High-density lipoproteins induce a rapid and transient release of Ca2+ in cultured fibroblasts

Several different cell types showed increased rates of proliferation and cholesterol mobilization in response to treatment with high-density lipoprotein (HDL). This would suggest that one main function of HDL is the activation of signal pathways in cells. In the current study we have used the fluorescent indicator fura-2 to monitor the level of cytosolic Ca2+ ([Ca2+]i) in human skin fibroblasts. Exposure of subconfluent as well as confluent fibroblasts to HDL3 (20-60 micrograms/ml) resulted in a rapid and transient increase in [Ca2+]i. Sequential additions of HDL3 resulted in diminished rises in [Ca2+]i. The transient rise in [Ca2+]i was observed with HDL prepared from plasma either by conventional ultracentrifugation or by precipitation with dextran sulphate. Chelation of the extracellular Ca2+ with EGTA prior to the addition of HDL3 did not prevent the HDL3-induced rise in [Ca2+]i, suggesting that the mobilized Ca2+ was derived mainly from intracellular stores. Covalent modification of the apoproteins of HDL3 with dimethyl suberimidate or tetranitromethane did not inhibit the HDL3-induced rise in [Ca2+]i. This indicates that the binding of HDL3 to cell surface receptors may not be necessary for the mobilization of intracellular Ca2+. Moreover, the Ca(2+)-releasing effect of HDL3 was not inhibited by the presence of albumin (1%, w/v) in the extracellular medium, suggesting that non-esterified fatty acids were not the cause of the increased [Ca2+]i. The exposure of fibroblasts to lysophosphatidic acid, a potent mitogen and Ca(2+)-releasing agent, before addition of HDL3 completely inhibited the HDL3-induced rise in [Ca2+]i. Furthermore, phorbol 12-myristate 13-acetate blocked the HDL3-induced rise in [Ca2+]i. The results of this study imply that exposure of cells to HDL generates an intracellular signal which is induced by a component of the lipid fraction.