As derivatives of the Hsp90-inhibitor and tumoricidal agent
geldanamycin move into phase II clinical trials, its potential for triggering adverse effects in non-
tumor cell populations requires closer examination. In this report, the effect of
geldanamycin on the differentiation and survival of C2C12 myoblasts was investigated. Treatment of differentiating C2C12 myoblasts with
geldanamycin blocked
myogenin expression, inhibited myotubule formation, and led to the depletion of three Hsp90-dependent
protein kinases, ErbB2, Fyn, and Akt, and induction of apoptosis. ErbB2 levels declined rapidly, while Fyn and Akt levels decreased at a slower rate.
Geldanamycin blocked the interaction of Hsp90 and its "
kinase-specific" co-chaperone Cdc37 with Fyn, indicating that Fyn is an Hsp90-dependent
kinase. Pulse-chase experiments indicated that
geldanamycin caused newly synthesized Akt and Fyn to be degraded rapidly, but
geldanamycin had little effect on the turnover rate of mature Fyn and Akt. Curiously, total cellular Src (c-Src)
protein levels and the turnover rate of newly synthesized c-Src were unaffected by
geldanamycin. While,
geldanamycin had no effect on the levels of the putative Hsp90 client
protein MyoD expressed in C2C12 cells,
geldanamycin disrupted the interaction of Cdc37 with MyoD. Thus, inhibition of Hsp90 caused C2C12 cells to become depleted of multiple signal transduction
proteins whose functions are essential for myoblast differentiation, and muscle cell survival, suggesting that
geldanamycin derivatives may have the prospective of adversely affecting the physiology of certain sensitive muscle cell populations in vivo.