Since 1990, the National Cancer Institute (NCI) has screened more than 60,000 compounds against a panel of 60 human cancer cell lines. The 50-percent growth-inhibitory concentration (GI50) for any single cell line is simply an index of cytotoxicity or cytostasis, but the patterns of 60 such GI50 values encode unexpectedly rich, detailed information on mechanisms of drug action and drug resistance. Each compound's pattern is like a fingerprint, essentially unique among the many billions of distinguishable possibilities. These activity patterns are being used in conjunction with molecular structural features of the tested agents to explore the NCI's database of more than 460,000 compounds, and they are providing insight into potential target molecules and modulators of activity in the 60 cell lines. For example, the information is being used to search for candidate anticancer drugs that are not dependent on intact p53 suppressor gene function for their activity. It remains to be seen how effective this information-intensive strategy will be at generating new clinically active agents.

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Myosin has been isolated from cloned mouse fibroblasts, line L-929. Fibroblast myosin: (i) binds to rabbit muscle actin and is dissociated from it by ATP, (ii) has an ATPase activity that is suppressed by Mg2+ in 0.6 M KCI and is activated by rabbit muscle actin in the presence of Mgg+ in 14 mM KCI, (iii) forms thin bi. polar aggregates in 0.1 M KCI when viewed in the electron microscope, (iv) possesses a heavy chain with the same mobility as muscle myosin (molecular weight 200,000) in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In these respects, fibroblast myosin appears to be similar to muscle myosin in structure and function.Since the work of Ishikawa et al. (1) on the presence of actinlike filaments in fibroblasts, the possible existence of a myosin counterpart in these cells has been raised. Recently, Yang and Perdue (2) succeeded in isolating and purifying actin from tertiary cultures of chick embryo cells and found it similar to muscle actin in its molecular properties. Furthermore, the the work of Bray (3) has indicated that cytoplasmic actin can be isolated from several different embryonic sources.cAMP has a marked effect on the motility, adhesion, morphology, and growth of fibroblasts grown in vitro (4-9). It is possible that some of the effects of cAMP could be mediated through one or more of the "contractile" proteins (actin, myosin, and troponin-tropomyosin). To explore this hypothesis, we began by attempting to isolate a myosin-like protein from cloned fibroblasts. Using a modification of a technique developed for the isolation of platelet myosin (10), we isolated myosin from cloned mouse fibroblasts grown in vitro. This protein has been characterized by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, ATPase assay with and without muscle actin, actin binding, and electron microscopy. MATERIALS AND METHODSCloned mouse (L-929) fibroblasts were grown either as monolayers or in spinner bottles. The cells were scraped off the sides of the bottles or sedimented from the suspended culture. They were washed three times in ten volumes of 0.9% NaCl-0.3% sodium citrate-5 mM dithiothreitol (S2threitol)-1 mM EDTA, and sedimented each time at 13,000 X g for 15 min.

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