GROWTH AND DIFFERENTIATION OF C2 MYOGENIC CELLS ARE DEPENDENT ON SERUM RESPONSE FACTOR

In order to study to what extent and at which stage serum response fac tor (SRF) is indispensable for myogenesis, we stably transfected C2 my ogenic cells with, successively, a glucocorticoid receptor expression vector and a construct allowing for the expression of an SRF antisense RNA under the direction of the mouse mammary tumor virus long termina l repeat. In the clones obtained, SRF synthesis is reversibly do,vn-re gulated by induction of SRF antisense RNA expression by dexamethasone, whose effect is antagonized by the antihormone RU486. Two kinds of pr oliferation and differentiation patterns have been obtained in the res ulting clones. Some clones with a high level of constitutive SRF antis ense RNA expression are unable to differentiate into myotubes; their g rowth can be blocked by further induction of SRF antisense RNA express ion by dexamethasone. Other clones are able to differentiate and are a ble to synthesize SRF, MyoD, myogenin, and myosin heavy chain at confl uency, When SRF antisense RNA expression is induced in proliferating m yoblasts by dexamethasone treatment, cell growth is blocked and cyclin A concentration drops, When SRF antisense RNA synthesis is induced in arrested confluent myoblasts cultured in a differentiation medium, ce ll fusion is blocked and synthesis of not only SRF but also MyoD, myog enin, and myosin heavy chain is inhibited. Our results show, therefore , that SRF synthesis is indispensable for both myoblast proliferation and myogenic differentiation.