Role of intracellular calcium and protein kinase C in the endocytosis of transferrin and insulin by HL60 cells

Abstract

The role of the cytosolic free calcium concentration ([Ca2+](i)) and of protein kinase C on the internalization of transferrin and insulin in the human promyelocytic cell line HL60 was investigated. [Ca2+](i) was selectively monitored and manipulated by the use of the fluorescent Ca2+ indicator and buffer quin2, while receptor-ligand internalization was studied directly by quantitative electron microscope autoradiography. Decreasing the [Ca2+](i) up to 10-fold below resting level had no effect on the internalization of transferrin or insulin. Similarly, a 10-fold elevation of the [Ca2+](i) using the calcium ionophore ionomycin caused little or no change in the endocytosis of the two ligands. In contrast, activation of protein kinase C by phorbol myristate acetate markedly stimulated the internalization of both occupied and unoccupied transferrin receptors, even in cells with very low [Ca2+](i). The insulin receptor was found to behave differently in response to phorbol myristate acetate, however, in that only the occupied receptors were stimulated to internalize. We conclude that the [Ca2+](i) plays only a minor role in regulating receptor-mediated endocytosis, whereas internalization depending on receptor type and occupancy.