, and vice versa. In spite of the changes in flux values following administration of a skin extract, the 3(H)TdR labelling indices were reduced only after a delay of 10 h, confirming previous results.

The investigation in vivo of the tissue specificity of pig skin fractions

Using an established assay system, a whole aqueous extract of pigskin and the fractions derived from it by ultrafiltration were investigated for their ability to inhibit proliferation in epidermal basal cells of mouse pinna skin in vitro. The fraction between 30KD and 50KD was found to inhibit both mitosis and DNA synthesis specifically in epidermal cells. Furthermore, a dose dependent relationship was found between the degree of inhibition of DNA synthesis and the amount of 30-50KD fraction given. The results are discussed in relation to the cytospecific, tissue and organ specific activity of the fractions arising from the assay methods used; furthermore, the implication of interpreting the extent of specificity in this manner is discussed in relation to the definition of the term 'chalone'.

Epidermis extracts (chalone) inhibit cell flux at the the G1-S, S-G2 and G2-M transitions in mouse epidermis

Epidermal cell flux at the G1-S, S-G2 and G2-M transition was examined during the first 4 hr after injection of epidermis extract. The flux parameters were estimated by a combination of several methods. The G1-S and S-G2 transit rates were calculated on the basis of a double labelling technique with [3H]TdR, the G2-M flux by means of colcemid and the relative proportion of cells in the S or G2 phase by means of flow cytometry. All experiments were performed both in early morning and late evening, corresponding to maximum and minimum rates of epidermal cell proliferation in the hairless mouse. The epidermis extract inhibited the S-G2 and G2-M transit rates to the same degree, while the inhibition of cell flux at the G1-S transit was consistently stronger. In general, the inhibition of cell flux at the different transitions was most pronounced when the rate of cell proliferation was low and vice versa.

Cell proliferation kinetics of epidermis and sebaceous glands in relation to chalone action

Median S-phase lengths of pinna epidermis and sebaceous glands, and of epithelia from the oesophagus and under surface of the tongue of Albino Swiss S mice were estimated by the percentage labelled mitoses method (PLM). The 18.4 and 18,8 hr for the median length of S-phase for pinna epidermis and sebaceous glands respectively made it possible for these two tissues to be used experimentally for testing tissue specificity in chalone assay experiments. The 10.0 and 11.5 hr for oesophagus ang tongue epithelium respectively made experimental design for chalone assay difficult when pinna epidermis was the target tissue. The results of the Labelling Index measured each hour throughout a 24-hr period showed no distinct single peaked diurnal rhythm for pinna epidermis and sebaceous glands. Instead a circadian rhythm with several small peaks occurred which would be expected if an S-phase of approximately 18 hr was imposed on the diurnal rhythm. This indicates that there may be very little change in the rate of DNA synthesis. The results are given for the assay in vivo of purified epidermal G1 and G2 chalones, and the 72--81% ethanol precipitate of pig skin from which they could be isolated. These experiments were performed over a time period which took into account the diurnal rhythm of activity of the mice as well as the S-phase lengths. Extrapolating the results with time of action of the chalone shows that the G1 chalone acts at the point of entry into DNA synthesis and that the S-phase length was approximately 17 hr for both the pinna epidermis and sebaceous glands. This may be a more correct value since the PLM method overestimates the median S-phase length as it is known that in pinna skin the [3H]TdR is available to the tissues for 2 hr and true flash labelling does not take place. The previous reports that epidermal G1 chalone acts some hours prior to entry into S-phase resulted from experiments on back skin where the S-phase is shorter and there is a pronounced diurnal rhythm which could mask the chalone effect. The epidermal G2 chalone had no effect on DNA synthesis even at different times in the circadian rhythm. Thus the circadian rhythms and S-phase lengths of the test tissues need to be considered when experiments are performed with chalones. Ideally, the target tissues selected for cell line specificity tests should have the same cell kinetics for the easier and more accurate assessment and interpretation of results. When the tissues have markedly different cell kinetics, experimental procedures and results need to be evaluated accordingly. The point of action of G1 chalone can only be assessed if the effect is measured over the peak of incorporation of [3H]TdR into DNA. The results of the effects of skin extracts are analysed in relation to changes in the availability of [3H]TdR for the incorporation into DNA and to the possibility of there being two distinct populations of proliferating cells.

Growth regulation in X-irradiated mouse skin; the possible role of chalones

Extracts of hairless mouse skin were tested for their content of epidermal G1 inhibitor and G2 inhibitor at daily intervals after X-irradiation with 4 500 or 2 250 rad. After either dose the skin extracts lacked G1 inhibitory activity on days 5 and 6 respectively after irradiation. This coincided with the time when the epidermal mitotic rate again became normal and started a period of over-shoot. The time interval of 5-6 days corresponds to the turnover time of the differentiating cells in hairless mouse back epidermis. The findings indicate that the proliferating cells in epidermis can respond to changes in local chalone concentration, even after X-irradiation at the tested doses, and that the irradiated epidermal cell population still retains some important properties inherent in a cybernetically regulated system. The local G2-inhibitory activity also varied after irradiation, but these variations could not be directly related to the corresponding mitotic rates.

A simple method for measuring DNA synthesis in epidermis and sebaceous glands and its application in chalone assays

Liquid scintillation counting and autoradiography were used to measure DNA synthesis (3H-TdR uptake and incorporation) by the epidermis and the sebaceous glands in mouse pinna skin. Using this approach it has been possible to show that a chalone-like inhibitor of DNA synthesis acts rapidly to restrict the flow of epidermal G1 cells into S-phase. The point of action within the cell cycle is probably at the G1-S phase 'boundary'. The technique described is sufficiently sensitive to be used for measuring small alterations in DNA synthesis in short-term experiments and for long-term experiments in which the size of the progenitor cell population is altered by systemic or topical administration of compounds influencing DNA synthesis.

Growth kinetics in newborn mouse epidermis: response to epidermal chalone

Epidermal DNA synthesis, the epidermal mitotic rate, and the responsiveness to the epidermal G1 and G2 inhibitors were examined in newborn mice at different times after birth. The rate of epidermal cell renewal was in general low during the first two weeks of life. Later the two growth parameters increased and reached very high values at 32-33 days after birth. The rate of epidermal cell proliferation then decreased to a level comparable with that found in adult hairless mouse epidermis at 40-45 days. A single i.p. injection of skin extract containing the two epidermal growth inhibitors induced varying types of responses. The epidermal G2 inhibitor stimulated the mitotic rate on day 2 and day 10, but inhibited it on all other days. The epidermal G1 inhibitor brought about an increase in epidermal DNA synthesis on day 6 and possibly on the following days. No response at all seen at 2, 4, 17, and 32 days after birth. At the other examined times the inhibition was similar to that found in adult mice. These findings differed from those made in vitro on separated newborn mouse epidermal cells (our own unpublished data), and we suggest that the variability of newborn mouse epidermis could be an expression of the immaturity of the skin as a whole, and that dermis in some way modifies the response of epidermis to exogenous epidermal chalone. Our study did not support the theory that the nonresponsiveness of newborn mouse epidermal at certain times could be due to the presence of nonresponsive stem cells in epidermis.