Production of pyrimidine dimers in DNA of human skin exposed in situ to UVA radiation.

Cyclobutyl pyrimidine dimers, measured as sites recognized by the dimer-specific ultraviolet (UV) endonuclease from Micrococcus luteus, were produced in DNA of human skin exposed in situ to UVA (320-400 nm) radiation. The dimer yields produced by a broadband UVA source, by broadband UVA filtered to remove all light of wavelength less than 340 nm, and by narrow band radiation centered at 365 nm were similar, indicating that UVA radiation, and not stray shorter wavelength radiation, was responsible for dimer production. The identity of the UVA-induced DNA lesions was confirmed as pyrimidine dimers by photoreactivation of approximately 100% of the endonuclease-sensitive sites in vitro with the 40,000 dalton Escherichia coli photoreactivating enzyme.

Higher pyrimidine dimer yields in skin of normal humans with higher UVB sensitivity.

We have measured UVB (280-320 nm)-induced DNA damage in skin of individuals with different sensitivities to UVB irradiation as measured by minimal erythema dose (MED). The DNA damage was susceptible to cleavage by Micrococcus luteus UV endonuclease, which recognizes pyrimidine dimers in DNA. An alkaline agarose gel electrophoresis method was used to quantitate the number of M. luteus UV endonuclease-sensitive sites in nonradioactive DNA from skin biopsies of 7 individuals irradiated with UVB (0-180 mJ X cm-2). The production of sites correlated well with MED (correlation coefficient = 0.78). The slope of the dose response curve for the most UVB-sensitive individual (MED = 24 mJ X cm-2) and for the least UVB-sensitive individual (MED = 146 mJ X cm-2) were 11.5 X 10(-4) and 2.6 X 10(-4) sites per 1000 bases per mJ X cm-2, respectively. The UVB-induced DNA damage was determined to be pyrimidine dimers by its susceptibility to cleavage by M. luteus UV endonuclease and its photoreactivability by Escherichia coli photoreactivating enzyme.

Comparative protection efficiency of UVA- and UVB-induced tans against erythema and formation of endonuclease-sensitive sites in DNA by UVB in human skin.

UVA- and UVB-induced tans which were visually identical with each other were induced in separate sites on the lower back of 5 normal human volunteers of good tanning ability. Tanning was achieved by 4 exposures to UVA and UVB administered over an 8-day period. One week after the last exposure the protection afforded by the two types of tan against UVB-induced erythema and against UVB-induced DNA damage was measured. Protection against erythema was measured by comparison of the minimal erythema doses of UVB in tanned and untanned skin. Protection against DNA damage was assessed by comparing the numbers of endonuclease-sensitive sites in epidermal DNA extracted from biopsies taken from tanned and untanned sites exposed to the same dose of UVB. The UVB tans conferred significant protection (mean 2.98-fold) against UVB-induced erythema. UVA tans were not associated with significant protection (mean 1.4-fold). In contrast, both UVA- and UVB-induced tans were associated with a similar reduction in yield of endonuclease-sensitive sites in epidermal DNA (in UVA tan to 47% and in UVB tan to 45% of the yield in untanned skin). Protection conferred by the tans against erythema was therefore not paralleled by protection against DNA damage.

Specificity of hemin action in vivo at early stages of hematopoietic cell differentiation.

The effects of a single injection of hemin on murine marrow BFU-E and CFU-S were assessed to determine whether hemin is as effective in augmenting primitive (day 7) BFU-E levels in situ as it is in vitro and to assess hemin's action on transplantable pluripotent stem cells (CFU-S). The results show that hemin exerts a cell-specific enhancement of both BFU-E marrow levels and cell cycling within 6 h of its administration in vivo. No such effect on CFU-S was observed.

Further characterization of the hemin-induced enhancement of primitive erythroid progenitor cell growth in vitro.

We have previously demonstrated that hemin specifically enhances the in vitro plating efficiency of primitive murine erythroid progenitors (day 7 BFUE), whereas it does not appear to affect more mature progenitors (mature BFUE or CFUE). In this report, we further characterize the effects of hemin on marrow-derived day 7 BFUE growth in vitro. BFUE were enhanced by hemin in a dose-dependent manner and to a greater extent in methyl cellulose than in plasma clot cultures. That hemin might increase the rate of cell division was suggested by the greater size of colonies grown in its presence as well as their earlier appearance in culture. In contrast, the addition of hemin to marrow cell cultures did not appear to affect the survival rate of BFUE or their progeny. While significantly augmenting the frequency of BFUE, hemin had no consistent stimulatory effect on CFUGM. Lastly, hemin was equally capable of augmenting burst growth in adherent cell-depleted as in whole marrow cell preparations. These experiments suggest that hemin augments directly and in a cell-specific manner the proliferation and/or differentiation of primitive marrow erythroid progenitors in vitro.