Reversal of Terminal Differentiation Mediated by p107 in Rb ^-/- Muscle Cells

Abstract

The terminal differentiation of mammalian muscle cells requires the tumor suppressor retinoblastoma protein (Rb). Unlike their wild-type counterparts, multinucleated myotubes from mouse cells deficient in Rb (Rb-/-) were induced by serum to re-enter the cell cycle. Development of the myogenic phenotype in Rb-/- cells correlated with increased expression of p107, which interacted with myogenic transcription factors. Serum-induced cell cycle reentry, on the other hand, correlated with decreased p107 expression. Thus, although p107 could substitute for Rb as a cofactor for differentiation, it could not maintain the terminally differentiated state in Rb-/- myotubes.

The retinoblastoma gene family is differentially expressed during embryogenesis

“…RB1 transcripts were detected throughout myogenesis: in somites and myotomes at E8.5 ± 11.5 ( Figure 2a, c and d), in migrating myoblasts at E12.5 ± 13.5 (Figure 2e and f) and in di erentiating muscles at E14.5 ± 16.5 ( Figure 2g, h and i). Consistent with the expression of RB1 during myogenesis, both in vitro and in vivo studies indicate that pRb is required for terminal di erentiation of skeletal muscles (Gu et al, 1993;Schneider et al, 1994;Zacksenhaus et al, 1996a).…”

“…The double knockout RB 7/7 : p107 7/7 mice do show accelerated cell death in liver and CNS, indicating that lack of expression of both genes can have an additive e ect. Expression of p107 can partially substitute for pRb during di erentiation of RB 7/7 myoblasts in vitro (Schneider et al, 1994), therefore p107 may compensate for pRb in some tissue contexts. Whether the di erent e ects of the RB1 family depend on expression, rather than protein activity is yet to be determined.…”

Developmental defects and tumor predisposition in Rb mutant mice

“…Most notably is a defect in myogenesis, as was previously observed in mice carrying a hypomorph Rb allele (Zacksenhaus et al, 1996;Riley et al, 1997). A role for pRb in myogenic di erentiation is supported by the observation that Rb-de®cient ®broblasts induced to di erentiate by MyoD are impaired in the expression of late di erentiation markers and unable to maintain the di erentiated phenotype (Schneider et al, 1994;Novitch et al, 1996). Rb-de®cient ®broblasts also fail to undergo adipocyte di erentiation, which is related to a direct interaction between pRb and the CCAAT/enhancer binding proteins (C/EBP) (Chen et al, 1996).…”

Looking proximally and distally: 100 years of limb regeneration and beyond

“…Reversal of differentiation by serum stimulation was also reported for myotubes differentiated from CC42 muscle cell lines derived from embryonic lethal Rb À/À mice (Schneider et al, 1994). The differentiation of wild-type skeletal muscle requires pRb (Huh et al, 2004), but p107 can substitute for pRb in pRb À/À muscle differentiation (Schneider et al, 1994). This alternate differentiation pathway apparently does not involve placing a lock on cell cycle re-entry like it does in the differentiation of normal muscle.…”

“…However, C2C12 myotubes are derived from immortalized myoblasts and have a deletion in the ink4a locus that expresses ARF (Pajcini et al, 2010). Reversal of differentiation by serum stimulation was also reported for myotubes differentiated from CC42 muscle cell lines derived from embryonic lethal Rb À/À mice (Schneider et al, 1994). The differentiation of wild-type skeletal muscle requires pRb (Huh et al, 2004), but p107 can substitute for pRb in pRb À/À muscle differentiation (Schneider et al, 1994).…”