TROPOMYOSIN-2 CDNA LACKING THE 3'-UNTRANSLATED REGION RIBOREGULATOR INDUCES GROWTH-INHIBITION OF V-KI-RAS-TRANSFORMED FIBROBLASTS

The levels of high molecular weight isoforms of tropomyosin (TM) are m arkedly reduced in ras-transformed cells. Previous studies have demons trated that the forced expression of tropomyosin-1 (TM-1) induces reve rsion of the transformed phenotype of ras-transformed fibroblasts. The effects of the related isoform TM-2 on transformation are less clear. To assess the effects of forced expression of the TM-2 protein on ras -induced tumorigenicity, we introduced a TM-2 cDNA lacking the 3' untr anslated region riboregulator into ras-transformed NIH 3T3 fibroblasts . TM-2 expression resulted in a flatter cell morphology and restoratio n of stress fibers. TM-2 expression also significantly reduced growth rates in low serum, soft agar, and nude mice. The reduced growth rates were associated with a prolongation of G(0)-G(1). To identify the mec hanism of TM-2-induced growth inhibition, we analyzed the effects of T M-2 reexpression on ERK and c-jun N-terminal kinase (JNK) activities. Levels of ERK phosphorylation and activity in TM-2-transfected tumor c ells were comparable to those in mock-transfected tumor cells. JNK act ivity was only modestly increased in ras-transformed cells relative to untransformed NIH 3T3 cells and only slightly reduced as result of fo rced TM-2 expression. We conclude that the partially restored expressi on of the TM-2 protein induces growth inhibition of ras-transformed NI H 3T3 cells without influencing ERK or JNK activities. Furthermore, th e 3' untranslated region riboregulator of the alpha-tropomyosin gene i s not needed for the inhibition of ras-induced growth.