INTRACELLULAR CALCIUM SIGNALING AFTER BINDING OF LOW-DENSITY-LIPOPROTEIN TO CONFLUENT AND NONCONFLUENT CULTURES OF AN ENDOTHELIAL-CELL LINE, EAHY 926

The effect of lipoproteins on cellular calcium ion concentration ([Ca2+]i) in EA.hy 926 endothelial cells was investigated, particularly with respect to the difference in response on [Ca2+]i between native low-density lipoprotein (LDL) which binds to the apo B/E receptor, and acetylated LDL (AcLDL) which binds to the scavenger receptor. The scavenger receptor recognizes chemically or cell-induced modified LDL. LDL as well as AcLDL caused a transient increase of [Ca2+]i lasting 1-2 min. On a protein basis, LDL was more effective that AcLDL in raising [Ca2+]i. Preincubation of confluent cultures in growth medium with a reduced fetal bovine serum content (2% FBS instead of 10%) increased the potency of LDL to increase [Ca2+]i. The LDL-induced peak [Ca2+]i was dependent on cell density. The effect of AcLDL on [Ca2+]i did not differ between confluent and nonconfluent cultures. Also, preincubation with 2% FBS did not modify the AcLDL-induced calcium response. We conclude that binding of lipoproteins to membrane lipoprotein receptors is responsible for the transient rise of [Ca2+]i although the characteristics of the calcium response are dependent on the receptor involved, i.e. the apo B/E (LDL) receptor or the scavenger receptor. We suggest that Ca2+ acts as a second messenger, and that the LDL-induced calcium response is controlled by the proliferative state of the cells.