PROTEIN EXPRESSION OF THE EPSILON SUBSPECIES OF PROTEIN-KINASE-C CEASES AS SWISS 3T6 FIBROBLASTS INCREASE IN CELL-DENSITY EVEN THOUGH MESSAGE FOR THE PROTEIN IS STILL PRESENT
P. Littlebury et al., PROTEIN EXPRESSION OF THE EPSILON SUBSPECIES OF PROTEIN-KINASE-C CEASES AS SWISS 3T6 FIBROBLASTS INCREASE IN CELL-DENSITY EVEN THOUGH MESSAGE FOR THE PROTEIN IS STILL PRESENT, FEBS letters, 400(3), 1997, pp. 304-308
We have noted previously that growth of C6 glioma cells from low cell
density to confluency and quiescence in serum in accompanied by change
s in protein content of different protein kinase C (PKC) subspecies. H
ere we show that the same occurs as non-contact-inhibiting Swiss 3T6 f
ibroblasts grow to high density in the presence of serum. Protein expr
ession of PKC subspecies alpha and delta increases as the cells increa
se in density while that of PKC-zeta remains the same. Unusually, prot
ein expression of PKC-epsilon is completely down-regulated as cells gr
ow beyond about 50% confluency and no PKC-epsilon protein can be detec
ted in 3T6 fibroblasts at high density by Western blotting. However, m
RNA for PKC-epsilon is expressed at all stages of fibroblast growth as
revealed by RT-PCR. When high-density 3T6 fibroblasts are passaged to
low density in fresh medium, re-expression of PKC-epsilon protein is
observed within 15 min and becomes down-regulated as cells become more
dense. This very rapid synthesis of PKC-epsilon is not blocked by the
transcription inhibitor actinomycin D but is inhibited by cycloheximi
de. PKC-epsilon has some characteristics of a novel 'early response' p
rotein whose synthesis in newly passaged 3T6 cells is regulated at the
translational level.