A. Lopezgirona et al., ADDITION OF CALMODULIN ANTAGONISTS TO NRK CELLS DURING G1 INHIBITS PROLIFERATING CELL NUCLEAR ANTIGEN EXPRESSION, Cell calcium, 18(1), 1995, pp. 30-40
The mRNAs of most proteins involved in DNA synthesis show an S phase c
orrelated expression when mammalian cells are stimulated to proliferat
e from G0. This is the case for proliferating cell nuclear antigen (PC
NA), a cofactor of DNA polymerase delta that is essential for the synt
hesis of the leading and tagging strands of DNA. Normal rat kidney cel
ls re-entering the cell cycle from quiescence start DNA synthesis at 1
2 h and reach a maximum at 20 h. The expression of PCNA parallels the
synthesis of DNA, Progression through the S phase was inhibited by add
ition of the anticalmodulin drug W13 to the cells during G1, 5 h after
activation. W13 also inhibited the increase in both PCNA protein and
mRNA indicating that calmodulin regulates its expression, Using TK(-)t
s13 cells transfected with a plasmid containing the thymidine kinase g
ene under the control of the human 2.8 kb PCNA promoter, we demonstrat
ed that this promoter is not regulated by calmodulin. The half-life of
PCNA mRNA during G1/S transition was not modified by the treatment wi
th W13, indicating that the decrease in the mRMA found when calmodulin
was inhibited is not due to changes in its stability. Run-on assays r
evealed that control cells produced predominantly complete PCNA transc
ripts during S phase, while short incomplete transcripts were generate
d in W13-treated cells at the same time. These results indicate that c
almodulin participates in a more direct or indirect way during G1 in t
he activation of PCNA expression, From data presented here it can be s
uggested that calmodulin activates the release of a transcriptional bl
ock reading to an increase in the amount of PCNA during S phase.