We discovered that LZAP binds the alternate reading frame protein ARF (p14ARF in humans, p19ARF in mice) and described LZAP as an activator of p53, both dependent and independent of ARF. We also described LZAP inhibition of RelA (NF-?B) and that loss of LZAP accelerates tumor cell invasion, as well as xenograft tumor growth and angiogenesis. Mechanistically, we found that LZAP decreases RelA phosphorylation and inhibits NF-?B transcription. Other groups have shown that LZAP accelerates cell death in response to chemotherapeutic agents and alters the G2/M cell cycle checkpoint through inhibition of Chk1 and Chk2. The potential tumor suppressor activities of LZAP have continued to emerge with our description that LZAP expression is lost in 30% of human HNSCC and loss of LZAP is associated with increased expression of select NF-?B targets. Human tumor and xenograft mouse tumor data, as well as, LZAP activities as an activator of p53 and suppressor of RelA suggest that LZAP may function as a tumor suppressor;however, validation of LZAP tumor suppressor status has been lacking. We now have preliminary data that mice with heterozygous loss of LZAP develop lung tumors at higher multiplicity and incidence compared to wild-type littermates. The number of cancer-centric proteins with seemingly unrelated activities that LZAP regulates is remarkable (e.g. p53, MDM2, RelA, Chk1, Chk2, p38MAPK). We noted that without exception, all of these LZAP targets are also targets of the wild-type p53-induced phosphatase 1 (Wip1). Wip1 was originally described as a p53- transcriptional target, but its oncogenic activity as a potent inhibitor of p53 and p38MAPK (p38) was soon recognized. More recently, Wip1 inhibition of RelA suggests that Wip1 may also have tumor suppressive activities. Wip1 activity has been reported to be controlled at the level of expression, but our recently published data show that LZAP regulates Wip1 ability to bind and inhibit one target (p38). Because both p53 and RelA are regulated by LZAP and serve critical roles in lung tumorigenesis, we will leverage our LZAP murine model of lung cancer to determine if p53 and RelA are critical for tumor development in these mice. Given that LZAP regulates Wip1 activity toward p38, we will determine if LZAP also regulates Wip1 activity toward other Wip1 substrates (e.g. p53, MDM2, RelA, Chk1, Chk2) and in doing so will develop a more comprehensive understanding of LZAP activity. In related, but not dependent, studies we will determine if all or a portion of LZAP tumor suppressive activities depend on Wip1 and if Wip1 regulates LZAP phosphorylation, ubiquitination and expression. These innovative and mechanistic studies will significantly advance the LZAP field by establishing LZAP as a tumor suppressor, by identification of LZAP as the first regulator of the oncogenic phosphatase Wip1, by determining if p53 and RelA are critical targets of LZAP for murine tumorigenesis, and by determining the role of Wip1 as a major effector and potentially as a regulator of LZAP activities.
LZAP induces cell cycle arrest, inhibits apoptosis and invasion, and expression of LZAP is lost in 30% of human head &neck squamous cell carcinomas and in a murine lung tumor model. LZAP regulates many tumor-centric proteins (p53, MDM2, RelA, Chk1, Chk2, p38MAPK), and now, we have data suggesting that LZAP activity is at least partially dependent the oncogenic phosphatase Wip1. Based on our preliminary data and using murine models and LZAP-specific reagents, we will explore mechanisms of LZAP action which may lead to new strategies to target tumors dependent on loss of LZAP expression.
