09-14-2021-0417 - p14ARF (also called ARF tumor suppressor, ARF, p14ARF alternate reading frame protein product of the CDKN2A locus (i.e. INK4a/ARF locus) ARF rb Rb Robert Bettey

 p14ARF (also called ARF tumor suppressor, ARF, p14^ARF) is an alternate reading frame protein product of the CDKN2A locus (i.e. INK4a/ARF locus).^[1] p14ARF is induced in response to elevated mitogenic stimulation, such as aberrant growth signaling from MYC and Ras (protein).^[2] It accumulates mainly in the nucleolus where it forms stable complexes with NPM or Mdm2. These interactions allow p14ARF to act as a tumor suppressor by inhibiting ribosome biogenesis or initiating p53-dependent cell cycle arrest and apoptosis, respectively.^[3] p14ARF is an atypical protein, in terms of its transcription, its amino acid composition, and its degradation: it is transcribed in an alternate reading frame of a different protein, it is highly basic,^[1] and it is polyubiquinated at the N-terminus.^[4]

Both p16INK4a and p14ARF are involved in cell cycle regulation. p14ARF inhibits mdm2, thus promoting p53, which promotes p21 activation, which then binds and inactivates certain cyclin-CDK complexes, which would otherwise promote transcription of genes that would carry the cell through the G₁/S checkpoint of the cell cycle. Loss of p14ARF by a homozygous mutation in the CDKN2A (INK4A) gene will lead to elevated levels in mdm2 and, therefore, loss of p53 function and cell cycle control.

The equivalent in mice is p19ARF.

This article is about a tumor suppressor gene. For ARF small GTP-binding protein, see ADP ribosylation factor.

Very commonly, cancer is associated with a loss of function of INK4a, ARF, Rb, or p53.^[11] Without INK4a, Cdk4/6 can inappropriately phosphorylate Rb, leading to increased E2F-dependent transcription. Without ARF, Mdm2 can inappropriately inhibit p53, leading to increased cell survival.

The INK4a/ARF locus is found to be deleted or silenced in many kinds of tumors. For example, of the 100 primary breast carcinomas, approximately 41% have p14^ARF defects.^[12] In a separate study, 32% of colorectal adenomas (non-cancerous tumors) were found to have p14^ARF inactivation due to hypermethylation of the promoter. Mouse models lacking p19^Arf, p53, and Mdm2 are more prone to tumor development than mice without Mdm2 and p53, alone. This suggests that p19^Arf has Mdm2- and p53-independent effects, as well.^[13] Investigating this idea lead to the recent discovery of smARF.^[14]

Homozygous deletions and other mutations of CDK2NA (ARF) have been found to be associated with glioblastoma.^[15]

smARF[edit]

Until recently, the two known effects of ARF were growth inhibition by NPM interactions and apoptosis induction by Mdm2interactions. The function of ARF involving p53-independent death, has now been attributed to the small mitochondrial isoform of ARF, smARF.^[14] While full-length ARF inhibits cell growth by cell cycle arrest or type I apoptotic death, smARF kills cells by type II autophagic death. Like ARF, the expression of smARF increases when there are aberrant proliferation signals. When smARF is overexpressed, it localizes to the mitochondrial matrix, damaging the mitochondria membrane potential and structure, and leading to autophagic cell death.^[16]

The translation of the truncated ARF, smARF, is initiated at an internal methionine (M45) of the ARF transcript in human and mouse cells. SmARF is also detected in rats, even though an internal methionine is not present in the rat transcript. This suggests that there is an alternate mechanism to form smARF, underscoring the importance of this isoform.^[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.^[3] In some cell types, however, full-length ARF can also localize to the mitochondria and induce type II cell death, suggesting that in addition to autophagy being a starvation or other environmental response, it may also be involved in responding to oncogene activation.^[2]

Biochemistry[edit]

ARF expression is regulated by oncogenic signaling. 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.^[8] ARF can also be induced by enforced E2Fexpression. Although E2F expression is increased during the cell cycle, ARF expression probably is not because the activation of a second, unknown transcription factor might be needed to prevent an ARF response to transient E2F increases.^[11] ARF is negatively regulated by Rb-E2F complexes ^[11] and by amplified p53 activation.^[8] Aberrant growth signals also increase smARF expression.^[16]

ARF is a highly basic (pI>12) and hydrophobic protein.^[8] Its basic nature is attributed to its arginine content; more than 20% of its amino acids are arginine, and it contains little or no lysine. Due to these characteristics, ARF is likely to be unstructured unless it is bound to other targets. It reportedly complexes with more than 25 proteins, although the significance of each of these interactions is not known.^[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. This process involves a three-enzymecascade similar to the way ubiquitylation occurs. E1 is an activating enzyme, E2 is a conjugation enzyme, and E3 is a ligase. ARF associates with UBC9, the only SUMO E2 known, suggesting ARF facilitates SUMO conjugation. The importance of this role is unknown, as sumoylation is involved in different functions, such as protein trafficking, ubiquitylation interference, and gene expression changes.^[1]

