Cations and calmodulin in normal and neoplastic cell growth regulation

Abstract

The purpose of this presentation is to review pertinent literature pertaining to the role of divalent cations and calmodulin in regulating growth of nonneoplastic and neoplastic cells and to examine the anticancer efficacy of some calmodulin inhibitors. Although normal eukaryotic cell replication and proliferation is closely controlled by a complex system of endogenous substances, it is likely that the coordination of purposeful interactions between these substances is the ultimate responsibility of two groups of cellular components, namely the divalent cations Ca 2+ and Mg 2+ and the versatile intracellular Ca 2+ -binding protein calmodulin (CaM). When free Ca 2+ enters normal cells, it acts as a positive signal for proliferation; this action appears to be specifically associated with the late G 1 phase, just prior to DNA synthesis. This period is designated G 1 /S and is considered to contain Pardee's "restriction point." Reduction of extracellular Ca 2+ concentrations between physiological levels (1–0.1 mM) results in gradually reduced rates of cell proliferation; at Ca 2+ concentrations of 0.1 mM or less, normal cell proliferation is reversibly inhibited. Since an extracellular concentration of about 0.7 mM Mg 2+ is required for Ca 2+ to initiate cell replication, it has been proposed that Ca 2+ and Mg 2+ act in concert via a common mechanism, however, in contrast to Ca 2+ , Mg 2+ appears to be required throughout the entire cell cycle. Intracellular Ca 2+ can activate CaM which, in turn, can modulate various cellular processes that affect cell proliferation, including cyclic nucleotide metabolism, protein phosphorylation, polyamine metabolism, prostaglandin metabolism, Ca 2+ transport, DNA synthesis, and microtubular function including mitosis. Following mitosis, the CaM levels in daughter cells are only about one-half of the premitotic levels; however, during the late G 1 /S period the CaM levels rise rapidly and are maintained at maximum levels during G 2 and M phases. These findings suggest that, in addition to playing a significant role in the function of the mitotic spindle, CaM (in association with Ca 2+ ) also appears to be involved in initiating cell replication during the G 1 /S period. A characteristic feature of neoplastic cells is their relative autonomy from the effects of endogenous growth regulators. In contrast to normal cells, neoplastic cells continue to proliferate in the presence of suboptimal concentrations of extracellular Ca 2+ and intracellular Mg 2+ . Since cancer cells have been shown to have elevated levels of Ca 2+ (100–400% increase), Mg 2+ (25–50% increase), and CaM (50–400% increase), it is possible that the inability of cancer cells to respond to growth regulators is due mainly to the sustained elevated levels of Ca 2+ and (or) CaM during the G 1 /S period. To investigate whether CaM does indeed play a role in promoting neoplastic cell proliferation, recent studies are reported in which the growth-inhibitory effects of anti-CaM agents such as trifluoperazine (TFP) and naphthalenesulfonamides (W-7 and W-13) have been tested on a human breast cancer cell line (MDA-MB-231) using the soft-agar clonogenic assay. The results indicate that all three anticalmodulin agents inhibit colony formation at relatively low concentrations. The IC 50 values, with continuous drug exposure, are as follows: 18, 30, and 38 μM for TFP, W-7, and W-13, respectively. These promising findings suggest that anticalmodulin agents should be investigated more extensively for their possible value in the treatment of cancer.