Identification of sterol acceptors that stimulate cholesterol efflux from human spermatozoa during in vitro capacitation.
The nature of cholesterol-binding proteins acting upon human spermatozoa during in vitro capacitation was determined by measuring the efflux of [3H]cholesterol and of [3H]cholesteryl sulfate from labeled spermatozoa. Efflux of [3H]sterols was stimulated when the labeled gametes were incubated in Ham's F-10 medium supplemented with female serum or follicular fluid. Upon centrifugation of capacitated spermatozoa and application of the supernatant to density-gradient ultracentrifugation for lipoprotein analysis, both [3H]cholesterol and [3H]cholesteryl sulfate were found to be carried by very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), high-density lipoproteins (HDL), as well as the albumin fraction (d greater than 1.21) in serum. When the capacitation medium was supplemented with follicular fluid, the [3H]sterols were bound to HDL's and to the albumin fraction; when the latter fraction was analysed by molecular sieve chromatography, 60-70% of the radioactivity eluted in fractions with a mean molecular weight corresponding to that of human serum albumin. Sperm cholesterol efflux was also stimulated when serum or follicular fluid was added to a simplified medium (50 mM Tris-HCl, 0.56% NaCl, pH 7.8); efflux of [3H]cholesterol from labeled gametes progressed in a time-dependent manner, but was low in the absence of serum components. The [3H]cholesterol/cholesterol ratios were higher in the albumin and HDL fractions, indicating some degree of specificity of these sterol acceptors. It was observed that follicular fluid albumin has a [3H]sterol binding capacity that is 2-3-fold higher than that of serum albumin. Commercial human serum albumin also promoted sperm cholesterol efflux. These results provide new information concerning those components of follicular fluid which may play a role in human sperm capacitation and provide further support for the concept that loss of cholesterol from the sperm plasma membrane is an important component of the capacitation process.
Langlais
J
J
Laboratoire d'Ontogénie et Reproduction, Centre Hospitalier de l'Université Laval, Québec, Canada.
Kan
F W
FW
Granger
L
L
Raymond
L
L
Bleau
G
G
Roberts
K D
KD
D000177:Acrosome / Q000502:physiology; D001826:Body Fluids / Q000502:physiology*; D002352:Carrier Proteins / Q000378:metabolism*; D002784:Cholesterol / Q000378:metabolism*; D005260:Female; D006801:Humans; D008074:Lipoproteins / Q000378:metabolism*; D008297:Male; D006080:Ovarian Follicle / Q000502:physiology*; D010196:Pancreatic Elastase; D013075:Sperm Capacitation; D013094:Spermatozoa / Q000378:metabolism*
D002352:Carrier Proteins; D008074:Lipoproteins; D002784:Cholesterol; D010196:Pancreatic Elastase; C017130:cholesterol-binding protein