Protein degradation during terminal cytodifferentiation. Studies on mammary gland in organ culture

Abstract

In [rabbit] mammary gland explants subjected to experimental manipulation, average rates (during 24 h periods) of degradation of fatty acid synthase, casein and cytosol-fraction proteins were measured by a double-isotope method. Rates of fatty acid synthase degradation were computed from measurements of changing enzyme amount and rate of synthesis. During the period of most rapid enzyme accumulation there is a transient decrease in the computed fatty acid synthase degradation rate. Removal of hormones [insulin, prolactin, cortisol] produces a rapid increase in the computed rate of degradation of the enzyme. The average fatty acid synthase degradation rate measured by the double-isotope method is low in the presence of hormones and increases on hormone removal. This increase in degradation rate is inhibited by adrenaline [epinephrine] and further blocked by insulin. NH4Cl (10 mM) partially inhibits the increase in protein degradation on hormone removal. The pattern of changes in the average cytosol-fraction protein degradation rate is similar to that for fatty acid synthase alone. There is no relationship between subunit MW and degradation rate under all experimental conditions. Isotope ratios for resolved cytosol protein mixtures are transformed logarithmically to make the standard deviations an estimate of degradation rate heterogeneity. By this analysis, in some conditions there appears to be measurable degradation rate heterogeneity. Little casein degradation is measured in the presence of hormones, but a marked increase in the rate of degradation can be measured when hormones are removed. Whereas at 24-48 h NH4Cl (10mM) has little effect on this enhanced rate of degradation, at 48-72 h it causes a large decrease in degradation rate. Results are discussed in terms of a 2-component degradation system in mammary gland explants.