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Acetylcholine receptor: effects of proteolysis on receptor metabolism
Abstract:
Previous studies (Miskin, R., T. G. Easton, and E. Reich, 1970, Cell. 15:1301-1312) have shown that sarcoma virus transformation and tumor promoters reduced the cell surface concentration of acetylcholine receptors (AChR) in differentiating chick embryo myogenic cultures. Both of these agents also induced high rates of plasminogen activator (PA) synthesis in myogenic cultures (Miskin, R., T. G. Easton, A. Maelicke, and E. Reich, 1978, Cell. 15:1287-1300), and the present work was performed to establish whether proteolysis might significantly affect receptor metabolism. Proteolysis in myogenic cultures was modulated by one or more of the following: stimulation of PA synthesis, direct addition of plasmin, removal of plasminogen, or addition of plasmin inhibitors. The results were: (a) When the rates of proteolysis were raised either by addition of plasmin or by stimulating PA synthesis in the presence of plasminogen, both the steady-state concentration and the half-life of surface AChR decreased, but the rate of receptor synthesis was unaffected. (b) The magnitude of these effects, and their dependence on added plasminogen, indicated that proteolysis initiated by plasminogen activation could account almost entirely for the reduction in receptor half-life produced by sarcoma virus transformation and phorbol ester. (c) The rate of receptor synthesis, which is also reduced by viral transformation and tumor promoters, was not modified by proteolysis; hence plasmin action may be responsible for a large part, but not all of the change in surface receptor under these conditions. (d) The plasmin catalysed changes in receptor parameters appear to occur in response to modified membrane metabolism resulting from proteolysis of surface components other than AChR itself.
Insights
Proteolysis, triggered by plasminogen activation, significantly reduces acetylcholine receptors (AChR) on cell surfaces. This proteolysis explains most of the receptor loss seen with sarcoma virus and tumor promoters.
Area of Science:
- Cell Biology
- Biochemistry
- Molecular Biology
Background:
- Sarcoma virus transformation and tumor promoters decrease cell surface acetylcholine receptors (AChR) in myogenic cultures.
- These agents also increase plasminogen activator (PA) synthesis, suggesting a role for proteolysis in receptor regulation.
Purpose of the Study:
- To investigate whether proteolysis significantly affects acetylcholine receptor (AChR) metabolism.
- To determine the extent to which proteolysis contributes to AChR reduction induced by viral transformation and tumor promoters.
Main Methods:
- Modulating proteolysis in myogenic cultures via PA synthesis stimulation, direct plasmin addition, plasminogen removal, or plasmin inhibitor addition.
- Measuring steady-state concentration, half-life, and synthesis rate of surface AChR under different proteolysis conditions.
Main Results:
- Increased proteolysis (via plasmin or PA synthesis) decreased AChR concentration and half-life, without affecting synthesis rate.
- Proteolysis initiated by plasminogen activation largely accounted for the AChR half-life reduction caused by sarcoma virus and phorbol ester.
- Proteolysis did not alter AChR synthesis rate, indicating it's responsible for only part of the receptor changes induced by viral transformation and tumor promoters.
- Plasmin-catalyzed changes in AChR parameters likely result from proteolysis of other surface components, altering membrane metabolism.
Conclusions:
- Proteolysis, particularly plasmin-mediated, plays a significant role in the reduction of cell surface acetylcholine receptors (AChR).
- Plasminogen activation and subsequent proteolysis are key mechanisms underlying AChR loss induced by oncogenic transformation and tumor promoters.
- Altered membrane metabolism due to proteolysis of non-AChR surface components contributes to changes in AChR levels.