Expression and synthesis of high mobility group chromosomal proteins in different rat skeletal cell lines during myogenesis

The synthesis, turnover, and expression of all the major high mobility group (HMG) chromosomal proteins was studied in different rat skeletal myogenic cell lines. Whereas pulse-chase experiments revealed a similar half-life (greater than 2 cell generations) for all the HMG proteins in both L8 myoblasts and myotubes, [3H]lysine incorporation data indicated a 2- to 4-fold greater incorporation of the label in the HMG proteins in proliferating myoblasts relative to the nondividing myotubes. Analysis of the HMG-1, -14, and -17 mRNAs during myogenesis showed a significant down-regulation in L6 and L8 myotubes compared to the myoblasts. However, the timing of the shift and the extent of down-regulation was cell type-dependent, being more pronounced in L6 myotubes at fusion compared to 4 days postfusion in L8 myotubes. By contrast, L8-derived fusion-defective fu-1 cells over the same period of growth showed no change in HMG-14/17 mRNA levels. HMG-I(Y) protein isoforms, noted for the first time in rat myoblasts, like their counterparts, seemed to be stable and showed a precipitous reduction in their mRNAs during myogenesis. The results suggest a cell type-specific correlation between HMG expression and cell proliferation; they also argue for their role in maintenance of the cell's state of differentiation.