Ultrastructural and biochemical evidence for the presence of mature steroidogenic acute regulatory protein (StAR) in the cytoplasm of human luteal cells.
The distribution of the steroidogenic acute regulatory protein (StAR) inside thecal and granulosa-lutein cells of human corpus luteum (CL) was assessed by immunoelectron microscopy. We found greater levels of StAR immunolabeling in steroidogenic cells from early- and mid-than in late luteal phase CL and lower levels in cells from women treated with a GnRH antagonist in the mid-luteal phase. Immunoelectron microscopy revealed significant levels of StAR antigen in the mitochondria and in the cytoplasm of luteal cells. The 30 kDa mature StAR protein was present in both mitochondria and cytosol (post-mitochondrial) fractions from homogenates of CL at different ages, whereas cytochrome c and mitochondrial HSP70 were detected only in the mitochondrial fraction. Therefore, we hypothesized that either appreciable processing of StAR 37 kDa pre-protein occurs outside the mitochondria, or mature StAR protein is selectively released into the cytoplasm after mitochondrial processing. The presence of mature StAR in the cytoplasm is consonant with the notion that StAR acts on the outer mitochondrial membrane to effect sterol import, and that StAR may interact with other cytoplasmic proteins involved in cholesterol metabolism, including hormone sensitive lipase.
Sierralta
Walter D
WD
Laboratorio de Estructuras, INTA-Universidad de Chile, Macul 5540, PO Box 138, Santiago 11, Chile. wsierra@inta.cl
Kohen
Paulina
P
Castro
Olga
O
Muñoz
Alex
A
Strauss
Jerome F
JF
Devoto
Luigi
L
D015153:Blotting, Western; D002352:Carrier Proteins / Q000378:metabolism; D045304:Cytochromes c / Q000378:metabolism; D003593:Cytoplasm / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors / Q000494:pharmacology; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D006801:Humans; D008184:Luteal Cells / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D008183:Luteal Phase; D008565:Membrane Proteins / Q000378:metabolism; D016253:Microscopy, Immunoelectron; D008928:Mitochondria / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism* / Q000648:ultrastructure; D000098682:Steroidogenic Acute Regulatory Protein
D002352:Carrier Proteins; D018840:HSP70 Heat-Shock Proteins; D008565:Membrane Proteins; D010750:Phosphoproteins; C105684:STARD3 protein, human; D000098682:Steroidogenic Acute Regulatory Protein; D007987:Gonadotropin-Releasing Hormone; D045304:Cytochromes c; C062876:cetrorelix