CHROMATIN CHANGES IN CELL-TRANSFORMATION - PROGRESSIVE UNFOLDING OF THE HIGHER-ORDER STRUCTURE DURING THE EVOLUTION OF RAT HEPATOCYTE NODULES - A DIFFERENTIAL SCANNING CALORIMETRY STUDY

Using differential scanning calorimetry and complementary ultrastructu ral observations, we have characterized the status of chromatin during the transformation of rat hepatocytes in the resistant hepatocyte mod el of Solt and Farber (1976. Nature (Lond.). 263:701-703). Differentia l scanning calorimetry affords a measure of the degree of condensation of chromatin in situ and has therefore been used in this work for the purpose of establishing the nature of the structural changes associat ed with the emergence of successive cellular populations. Since the re sistant hepatocyte model generates a series of synchronous phenotypic changes, it was possible to determine unambiguously the content of het erochromatin at each step of the process. The higher-order structure u ndergoes a partial relaxation in early developing nodules, isolated 16 weeks after initiation; the thermal transition at 90-degrees-C, which is characteristic of noninteracting core particles, increases with re spect to control hepatocytes. Dramatic changes occurr in persistent (4 6-week) nodules. The 90-degrees-C endotherm dominates the thermogram, while the transition at 107-degrees-C, corresponding to the denaturati on of the core particle packaged within the heterochromatic domains, d isappears. The complete loss of the higher-order structure at this sta ge of transformation has been further verified by ultrastructural obse rvations on thin nuclear sections. Ten-nm filaments, having a beaded a ppearance, are scattered throughout the nucleoplasm and clearly result from the decondensation of 30-nm-thick fibers. This catastrophic rela xation process cannot be related to an effective increase in gene acti vity. Rather, our observations suggest that during transformation chro matin is in a state of high transcriptional competence associated with the alert of general cellular programs. This view is consistent with the finding that in persistent nodules the DNA is extensively hypometh ylated with respect to normal liver.