Terminally differentiated skeletal myotubes are not confined to G 0 but can enter G 1 upon growth factor stimulation

Terminally differentiated cells are specialized cells unable to proliferate that constitute most of the mammalian body. Despite their abundance, little information exists on the characteristics of cell cycle control in these cells and the molecular mechanisms that prevent their proliferation. They are generally believed to be irreversibly restricted to the G ₀ state. In this report, we define some features of a paradigmatic terminally differentiated system, the skeletal muscle, by studying its responses to various mitogenic stimuli. We show that forced expression of a number of cell cycle-regulatory genes, including erbB-2, v-ras, v-myc, B- myb, Id-1, and E2F-1, alone or in combinations, cannot induce terminally differentiated skeletal muscle cells (myotubes) to synthesize DNA. However, serum-stimulated myotubes display a typical immediate-early response, including the up-regulation of c-fos, c-jun, c-myc, and Id-1. They also elevate the expression of cyclin D1 after 4 hours of serum treatment. All these events take place in myotubes in a way that is indistinguishable from that of quiescent, undifferentiated myoblasts reactivated by serum. Moreover, pretreatment with serum shortens the time required by E1A to induce DNA synthesis, confirming that myotubes can partially traverse G ₁. Serum growth factors do not activate late-G ₁ genes in myotubes, suggesting that the block that prevents terminally differentiated cells from proliferating acts in mid- G ₁. Our results show that terminally differentiated cells are not confined to G ₀ but can partially reenter G ₁ in response to growth factors; they contribute to a much-needed definition of terminal differentiation. The important differences in the control of the cell cycle between terminally differentiated and senescent cells are discussed.