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 Ca2+ and Mg2+ and the versatile intracellular Ca2+-binding protein calmodulin (CaM). When free Ca2+ enters normal cells, it acts as a positive signal for proliferation; this action appears to be specifically associated with the late G1 phase, just prior to DNA synthesis. This period is designated G1/S and is considered to contain Pardee's "restriction point." Reduction of extracellular Ca2+ concentrations between physiological levels (1–0.1 mM) results in gradually reduced rates of cell proliferation; at Ca2+ concentrations of 0.1 mM or less, normal cell proliferation is reversibly inhibited. Since an extracellular concentration of about 0.7 mM Mg2+ is required for Ca2+ to initiate cell replication, it has been proposed that Ca2+ and Mg2+ act in concert via a common mechanism, however, in contrast to Ca2+, Mg2+ appears to be required throughout the entire cell cycle. Intracellular Ca2+ 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, Ca2+ 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 G1/S period the CaM levels rise rapidly and are maintained at maximum levels during G2 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 Ca2+) also appears to be involved in initiating cell replication during the G1/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 Ca2+ and intracellular Mg2+. Since cancer cells have been shown to have elevated levels of Ca2+ (100–400% increase), Mg2+ (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 Ca2+ and (or) CaM during the G1/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 IC50 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.