Rh. Warren, TGF-ALPHA-INDUCED BREAKDOWN OF STRESS FIBERS AND DEGRADATION OF TROPOMYOSIN IN NRK CELLS IS BLOCKED BY A PROTEASOME INHIBITOR, Experimental cell research, 236(1), 1997, pp. 294-303
Treatment of NRK cells with TGF-alpha in the presence of serum initiat
es disassembly of cytoskeletal stress fibers and suppresses the synthe
sis of tropomyosin isoforms (TMs) 1, 2, and 3 but not TMs 4 and 5 (Coo
per et al., Cancer Res. 47, 4493-4500, 1987). In order to determine ho
w the loss of tropomyosin is induced and what role it plays in cytoske
letal disruption, the turnover of tropomyosin was studied in the prese
nce of the transforming growth factor and protease inhibitors. Cells w
ere pulse-labeled with [S-35]methionine and chased in the absence or t
he presence of the growth factor. It was found that TMs 1, 2, and 3 ar
e degraded at about twice the rate of TMs 4 and 5 in control cells and
that the rate of degradation of TMs 1-3 is accelerated by the growth
factors. Degradation of TMs in control and growth factor-treated cells
is blocked by a membrane-permeable inhibitor of cysteine proteases (L
LnL) that acts upon calpains and proteasomes, and the cells maintain a
flattened shape with a normal complement of stress fibers. Applicatio
n of inhibitors that block calpains but not proteasomes does not block
TM degradation. Treatments (suspension culture or cytochalasin B) tha
t disrupt stress fibers without application of the growth factors also
accelerate TM degradation, suggesting that acceleration of TM degrada
tion is a consequence of its release from stress fibers during their b
reakdown. The normally more rapid turnover of the TM isoforms 1-3 that
are lost in the phenotypically transformed cells could serve to facil
itate the cytoskeletal reorganization that follows the activation of s
ignal transduction pathways by the transforming growth factors observe
d in this study or during other rearrangements of the cytoskeleton suc
h as occur during cell migration or mitosis. (C) 1997 Academic Press.