When the pair steeped human fibroblasts in EGF tagged with radioactive iodine, they found that the amount of radioactivity affixed to the cell's surface peaked after ∼30 to 40 min, and then plummeted (Carpenter and Cohen, 1976). To track the missing radioactivity, Carpenter and Cohen soaked cells in labeled EGF before shifting them to a hormone-free mixture. The hot iodine returned to solution, the researchers discovered, but not as part of EGF. Almost all of it had transformed into monoiodotyrosine and diiodotyrosine, breakdown products of EGF. That finding provided strong circumstantial evidence that after EGF binds to a receptor, cells take in the hormone, chop it up, and eject the fragments.
To bolster that conclusion, the team added antibodies that target EGF to a solution of cells bathed in the hormone. The longer the experiment ran, the fewer antibodies attached to the cells, implying that EGF was vanishing from the plasma membrane. Carpenter and Cohen's results suggested the hormone was ending up in the lysosomes for demolition. When they combined EGF-laden cells with chloroquine, which hinders the organelle's protein-slicing enzymes, the breakdown of EGF slowed. “The key experiment was showing that lysosomal inhibitors prevented degradation [of the hormone],” says Carpenter.
The findings supported the notion that the hormone's receptors are “swallowed” and replaced by fresh proteins—an inference later studies substantiated. The team determined that cells required 10 h to regain their full EGF-binding capacity. But the recovery stagnated if the researchers mixed in molecules that inhibit protein or RNA synthesis. Further work showed that cells absorbed and processed more than just protein hormones. For example, research led by Nobel laureates Michael Brown and Joseph Goldstein demonstrated that cells also engulf low-density lipoproteins and recycle the receptors (Anderson et al., 1976, 1977, 1982). And multiple studies in recent years have emphasized that a lot of signaling occurs even after uptake of receptors into cells.