Altered endocytosis in a mutant of LM fibroblasts defective in cell-cell fusion.

Mutants of LM fibroblasts selected for their decreased ability to undergo polyethylene glycol-induced cell-to-cell fusion (F40 subline) were examined for possible alterations of their ability to carry out endocytosis. Both fluid-phase endocytosis of inulin and horseradish peroxidase and nonreceptor mediated adsorptive endocytosis of poly(L-lysine) were reduced to 60% of control values. Comparable results were obtained when the uptake of poly(L-lysine) was measured as internalization of surface-bound label in label-free medium or following continuous exposure. Accelerated breakdown of internalized label was ruled out as a cause for decreased label accumulation. Accelerated exocytosis is an unlikely cause, and it is suggested that the decreased uptake is due to a decrease in the constitutive membrane vesiculation process that leads to the formation of endocytotic vesicles. The capacity of F40 cells to degrade internalized horseradish peroxidase and poly(L-lysine) was not impaired, nor was their susceptibility to the cytotoxic action of methotrexate-poly(L-lysine). This drug conjugate must be degraded inside cells and release small molecular methotrexate in order to be cytocidal. These data suggest that only the first step of nonspecific endocytosis is impaired, while the subsequent steps that require fusion of endosomes to lysosomes proceed normally. Since the formation of primary endosomes requires membrane fusion through the external aspect of the plasma membrane and in that respect resembles cell-cell fusion, we propose the hypothesis that the observed decrease in endocytosis is related to the decreased ability of F40 cells to fuse with each other, and reflects a decreased efficiency of fusion processes at the external face of the plasma membrane.