NIAM's tangled web of growth control.

Aside from its canonical activity activating p53 in response to oncogene activation, the tumor suppressor (alternate reading frame) ARF exhibits p53-independent antiproliferative and tumor-suppressive activities. In search of the mechanism underpinning these activities, a number of ARF-interacting proteins have been identified in the years since the ARF/MDM2 interaction was characterized as the means by which ARF activates p53. These novel ARF interactors include ARF-binding protein1 (ARF-BP1), nucleophosmin, C-terminal binding protein (CtBP)-family co-repressors, and the nuclear interactor of ARF and MDM2 (NIAM). Of these known ARF interactors, NIAM is unique in its action as a putative tumor suppressor and suppressor of cell growth, rather than an oncogene or growth/survival factor that is antagonized by ARF.

In the April 15, 2014 issue of Cell Cycle, Reed et al. investigated the pathways downstream of NIAM required for its growth-suppressive properties. What little was previously known about NIAM strongly suggested its ability to induce cell cycle arrest channels through multiple parallel pathways. NIAM promotes both ARF-dependent and -independent activation of p53 transcriptional activity and is downregulated in a number of pancreatic adenocarcinoma cell lines and human tumors. Mysteriously, however, NIAM also promotes cell cycle arrest through a p53-independent pathway. These observations suggest an unusually complex interplay among NIAM, ARF, p53, and the cell cycle-regulatory machinery.

Reed et al. attempt to elucidate the mechanisms underlying NIAM’s control of the p53-dependent arm of its cell cycle-inhibitory activities, discovering that physiologic levels of NIAM are necessary for full p53 transactivation of the p21 promoter, and that NIAM promotes acetylation of lysine 120 of p53, a modification regarded as critical for activation of the apoptotic arm of the p53 transcription program., The authors further demonstrate that p53 activation by NIAM occurs through 2 distinct pathways: disruption of the p53–MDM2 complex, and association of NIAM with the Tip60 acetyltransferase.

In a surprising twist, the authors show that depletion of Tip60 only partially reverses NIAM-mediated K120 acetylation, and that an N-terminal NIAM truncation mutant, competent for MDM2 binding, Tip60 binding, chromatin association, and cell cycle arrest, is unable to drive K120 acetylation of p53. While it is possible that residual Tip60 or the presence of hMOF is sufficient for acetylation of a limited pool of p53, the inability of the truncation mutant to promote p53 acetylation despite retaining robust Tip60 interaction is surprising and may suggest that the NIAM C terminus recruits additional functions necessary for in vivo acetylation of p53 at sites of p53-dependent transcription. The data are also a clear indication that the K120 acetylation driven by NIAM is dispensable for its broader functions in inhibiting MDM2–p53 interaction and promoting cell cycle arrest.

Together, Reed et al.’s data support a model whereby one pool of NIAM sequesters MDM2 from p53 and allows p53 activation, while another pool promotes activation and association with the p53 acetylase and coactivator Tip60. However, advances in our understanding often raise as many questions as they provide answers, and regulation of proliferation by NIAM is no exception. For example, if K120 acetylation by NIAM is separable from Tip60 binding and p53 activation, how is NIAM regulating Tip60 function? It is possible that Tip60 in this model functions as a canonical histone acetyltransferase, acetylating histones in the vicinity of p53 target promoters, and that NIAM facilitates this activity through enhancing DNA binding of Tip60, or modulating its interaction with negative regulators.

Interestingly, Tip60 is known to interact with MDM2 and negatively regulate MDM2-mediated NEDDylation, while NIAM interacts with both of these factors but, notably, not p53. It is also interesting to note that the NIAM N terminus encodes all functions necessary for p53-dependent growth arrest, including binding of Tip60 and MDM2, while the C terminus, upon which K120 acetylation is dependent, seems to provide some function necessary to direct the acetylase activity of Tip60 toward p53. Future work will be instrumental in determining whether NIAM regulation of p53 activities occurs through regulation of a Tip60–MDM2 interaction.

Of broader interest will be the exploration of NIAM’s role in tumorigenesis—its downregulation suggests that NIAM activity may be selected against during tumorigenesis, so it will be critical to develop animal models to probe NIAM function and to carefully analyze the wealth of available high-throughput sequence and microarray data to determine whether NIAM functions as a tumor suppressor in certain tumors, perhaps in a mutually exclusive fashion with Tip60 and p53.