The role of
protein kinase C (PKC) in regulation of the cell cycle and induction of apoptosis was examined in a rat
myeloid leukemia cell line--
chloroma. Exposure of
chloroma cells to
calphostin C (a specific PKC inhibitor) led to inhibition of cell proliferation, caused by (i) partial and transient arrest of cells in the G1 phase of the cell cycle and (ii) induction of apoptosis in part of the cell population. The
calphostin-C-induced inhibition of PKC activity was accompanied by changes in the expression of
proliferating cell nuclear antigen (
PCNA)--a
protein known to participate in regulation of DNA replication and repair. Flow cytometry and western blot analyses of the
PCNA expression showed that, following the
calphostin C treatment, expression of
PCNA declined and attained the lowest value on day 2, followed by recovery to the control level. The decline of the
PCNA expression was accompanied by changes in the molecular weight of the
protein suggesting either its degradation or release of unfinished
protein. The decline/recovery of the
PCNA expression followed the same time-pattern as the cell cycle arrest and apoptosis. Cell apoptosis is often accompanied by cell cycle arrest, therefore, we further examined whether G1 arrest would, by itself, cause apoptosis. Exposure of the
chloroma cells to various doses of
hydroxyurea, a G1 arrest inducing agent, caused cell apoptosis within 24-48 h
after treatment. This suggests that PKC might indirectly be responsible for the
chloroma cell apoptosis induced by
calphostin C. In conclusion, it appears that PKC regulates cell cycle progression by modulating the G1 to S phase transition. One of the possible targets of the PKC effects observed here might be, among others (?),
PCNA--a
protein involved in regulation of both cell proliferation and apoptosis.