ULTRAVIOLET-RADIATION INCREASES TROPOELASTIN ACCUMULATION BY A POSTTRANSCRIPTIONAL MECHANISM IN DERMAL FIBROBLASTS

Chronically sun-damaged human skin is characterized by dermal connecti ve tissue damage that includes the massive accumulation of abnormal el astic fibers. The content of elastin, the major protein component of e lastic fibers, is increased two- to sixfold in sun-damaged skin, The a im of this study was to determine the mechanism responsible for the in crease in elastin levels after ultraviolet (UV) irradiation, Confluent cultures of normal dermal fibroblasts were irradiated with 4.5 mJ/cm( 2) of UVB; sham-treated cells served as the control group, The accumul ation of tropoelastin was determined at 5 d after treatment by measuri ng the incorporation of C-14-proline into radiolabeled tropoelastin is olated from cell layers and media, UV irradiation increased radiolabel ed tropoelastin accumulation approximately twofold without affecting D NA content, the total amount of radiolabeled protein, or tropoelastin secretion. Moreover, the steady-state levels of tropoelastin mRNA, as determined by slot blot hybridizations, were unaffected by UV treatmen t, However, the translation of tropoelastin mRNA was increased when to tal RNA from irradiated cells was used in cell-free translation experi ments. These results suggest that altered translational efficiency may account for the increase in tropoelastin accumulation after UV irradi ation, In support of this hypothesis, nucleotide sequences were derive d from tropoelastin mRNA isolated from UV-irradiated and nonirradiated dermal fibroblasts, Almost a 12% substitution rate was observed in nu cleotide sequences derived from the 3' untranslated region of tropoela stin mRNA from the UV-treated cells, In contrast, a coding domain of t ropoelastin did not contain base-substitution mutations, These multipl e base substitutions in a noncoding domain of tropoelastin mRNA may be responsible for the post-transcriptional increase in tropoelastin acc umulation after UV irradiation.