MuRF-1 and Atrogin-1 Protein Expression and Quadriceps Fiber Size and Muscle Mass in Stable Patients with COPD

Figures & data

Table 1.  Clinical characteristics of COPD patients and controls

	COPD (n = 32)	Controls (n = 15)	p-value	Low FFMI (n = 16)	Normal FFMI (n = 16)	p- value
Age (yr)	65(8)	68(8)	0.19	64(8)	66(8)	0.60
Gender (% male)	57%	53%	1.0	44	71	0.16
Smoking history (pack-years)	42(29,75)	2(0,8)	<0.0001	38(23,55)	48(35,75)	<0.0001
Smoking status (% current: ex)	7:93	0:53	1.0	6:94	7:93	1.00
% on oral glucocorticoids	3%	0%	0.54	13	7	1.00
FEV1 (L)	0.92(0.64,1.29)	2.92(2.49,3.16)	<0.0001	0.77(0.58,1.13)	1.00(0.68,1.70)	0.10
FEV1 (% predicted)	35(25,51)	111(101,122)	<0.0001	30(25,41)	46(24,60)	0.24
TLCO (% predicted)	39(16)	93(16)	<0.0001	32(14)	46(16)	0.02
PaO2 (kPa)	9.2(1.1)	10.9(1.6)	<0.0001	9.3(1.1)	9.1(1.2)	0.65
PaCO2 (kPa)	5.3(0.5)	5.2(0.5)	0.44	5.3(0.6)	5.4(0.5)	0.81
Body Mass Index (kg/m^2)	23.2(3.9)	25.6(4.6)	0.09	20.6(2.6)	26.2(2.9)	<0.0001
Fat-free mass(kg)	44(9)	47(9)	0.33	39(6)	49(8)	<0.0001
Fat-free mass index (kg/m^2)	15.7(2.0)	17.0(2.1)	0.02	14.1(1.0)	17.0(1.6)	<0.0001
Quadriceps MVC (kg)	27(10)	36(11)	0.01	22(8)	33(8)	0.001
Quadriceps twitch force (kg)	7.5(3.1)	9.4(3.2)	0.08	6.5(3.1)	8.6(4.1)	0.07
Locomotion time (min/12h)	38(22,55)	96(61,128)	<0.0001	30(23,41)	41(17,66)	0.19
6-minute walk distance (m)	391(137)	616(97)	<0.0001	384(140)	398(139)	0.79
Peak VO2 (ml/kg/min)	12.6(9.6,14.6)	21.1(17.3,26.9)	<0.0001	11.1(9.4,14.1)	13.6(9.7,15.1)	0.27

Results are mean(standard deviation), compared using the t test, or median(25^th percentile, 75^th percentile), compared using the Mann–Whitney U-test. Abbreviations: COPD Chronic Obstructive Pulmonary Disease, FEV₁ = Forced Expiratory Volume in 1 second, TL_CO = carbon monoxide diffusing capacity, PaO₂ = partial pressure of oxygen in arterial blood, PaCO₂ = partial pressure of carbon dioxide in arterial blood, MVC = Maximal Voluntary Contraction, VO₂ = oxygen consumption during maximal incremental cycle ergometry.

Table 2.  Quadriceps muscle fiber cross-sectional areas in COPD patients and controls

	COPD (n = 32)	Controls (n = 15)	p-value	COPD low FFMI (n = 16)	COPD normal FFMI (n = 16)	p-value
Type I fiber CSA (μm^2)	5020(3670,6060)	5130(4430,5850)	0.35	4650(3630,7230)	5070(3670,5720)	1.00
Type I/IIa fiber CSA (μm^2)	5390(3260,5860)	4770(4410,6000)	0.84	4690(2640,5740)	5470(5060,7640)	0.16
Type IIa fiber CSA (μm^2)	4030(2920,4750)	4230(30505350)	0.35	3520(2220,4850)	4130(3670,4750)	0.26
Type IIx fiber CSA (μm^2)	2490(1830,3240)	4790(2930,6190)	0.003	2550(1650,3370)	2490(1930,3920)	0.59
Proportion of type I fibers (%)	28(19)	58(14)	<0.0001	24(19)	32(19)	0.28
Proportion of type I/IIa fibers (%)	4(1,6)	2(0,6)	0.19	5(0,7)	3(0,5)	0.23
Proportion of type IIa fibers (%)	62(17)	37(14)	<0.0001	65(18)	59(14)	0.28
Proportion of type IIx fibers (%)	4(1,8)	0(0,4)	0.009	4(1,8)	4(2,9)	0.61

Results are mean(standard deviation), compared using the t test, or median(25^th percentile, 75^th percentile), compared using the Mann–Whitney U-test. Abbreviations: COPD Chronic Obstructive Pulmonary Disease, CSA: cross-sectional area, FFMI: fat-free mass index Muscle fiber proportions do not add up exactly to a total of 100% as these are median values.

Figure 1.  Box plots of atrogin-1 and MuRF-1 protein levels in COPD patients with and without a low FFMI and healthy age-matched controls. Atrogin-1 protein levels in quadriceps muscle were lower in COPD patients than controls [0.64(0.31,1.24)AU vs 1.41(0.68,2.06)AU, p = 0.03, A] but not significantly different in COPD patients with a reduced FFMI compared to patients with a normal [0.73(0.34,1.61)AU vs 0.74(0.12,1.34)AU, p = 0.46, A]. There was a subset of COPD patients with relatively high protein levels of these mediators compared to the controls but these patients were not confined to the low FFMI group. MuRF-1 protein levels were not significantly different in quadriceps muscle from COPD patients compared to controls [0.56(0.45,1.30)AU vs 0.92(0.74,1.50)AU, p = 0.12, B], nor between COPD patients with and without a low FFMI [0.55(0.42,0.92)AU vs 0.59(0.54,1.62)AU, p = 0.35, B].

Figure 2.  Representative images of western blots for atrogin-1 and MuRF-1 protein in COPD patients and controls. The atrogin-1 band and MuRF-1 bands were seen on separate blots just below the 50 kDA marker. The atrogin bands are denser in the control samples than the COPD samples, with considerable variability between patients, while the MuRF-1 bands show no consistent density difference between COPD samples and controls.