3.2  Cellular proliferation and survival in DT40 B cells lacking expression of PKD family kinases
PKD enzymes have previously been linked to the regulation of cell proliferation and survival (reviewed in [20]). To investigate the effect that loss of PKD kinases had on B cell survival and/or proliferation we cultured wild-type and PKD-null cells in the presence (PKD1/3−/−: Flag-PKD3+ve) or absence (PKD1/3−/−) of doxycycline and monitored exponential growth. As shown in Fig. 2A, PKD1/3−/− cells proliferated exponentially and re-expression of Flag-PKD3 in these cells had no impact on the rate of proliferation. Furthermore, the viability of PKD1/3−/− B cells during routine culturing was not significantly different from that of wild-type B cells (data not shown). It was noted that the population doubling time of PKD1/3−/− cells was slightly slower than that of wild type DT40 cells (12.7 ± 2.8 h versus 10.2 ± 0.4 h) but the failure of PKD3 re-expression to modify the proliferation rate of PKD1/3−/− cells suggests that these small differences were most likely the result of clonal variation and were not caused specifically by loss of PKD enzymes. Thus, PKD family enzymes are not essential for regulating basal survival and proliferation of DT40 B cells.
PKD enzymes, specifically PKD1 and PKD2, have previously been linked to a protective role against oxidative stress-induced injury in 3T3 fibroblast, HeLa and epithelial cell lines [17,30–32]. We therefore addressed the role of PKD family kinases in regulating B cell survival in response to oxidative stress and other stress stimuli. As shown in Fig. 2B, loss of PKD1/3 expression had no significant impact on the survival of DT40 B cells in response to mitochondrial stress stimuli (H2O2 or serum deprivation); DNA damaging agents (etoposide or doxorubicin); ER pathway stress due to calcium overload (thapsigargin) or following prolonged treatment with phorbol esters or Trichostatin A, an inhibitor of class I/II HDACs. Thus, PKD kinases do not play an essential role in regulating B cell survival in response to a range of different stress stimuli.