Differences in calmodulin levels of normal and transformed cells as determined by culture conditions (original) (raw)

Several studies have suggested that calmodulin (CAM) levels increase in cells as a consequence of transformation by RNA tumor viruses. This study examines factors affecting CAM levels in normal and transformed chick embryo fibroblasts. Significant differences in CAM levels of normal and transformed cells were observed as cells grew from subconfluent to confluent densities. These changes were not cell cycle dependent, nor did they correlate with the growth rate of the cultures. The most signifi cant difference between normal and transformed cultures was a lack of down-regulation of CAM levels in transformed cells as compared to normal chick embryo fibroblasts. This decrease in CAM levels in normal cells occurred in high density cultures that were allowed to grow undisturbed for several days without trypsinization and reseeding. These experiments do not support the contention that differences in the growth potential of cycling cultures of normal and transformed cells are regulated through modulation of CAM levels. INTRODUCTION The divalent cation Ca2+ is a major regulatory signal in eukaryotic cells. Calcium primarily exerts this regulatory role through interaction with the calcium-binding protein CAM.4 Upon binding Ca2+, CAM has been demonstrated to regulate a wide range of cellular activities including cyclic nucleotide metabolism, exocytosis, actomyosin-based motility, intermediary metabolism, and Ca2+ sequestration. (For a review of Ca2+-CAM as a multifunc tional, regulatory complex, see Refs. 8, 13, and 17.) It has been suggested that, since CAM plays such a funda mental role in the regulation of a wide range of cellular activities, its regulatory role in the cell would probably not depend upon changes in the intracellular levels of CAM (7, 29). It seems likely that the regulation of CAM-associated activities would be con trolled by either alterations in the levels of CAM-binding proteins or by changes in Ca2+ distribution. However, a 2-to 4-fold increase in CAM content has been reported in transformed versus normal cells (7, 14, 32) and in rapidly dividing versus quiescent normal fibroblasts (32). It must be noted that previous studies did not consider, in detail, parameters such as percentage of actively dividing cells in the population, cell volume, the effects of cell culture condi tions, and possible variation during cell cycling. The current work addresses these points, demonstrating that CAM levels do in deed vary differentially in normal and transformed cells as a function of culture condition. However, the variation is more 1To whom requests (or reprints should be addressed.