Role of autocrine motility factor in a 12-lipoxygenase dependent anti-apoptotic pathway.

Abstract

Apoptosis or programmed cell death, plays an indispensable role in embryonic development, maturation of the immune system and maintenance of tissue and organ homeostasis (1–2). A wide spectrum of molecular entities including oncogenes, tumor suppressor genes, signal transducers, cell cycle proteins, free radicals, cations and proteases have been implicated in apoptosis. Recent work from our laboratory (3) demonstrated that 12-lipoxygenase (12-LOX) may play an essential role in regulating apoptosis in rat Walker carcinosarcoma (W256) cells. Down regulation of 12-LOX gene expression by antisense oligonucleotides triggered time and dose dependent apoptosis. The W256 cell death, triggered by antisense oligos could be partially blocked by exogenous 12(S)-HETE. The antisense oligo treatment also resulted in a significant decrease in the ratio of Bcl-2/Bax proteins which are critical determinants of apoptosis. As expected, over expression of Bcl-2 protein provided partial death sparing effects (3). It also was demonstrated that exogenous arachidonic acid (AA) protected against apoptosis (4). Exogenous AA in a time and dose dependent fashion suppressed NDGA induced W256 apoptosis as well as DNA fragmentation. Serum withdrawal also caused W256 cells to undergo typical apoptosis which was not rescued by several growth factors commonly found in serum. However, exogenous AA suppressed serum starvation induced W256 cell apoptosis (4). Further studies demonstrated that the factor responsible for AA induced protection against apoptosis was the 12-LOX metabolite of AA 12(S)-HETE. 12-LOX has been demonstrated by our laboratory to be expressed in numerous tumor cells from human, rat and mouse (5). Factors controlling the regulation of 12-LOX are largely unknown. Several growth factors have been shown to stimulate 12-LOX activity in tumor cells. For example, in A431, human epidermoid carcinoma cells platelet type 12-LOX is expressed (6). Serum starvation induces a time and dose dependent reduction in the expression of 12-LOX protein (8). Another cytokine which has been demonstrated to regulate 12-LOX levels in tumor cells is autocrine motility factor (AMF). AMF stimulates cell motility and invasion via receptor mediated signaling pathways. AMF is also a potent growth factor. Our laboratory demonstrated that AMF induced cell motility was mediated by the lipoxygenase generation of 12(S)-HETE (7). In a subsequent report, we also demonstrated that AMF was able to increase the cellular protein levels of 12-LOX in high but not in low metastatic clones (8).