The half-life of ARF is about 6 hours,^[4] while the half-life of smARF is less than 1 hour.^[3] Both isoforms are degraded in the proteasome.^[1][4] ARF is targeted for the proteasome by N-terminus ubiquitylation.^[4] Proteins are usually ubiquinated at lysineresidues. Human [[p14^ARF]], however, does not contain any lysines, and mouse p19^Arf only contains one lysine. If the mouse lysine is replaced with arginine, there is no effect on its degradation, suggesting it is also ubiquinated at the N-terminus. This adds to the uniqueness of the ARF proteins, because most eukaryotic proteins are acetylated at the N-terminus, preventing ubiquination at this location. Penultimate residues affect the efficiency of acetylation, in that acetylation is promoted by acidic residues and inhibited by basic ones. The N-terminal amino acid sequences of p19^Arf (Met-Gly-Arg) and p14^ARF (Met-Val-Arg) would be processed by methionine aminopeptidase but would not be acetylated, allowing ubiquination to proceed. The sequence of smARF, however, predicts that the initiating methionine would not be cleaved by methionine aminopeptidase and would probably be acetylated, and so is degraded by the proteasome without ubiquination.^[1]

Full-length nucleolar ARF appears to be stabilized by NPM. The NPM-ARF complex does not block the N-terminus of ARF but likely protects ARF from being accessed by degradation machinery.^[4] The mitochondrial matrix protein p32 stabilizes smARF.^[16] This protein binds various cellular and viral proteins, but its exact function is unknown. Knocking down p32 dramatically decreases smARF levels by increasing its turnover. The levels of p19^Arf are not affected by p32 knockdown, and so p32 specifically stabilizes smARF, possibly by protecting it from the proteasome or from mitochondrial proteases.^[16]

https://en.wikipedia.org/wiki/P14arf

Tumor suppressor genes and Oncogenes

Ligand

Growth factors

ONCO

c-Sis/PDGF HGF

Receptor

Wnt signaling pathway

TSP

CDH1

Hedgehog signaling pathway

TSP

PTCH1

TGF beta signaling pathway

TSP

TGF beta receptor 2

Receptor tyrosine kinase

ONCO

ErbB/c-ErbB HER2/neu Her 3 c-Met c-Ret

JAK-STAT signaling pathway

ONCO

c-Kit Flt3

Intracellular signaling P+Ps

Wnt signaling pathway

ONCO

Beta-catenin

TSP

APC

TGF beta signaling pathway

TSP

SMAD2 SMAD4

Akt/PKB signaling pathway

ONCO

c-Akt

TSP

PTEN

Hippo signaling pathway

TSP

Neurofibromin 2/Merlin

MAPK/ERK pathway

ONCO

c-Ras HRAS c-Raf

TSP

Neurofibromin 1

Other/unknown

ONCO

c-Src

TSP

Maspin

Nucleus

Cell cycle

ONCO

CDK4 Cyclin D Cyclin E

TSP

p53 pRb WT1 p16/p14arf

DNA repair/Fanconi

TSP

BRCA1 BRCA2

Ubiquitin ligase

ONCO

CBL MDM2

TSP

VHL

Transcription factor

ONCO

AP-1 c-Fos c-Jun c-Myc

TSP

KLF6

Mitochondrion

Apoptosis inhibitor

SDHB SDHD

Other/ungrouped

c-Bcl-2 Notch Stathmin

hidevte

Cell cycle proteins

Cyclin

A (A1, A2) B (B1, B2, B3) D (D1, D2, D3) E (E1, E2)

CDK

1 2 3 4 5 6 7 8 9 10 CDK-activating kinase

CDK inhibitor

INK4a/ARF (p14arf/p16, p15, p18, p19) cip/kip (p21, p27, p57)

P53 p63 p73 family

p53 p63 p73

Other

Cdc2 Cdc25 Cdc42 Cellular apoptosis susceptibility protein E2F Maturation promoting factor Wee Cullin (CUL7)

Phases and

checkpoints

Interphase

G1 phase S phase G2 phase

M phase

Mitosis (Preprophase Prophase Prometaphase Metaphase Anaphase Telophase) Cytokinesis

Cell cycle checkpoints

Restriction point Spindle checkpoint Postreplication checkpoint

Other cellular phases

Apoptosis G0 phase Meiosis

Categories: Genes on human chromosome 9