| Popis: |
Mitogen regulated protein (MRP; also called proliferin) is a 34 IcDa glycoprotein secreted during murine gestation. Its temporal profile of synthesis and secretion and its homology to prolactin indicate an hormonal role in fetal development. To identify its target tissues in 125 vivo, we injected tracer amounts of I-labeled MRP into nonpregnant mice and mice at various days of gestation. At mid-gestation the clearance profile was biphasic and provided evidence for two half-lives for MRP of 0.6 hour and about 8 hours. Both half-lives of MRP were significantly shorter than the half-life of 10 hours measured for serum albumin. Our data show that mammary gland and uterus take up and degrade MRP whereas MRP is taken up but not degraded by liver, heart, spleen, and kidney. MRP degradation in the mammary gland and uterus is reduced by prior treatment with chloroquine which inhibits lysosomal-mediated degradation. We propose that MRP is cleared from the blood by receptormediated endocytosis with subsequent degradation by mammary gland and uterus. Because degradation by their target cells is a fate of many hormones and growth factors, we suggest that mammary gland and uterus are also target tissues for MRP action. |
