Busulfan selectively induces cellular senescence but not apoptosis in WI38 fibroblasts via a p53-independent but extracellular signal-regulated kinase-p38 mitogen-activated protein kinase-dependent mechanism

Busulfan (BU) is a unique alkylating agent that primarily targets slowly proliferating or nonproliferating cells in the body, leading to various normal tissue damage while killing leukemia cells. However, the mechanism(s) of action whereby BU injures normal cells has not been well defined and, therefore, was investigated in the present study by using the normal human diploid WI38 fibroblasts as a model system. We found that WI38 fibroblasts incubated with BU (from 7.5-120 μM) for 24 h underwent senescence but not apoptosis in a dose-independent manner, whereas cells incubated with 80 and 20 μM etoposide (Etop) were committed to apoptosis and senescence, respectively. The induction of WI38 cell senescence by Etop was associated with p53 activation and could be attenuated by down-regulation of p53 using α-pifithrin (α-PFT) or p53 small interference RNA (siRNA). In contrast, WI38 cell senescence induced by BU was associated with prolonged activation of extracellular signal-regulated kinase (Erk), p38 mitogen-activated protein kinase (p38), and c-Jun NH ₂-terminal kinase (JNK) and could be suppressed by the inhibition of Erk and/or p38 with PD98059 (2′-amino-3′-methoxyflavone) and/or SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H- imidazole], respectively. However, inhibition of p53 with α-PFT or p53 siRNA or JNK with SP600125 (1,9-pyrazoloanthrone) failed to protect WI38 cells from BU-induced senescence. These findings suggest that BU is a distinctive chemotherapeutic agent that can selectively induce normal human fibroblast senescence through the Erk and p38 pathways.