The senescence marker protein (SMP-2) of the rat liver: purification, immunochemical characterization and age-dependent regulation

In vitro translation of total rat hepatic mRNAs has identified a 31 kilodalton senescence marker protein (SMP-2) which is present in higher amounts in prepubertal and senescent males than in the post-pubertal adult male (more than 10-fold). SMP-2 is an androgen-repressible protein. The negative regulation of the SMP-2 gene activity by androgen accounts for its increased expression during the androgen insensitive states of the prepubertal and senescent livers, and its constitutive expression in the female liver. A combination of separation procedures including salt fractionation, chromatofocusing, ion-exchange chromatography and preparative gel electrophoresis have led to the purification of SMP-2 to apparent homogeneity. The purified protein showed the same electrophoretic mobility as the sex- and age-specific in vitro translation product of hepatic mRNAs. The polyclonal antibody to SMP-2 was produced in the rabbit. The antibody selectively reacted with the 31 kDa sex- and age-specific translation product of hepatic mRNAs. Western blot analysis of the liver cytosol confirms monospecificity of the antiserum, as well as age- and sex-dependent changes in the tissue level of SMP-2. Histochemical staining of liver sections with the antiserum reveals a preferential periportal localization of SMP-2 in the hepatocytes. This finding is in marked contrast to the androgen-inducible alpha 2u globulin which is preferentially synthesized and localized in the pericentral hepatocytes. Thus, the zonal distribution of SMP-2 correlates with polarized androgen sensitivity of the hepatocytes within the liver lobule.