Erbstatin (ERB), a specific inhibitor of tyrosine protein kinases (TPK) was
used to examine the effect of TPK inhibition in X-irradiated cells of two
sublines of murine lymphoma L5178Y. These sublines, LY-R and LY-S, differ i
n their sensitivity to ionizing radiation, UV-C radiation and oxidants. Tot
al tyrosine protein kinase (TPK) activity was higher in unirradiated LY-R t
han in LY-S cells, whereas its stimulation by irradiation was maintained lo
nger in LY-S than in LY-R cells. Relative cell number measured 24 h after E
RB treatment and post-treatment cell cycle distribution did not differ betw
een the sublines. Longer growth observations and flow cytometry revealed hi
gher susceptibility of LY-R cells to ERB as compared to LY-S cells. The inh
ibitor induced a marked apoptosis in LY-R cells, but only slight apoptosis
in LY-S cells, whereas ERB-treated and X-irradiated cells exhibited increas
ed sub G1 fraction and DNA degradation pattern typical for apoptosis, when
examined 24 h after treatment. The relative growth rate of LY-R cells subje
cted to combined (5 mu g/ml ERB+X-radiation) treatment was higher than that
of irradiated cells during the first week of post-irradiation incubation.
This protective effect was negligible in the radiosensitive LY-S cells. On
the contrary, with cloning efficiency as the end-point, the effect of combi
ned treatment was additive. We concluded that ERB caused an increase in cel
l death by apoptosis, and this was reflected in the post-irradiation growth
kinetics. However, the altered death mode did not affect the overall letha
l effect of X-rays.