Porcine smooth muscle cell-conditioned medium stimulates LDL receptor activity in Hep G2 cells

Paracrine factors may modulate low density lipoprotein (LDL) receptor activity in hepatocytes. To study this the effect of conditioned medium prepared from a range of cell types on the binding and internalisation of 125I-LDL in Hep G2 cells was studied. Seven of the fourteen conditioned media tested, including those from P388D1, U937, porcine smooth muscle (Pc SMC) Swiss 3T3, STO, = 48 and MDCK cells, were found to increase the binding and internalisation of 125I-LDL at 37 degrees C by Hep G2 cells (P < 0.01). The largest increase in LDL receptor activity was produced by conditioned medium from Pc SMC cells and was, therefore, selected for further analysis. The Pc SMC-conditioned medium increased LDL receptor number in Hep G2 cells by three-fold but had no effect on LDL receptor activity in human skin fibroblasts. DNA synthesis and cholesterol synthesis by Hep G2 cells were inhibited by Pc SMC-conditioned medium. Preliminary characterisation of the Pc SMC-derived factor(s) suggests that it is a protein(s) of low relative molecular mass.