Stimulation of the proliferation of the madin‐darby canine kidney (MDCK) epithelial cell line by high‐density lipoproteins and their induction of 3‐hydroxy‐3‐methylglutaryl coenzyme a reductase activity

Abstract

MDCK Cells seeded on extracellular matrix- (ECM-) coated dishes and exposed to medium supplemented with high density lipoproteins (HDLs, 750 μg protein/ml) and transferrin (10 μg/ml) have a proliferative rate, final cell density, and morphological appearance similar to those of cells grown in serum-supplemented medium. The mitogenic stimulus provided by HDLs is not limited by the initial cell density at which cultures are seeded, nor is it limited in time, since cells grown in medium supplemented with transferrin and HDLs grew for at least 50 generations. The presence of HDLs in the medium is required in order for cells to survive, since cells actively proliferating in the presence of medium supplemented with HDLs and transferrin begin to die within 2 days after being transferred to medium supplemented only with transferrin. Low-density lipoprotein (LDL) is mitogenic for MDCK cells when present at low concentrations (from 2.5 to 100 μg protein/ml). Above 100 μg protein/ml, LDL is cytotoxic and therefore cannot support cell proliferation at an optimal rate. The mitogenic effect of HDLs is also observed when cells are maintained on fibronectin-coated dishes. However, the proliferative rate of the cells is suboptimal and cultures cannot be passaged on this substrate indefinitely, as they can be on ECM-coated dishes. A close association between the ability of HDLs to support cell proliferation and their ability to induce the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase is observed. HMG CoA reductase activity is 18 times higher (70 pmoles/min /10⁶ cells) in proliferating cells than in confluent, nondividing cells (4 pmoles/min /10⁶ cells). The HMG CoA reductase activity of sparse cells is more sensitive to induction by HDLs (eight-fold higher than control cells) than is the enzyme activity of confluent cells (twofold higher than control levels). The dose-response relationships between the abilities of HDLs to support proliferation and to induce HMG CoA reductase activity are similar. The time course of the stimulation of proliferation and the increase in enzyme activity of sparse, quiescent cells after exposure to HDLs are parallel. The HMG CoA reductase activity of sparse MDCK cells is induced six-fold by exposure to compactin, a competitive inhibitor of HMG CoA reductase. This induction of HMG CoA reductase is prevented by mevalonic acid, not affected by LDL, and synergistically enhanced by simultaneous exposure to HDLs. HDLs effect a rescue from the cytotoxic effect of compactin, whereas LDL does not. More specifically, cells proliferating in the presence of HDLs are 100 times more resistant to the toxic effects of compactin than are cells exposed to LDL. These results taken together suggest that the induction of HMG CoA reductase activity by HDLs may play a role in mediating the proliferative effect of HDLs. The significance of the increased mevalonate made available by higher levels of HMG CoA reductase appears not to lie in the bulk provision of cellular cholesterol, but rather in the provision of a specific pool of endogenously synthesized sterol, or in one or more of the nonsterol products of mevalonate. In cells that proliferate in response to HDLs, the induction of HMG CoA reductase activity appears to be a consistent and essential feature of a possibly pleiotypic metabolic response to HDLs.