6-(1-Hydroxyimino-4-methylpentyl)5,8-dimethyoxy 1,4-naphthoquinone S-52 (DMNQ S-52) was reported to have cytotoxic activity against L1210 leukemia cells. In the present study, we investigated the apoptotic mechanism of DMNQ S-52 in vitro and in vivo in murine solid cancer cells. DMNQ S-52 exerted cytotoxicity against Lewis lung carcinoma (LLC) cells (IC50=12.3 microM). DMNQ S-52 increased Annexin V positive cell population in a concentration-dependent manner. DMNQ S-52 also induced apoptosis through caspase-mediated pathway, including activation of caspase-3, cleavage of Poly(ADP-ribose) polymerase (PARP) and decreased expression of Bcl-2 in LLC cells in a time and concentration-dependent fashion. DMNQ S-52 activated the phosphorylation of c-Jun N-terminal kinase (JNK) and p38 as well as abrogated the expression of extracellular signal-regulated kinase (ERK) in a time-dependent manner at 10 microM. Similarly, cell proliferation inhibition by DMNQ S-52 was masked by caspase inhibitor Z-Asp-Glu-Val-Asp-fluoromethylketone (Z-VAD-FMK), JNK inhibitor SP600125 and p38 inhibitor SB203580, but not by MEK inhibitor U0126. Furthermore, i.p. administration of DMNQ S-52 at 5 mg/kg resulted in a potent inhibition of the growth of LLC cells implanted on the right flank of C57BL/6 mice compared to untreated control. Immunohistochemical analysis revealed the decreased tumor cell proliferation and increased tumor cell apoptosis in DMNQ S-52 treated tumor sections using terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) and proliferation cell nuclear antigen (PCNA). Taken together, these findings demonstrate that DMNQ S-52 may exhibit anti-tumor activity by inducing apoptosis via caspases and mitogen activated protein (MAP) kinase-dependent pathways.