[1] p14ARF is induced in response to elevated mitogenic stimulation, such as aberrant growth signaling from MYC and Ras (protein).
These interactions allow p14ARF to act as a tumor suppressor by inhibiting ribosome biogenesis or initiating p53-dependent cell cycle arrest and apoptosis, respectively.
[8] Therefore, INK4a and INK4b serve as tumor suppressors by restricting proliferation though the inhibition of the CDKs responsible for Rb phosphorylation.
Because of this, INK4a and p14ARF have unrelated amino acid sequences despite overlapping coding regions and have distinct functions.
As mentioned above, INK4a inhibits proliferation by indirectly allowing Rb to remain associated with E2F transcription factors.
Mdm2 also has E3 ubiquitin ligase activity toward p53, and promotes its exportation from the cell nucleus to the cytoplasm for degradation.
By antagonizing Mdm2, ARF permits the transcriptional activity of p53 that would lead to cell cycle arrest or apoptosis.
[9] NPM is an acidic ribosomal chaperone (protein) involved in preribosomal processing and nuclear exportation independent of p53, and oligomerizes with itself and p14ARF.
[10] The larger size resulting from more ribosomes and protein is not associated with increased proliferation, however, and this ARF-null phenotype occurs even though the normal basal levels of Arf are usually low.
Therefore, the function of basal level ARF in the NPM/ARF complex appears to be to monitor steady-state ribosome biogenesis and growth independently of preventing proliferation.
[12] In a separate study, 32% of colorectal adenomas (non-cancerous tumors) were found to have p14ARF inactivation due to hypermethylation of the promoter.
When smARF is overexpressed, it localizes to the mitochondrial matrix, damaging the mitochondria membrane potential and structure, and leading to autophagic cell death.
[14] The role of smARF is distinct from that of ARF, as it lacks the nuclear localization signal (NLS) and cannot bind to Mdm2 or NPM.
Aberrant mitogenic stimulation, such as by MYC or Ras (protein), will increase its expression, as will an amplification of mutated p53 or Mdm2, or p53 loss.
[1] One of these interactions results in sumoylating activity, suggesting that ARF may modify proteins to which it binds.
The SUMO protein is a small ubiquitin-like modifier, which is added to lysly ε-amino groups.
The importance of this role is unknown, as sumoylation is involved in different functions, such as protein trafficking, ubiquitylation interference, and gene expression changes.
The N-terminal amino acid sequences of p19Arf (Met-Gly-Arg) and p14ARF (Met-Val-Arg) would be processed by methionine aminopeptidase but would not be acetylated, allowing ubiquination to proceed.
The levels of p19Arf are not affected by p32 knockdown, and so p32 specifically stabilizes smARF, possibly by protecting it from the proteasome or from mitochondrial proteases.
ARF Promotes MDM2 Degradation and Stabilizes p53: ARF-INK4a Locus Deletion Impairs Both the Rb and p53 Tumor Suppression Pathways.