DIFFERENTIAL PROTEIN LOCALIZATION IN SARCOMERIC AND NONSARCOMERIC CONTRACTILE STRUCTURES OF CULTURED CARDIOMYOCYTES

The use of cardiomyocyte cell culture models allows the identification of various cell mediators that bring about changes in subcellular str uctures and gene expression associated with hypertrophy. The effects o f insulin-like growth factor-I (IGF-I), basic fibroblast growth factor (bFGF), and triiodothyronine (T3) on gene expression and on the struc tural organization of myofibrillar and cytoskeletal proteins were comp ared in adult atrial (aARC) and ventricular (vARC) as well as in neona tal ventricular rat cardiomyocytes (VNRC) in long-term culture. Struct ural changes were evaluated by confocal microscopy and correlated to b iochemical alterations. In vARC, IQF-I enhanced myofibrillar growth, w hereas bFGF or T3 restricted sarcomere assembly to the central cell ar ea, forming a sharp boundary in more than 50% of the cells. However, m yosin occurred both in the cross-striated myofibrillar structures and in patches running along the nonsarcomeric fibrillar structures (also called stress fiber-like structures) in the cell periphery. In cells t reated with either bFGF or T3, the expression of cn-smooth muscle acti n (cu-sm actin) was greatly increased, This actin isoform was incorpor ated mainly into the nonsarcomeric contractile structures outside the area where myofibrils ended abruptly, alpha-sm actin protein increased up to 14- to 17-fold while the mRNA showed a moderate increase of 2- to 4-fold. This suggests that alpha-sm actin is mainly regulated at th e translational or posttranslational level. In contrast, the cytoskele tal proteins alpha-actinin and vinculin increased only moderately (les s than 2-fold) but also showed a relocalization in cells With restrict ed myofibrils. In aARC and in vNRC, alpha-sm actin was only moderately upregulated by bFGF or T3 and no drastic morphological changes were o bserved. In conclusion, IGF-I, bFGF, and T3 induced characteristic str uctural phenotypes depending on the type of cardiomyocyte. Large amoun ts of or-sm actin as expressed in bFGF and T3 treated vARC seem to be incompatible with sarcomere assembly. (C) 1998 Academic Press.