A comparison of lysosomal involvements in the degradation of normal and abnormal endogenous proteins of differing half-lives in MRC5 cells

The suppression of endogenous protein degradation by fractions of foetal calf serum: dialysed serum is less able to suppress degradation in aged cells

We have studied the effect of various fractions of foetal bovine serum upon the endogenous degradation of long labelled proteins in cultured MRC5 cells, and upon other cellular functions. Only heat-inactivated serum was capable of suppressing protein degradation to a similar extent to complete serum. Acid-treated and delipidized sera were moderately effective. Albumin on its own was able to replace 40 per cent of the effect of serum, indicating the exogenous protein might compete with endogenous protein for degradation in lysosomes. Albumin was not capable of supporting DNA synthesis. Dialysed serum showed an age-related effect suppressing protein degradation to a lesser extent and being less effective in supporting DNA synthesis or cellular proliferation in aged cells. All the effects noted were related to lysosomal protein degradation. Serum diffusate did not suppress protein degradation.

Messenger RNA turnover during bone marrow erythroid cell differentiation

Incubation of bone marrow cells from anaemic rabbits in the presence of actinomycin D led to a decrease in total protein synthesis and an increase in the relative synthesis of globin. This increase in the proportion of globin was observed with in vivo labelling of cellular proteins and in vitro translation of isolated RNA, which indicates that the messenger RNA for globin is much more stable than the other bone marrow cell messages. This was further shown by pulse-labelling the RNA and characterization of the different species by separation on a cDNA-oligo(dT)-cellulose column. Within 12 h after pulse-labelling the relative levels of globin mRNA had risen 10-fold, while a rapid decrease in the level of the poly(A)-rich RNA fraction was observed. Investigations into the mechanisms of this differential stability indicate that the more metabolically active cells from the early stages of erythropoietic development are more susceptible to inhibitors of RNA synthesis such as actinomycin D and alpha-amanitin. A preliminary study using a lysosomal inhibitor, chloroquine, indicates that there appear to be at least two degradative mechanisms, involving a lysosomal and a non-lysosomal pathway, with selective specificity for different messages.

Degradation of peptides and proteins of different sizes by homogenates of human MRC5 lung fibroblasts. Aged cells have a decreased ability to degrade shortened proteins

The degradation of haemoglobin and haemoglobin-derived peptide fragments by homogenates of MRC5 fibroblasts has been investigated. Results show that the smaller fragments were degraded more rapidly than larger substrates at both pH 5.5 and pH 7.5. Only the smallest of the soluble cyanogen bromide peptides (Mr 3500) was degraded at pH 7.5. Degradation at pH 5.5 proceeded more rapidly than that at pH 7.5 for all substrates tested but was more marked with the larger substrates. Homogenates prepared from aged cells degraded puromycin peptides and, to a lesser extent, cyanogen bromide peptides at a slower rate, at pH 7.5, than those prepared from younger cells. We suggest that cytosolic degradation is less selective and at least one cytosolic proteolytic activity decreases as cells age.

Abnormal proteins of shortened length are preferentially degraded in the cytosol of cultured MRC5 fibroblasts

Puromycyl peptides were degraded in MRC5 fibroblasts more rapidly than normal proteins labelled for the corresponding length of time for both long and short labelling periods. The degradation of the puromycyl peptides occurred almost exclusively in the cytosol of the cells. Even when the half-lives of normal and puromycyl peptides were manipulated to be similar, proportionally more of the normal proteins were degraded in the lysosomes. The rapid degradation of the puromycyl peptides was not due to the inhibition of protein synthesis brought about by puromycin but was due to the structure of the substrates themselves. The degree and intracellular site of degradation of puromycyl peptides closely mimic those of abnormal (missense) proteins containing amino acid analogues.