Stimulation of protein degradation by low pH in L6G8C5 skeletal muscle cells is independent of apoptosis but dependent on differentiation state.
BACKGROUND
In chronic renal failure, metabolic acidosis increases protein degradation (PD) in skeletal muscle, an effect which in vivo requires glucocorticoid (GC). This disorder is poorly understood, but can be studied in vitro using L6G8C5 rat skeletal muscle cells. Two potential confounding factors in studies of PD in culture are apoptosis and dedifferentiation, both of which resemble catabolic states. The aim of this study was to determine the extent to which these factors contribute to the observed effects of acid and GC on PD.


METHODS
PD was measured in intact cells by pre-labelling cell protein with [(14)C]phenylalanine. Apoptosis was assessed morphologically by staining DNA with Hoechst 33342, by terminal deoxynucleotide transferase-mediated nick-end labelling and by cell-surface binding of Annexin V. Differentiation was assessed morphologically from myotube fusion and from activity of the marker enzyme creatine phosphokinase (CPK).


RESULTS
In undifferentiated myoblasts, pH had no detectable effect on apoptosis provided that serum was present and GC (dexamethasone; 5 nmol/l) decreased apoptosis. In spontaneously fused cultures in 2% serum, inhibition of apoptosis with caspase-3 inhibitor (C3I; Ac-Asp-Met-Gln-Asp-CHO; 50 micro mol/l) only decreased PD by 9% at pH 7.4. In contrast, the proteasome inhibitor MG132 decreased PD by 79%. Acid (pH 7.1) increased PD, with no requirement for GC, and this effect was blocked by MG132, but not by C3I. Differentiation was unaffected by 1-4 days of exposure to acid or GC. However, differentiation to myotubes led to decreased sensitivity of PD to acid. This effect of acid was lost completely in highly fused myotubes, but was partly restored by 500 nmol/l dexamethasone.


CONCLUSIONS
Stimulation of PD in these cells by acid and GC is not an artefact of apoptosis or dedifferentiation, but differentiation state does determine whether PD responds spontaneously to acid or (as in vivo) only does so in the presence of GC.
Pickering
Warren
W
Department of Nephrology, Leicester General Hospital, Leicester, UK.
Cheng
Mai-Kim
MK
Brown
Jeremy
J
Butler
Heather
H
Walls
John
J
Bevington
Alan
A
D000138:Acidosis / Q000378:metabolism*; D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D002454:Cell Differentiation; D002478:Cells, Cultured; D003402:Creatine Kinase / Q000378:metabolism; D016895:Culture Media, Serum-Free; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D006727:Hormone Antagonists / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007976:Leupeptins / Q000494:pharmacology; D015735:Mifepristone / Q000494:pharmacology; D018482:Muscle, Skeletal / Q000166:cytology*; D051381:Rats; D011965:Receptors, Glucocorticoid / Q000037:antagonists & inhibitors; D013148:Spironolactone / Q000494:pharmacology
D016895:Culture Media, Serum-Free; D015853:Cysteine Proteinase Inhibitors; D006727:Hormone Antagonists; D007976:Leupeptins; D011965:Receptors, Glucocorticoid; D013148:Spironolactone; D015735:Mifepristone; D003402:Creatine Kinase; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde