LZAP: A break on phosphorylation

KEYWORDS:

• LZAP
• CDK5RAP3
• WIP1

This article refers to:

LZAP is a novel Wip1 binding partner and positive regulator of its phosphatase activity in vitro

LZAP (LXXLL/Leucine zipper-containing ARF-binding protein) is an evolutionally conserved protein that exhibits controversial functions in human cancers. It does not have an enzymatic domain or other well-characterized functional motifs. Hence, the roles of LZAP in carcinogenesis remain obscure. LZAP was first identified as a protein associated with the regulatory subunit of CDK5. Thus, it was also named as CDK5RAP3.¹ The Yarbrough group identified LZAP in a yeast 2-hybrid screening using human ARF (encoded by CDKN2A) as the bait.² This study further showed that LZAP promoted p53 stability and transcriptional activity. However, a more recent study using hepatocellular carcinoma (HCC) cells suggested that LZAP reduced ARF expression but did not modulate p53. Additional evidence emerges indicating that the activities of LZAP may vary in different cancer types. LZAP was downregulated in head and neck squamous cell carcinomas (HNSCCs)³ and gastric cancers⁴ relative to adjacent normal tissues. In these cancers, knockdown of LZAP impaired xenograft tumor growth, underscoring a tumor suppressor-like role. In contrast, LZAP appears to be overexpressed in lung adenocarcinoma. Perhaps the most puzzling observations came from HCC. The Ching group showed that LZAP was overexpressed in HCC and promoted metastasis,⁵ whereas Zhao and colleagues found that LZAP expression was reduced in HCC and correlated with favorable prognosis.⁶ Both studies performed immunohistological staining of LZAP in tens of patients, although different antibodies were used. Additional studies with rigorous antibody validation are necessary to clarify this controversy. Based on the Cancer Genome Atlas data, HNSCC has the second lowest levels of LZAP mRNA among all cancer types, while HCC is at the high end. Collectively, these results suggest that LZAP likely executes either tumor promoting or suppressive functions in a context-dependent manner. However, a lack of mechanistic understanding of LZAP substantially impedes the functional studies.

Accumulating data suggest that LZAP is implicated in regulation of protein phosphorylation. In HNSCCs, LZAP was shown to suppress NF-κB, at least in part, through reducing the phosphorylation of RelA.³ Two groups independently showed that LZAP also antagonized the WNT signaling by reducing phosphorylation of GSK3β and consequently accelerating β-catenin degradation. Other molecules affected by LZAP include p38MAPK, Chk1 and Chk2. Interestingly, these LZAP targets were all substrates of the wild-type p53-induced phosphatase 1 (Wip1). In a recent paper of Cell Cycle, Wamsley and colleagues demonstrated that LZAP directly bound to Wip1 and promoted its phosphatase activity in a cell-free system toward several LZAP targets.⁷ The same group has previously shown that depletion of Wip1 in U2OS cells abolished the ability of LZAP to suppress phosphorylation of p38MAPK. These observations collectively suggest that Wip1 is a major target through which LZAP modulates protein phosphorylation. Because LZAP binds to both Wip1 and its substrates, it is straightforward to hypothesize that LZAP promotes Wip1 activity by acting as a scaffold. However, ERK1, a Wip1 substrate that does not bind to LZAP, is also subject to LZAP-mediated regulation. Thus, direct association with the substrates may not be necessary for the crosstalk between LZAP and Wip1. Rather, association with LZAP may lead to conformational changes of Wip1 that stimulate its phosphatase activity. Nonetheless, phosphorylation of several LZAP-binding partners, such as p53, MDM2, Chk1, and p38MAPK, are more potently affected by LZAP than ERK1 in the cell-free system. As such, it is possible that proteins associated with LZAP are preferential substrates of Wip1. As such, LZAP may modulate not only Wip1 activity but also substrate selectivity in vivo. In summary, the study led by Wamsley and colleagues identified a novel mechanistic link between LZAP and Wip1, providing crucial mechanistic insights into the crosstalk of LZAP with several its key targets implicated in DNA damage, inflammation and oncogenesis. These findings established an important platform to further interrogate the functions of LZAP in cancer and other diseases.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.