Do p53 Repeat Fields Synchronize Target Cells and Natural Killer Cells?

Natural Killer cells face a remarkable biological problem. They must continuously distinguish healthy cells from stressed, infected or malignant cells and decide which should be destroyed. Increasing evidence suggests that p53 participates directly in coordinating that decision. A 2024 review in Frontiers in Immunology brings together evidence that p53 affects not only the internal fate and external immune visibility of the target cell, but also the homeostasis, receptor signalling and functional state of the Natural Killer cell itself.

This creates an intriguing question for Codondex. If p53 coordinates several different components of the NK-target interaction, could the unusually dense repeat structures that Codondex detects within TP53 help identify genomic regions involved in regulating that coordination? We describe these structures as High-Density Nested Repeat Fields, or HDNRFs: regions in which repeated, overlapping and nested short sequences become unusually concentrated. HDNRFs are not simply conventional tandem repeats. They represent a broader sequence geography that becomes visible when DNA is interrogated simultaneously across different sequence lengths and positions.

The Codondex methodology approaches this problem without first needing to know the biological function of the sequence. It divides genetic sequence into overlapping components, preserves their relative positions and relationships, and identifies sequence combinations that recur or associate unusually strongly. The important distinction is that Codondex can identify unusual sequence architecture first and ask what that architecture means biologically afterwards. The progression is therefore from sequence, to pattern, to biological association and finally, if experimentally demonstrated, to mechanism.

We have already seen an intriguing example of this process within TP53. Earlier Codondex analysis converged on the intron 3 region containing the known PIN3 polymorphism, or Polymorphism Intron 3. PIN3 is a 16-base-pair duplication situated within an unusual region of TP53 associated with structural and regulatory features affecting p53 RNA processing. The importance of the PIN3 observation is not that Codondex discovered PIN3 itself. The polymorphism was already known. What was interesting was the route by which Codondex arrived there. The algorithm was not instructed to search for PIN3 or for a known p53 regulatory region. It converged on the location because of an unusual concentration of relationships within the sequence itself. Conventional biology then provided an independent reason why that location could matter.

PIN3 therefore provided an early example of the utility we are now trying to test more systematically with HDNRFs: whether unusual repeat geography can lead to biologically significant regulatory locations before we understand why those locations are important. More recent Codondex analysis has strengthened that question by revealing concentrations of repeated and overlapping sequences elsewhere within TP53 and other genes, suggesting that repeat density may represent a genomic characteristic worth measuring in its own right.

A second observation made the p53 question considerably more interesting. In our previous analysis of 48 tumour sections, Codondex-derived sequence rankings were compared with several experimental immune measurements, including Chromium-release Natural Killer cytotoxicity under unstimulated pNK, IL-2-stimulated pNK and sNK conditions, together with IFN-γ and other measurements. When the sequences were examined in different ways, including duplicate occurrence, repeated short sequences and tandem or overlapping repeat relationships. pNK repeatedly emerged as one of the more consistent candidate associations.

This was unexpected because the unstimulated pNK experimental signals themselves were frequently weak. Many tumour sections showed little or no measured pNK activity, while stimulated NK measurements often generated much stronger absolute signals. Yet pNK continued to appear when independent measures of Codondex sequence structure were compared with the experimental results. We previously discussed the possibility that this recurrence may therefore be more informative than the absolute strength of the pNK measurement suggests. A weak biological signal that repeatedly aligns with independently derived sequence characteristics may be pointing to an underlying state rather than simply reflecting the magnitude of immune activity.

The Frontiers in Immunology review provides a biological framework capable of explaining why this might occur. In the target cell, wild-type p53 can increase expression of NK-activating ligands including ULBP1 and ULBP2, which interact with NKG2D, and can influence PVR/CD155 and other components of activating versus inhibitory NK recognition. p53 also participates in the apoptotic machinery determining whether an NK attack successfully kills its target, including BAX-dependent mitochondrial pathways and death-receptor mechanisms.

But importantly, p53 is not operating only within the target. The same review describes p53-related regulation within NK cells themselves. p53 participates in NK-cell cycle and apoptotic control, influences receptor expression and regulates SAP, which couples SLAM-family signalling through Fyn and Vav-1 to NK activation. The authors explicitly separate the regulatory role of p53 in NK cells from its role in tumour cells, and conclude that these functions contribute to NK recognition and targeting of malignant cells.

This means that p53 potentially operates on both sides of the immune synapse. In the target, it can contribute to making a damaged cell visible, engageable and susceptible to killing. In the NK cell, p53-related pathways can influence whether the immune cell is functionally competent to recognize and respond to that target. Successful cytotoxicity may therefore depend upon the state of two interacting cellular programmes becoming appropriately aligned.

We have described this concept as p53-driven NK-target synchronicity. Until recently, however, the word “synchronicity” was principally a functional description of several p53-dependent variables moving together. New experimental work published in Molecular Systems Biology now gives that term a much more literal biological foundation.

Venkatachalapathy and colleagues demonstrated that p53 behaves as a dynamic cellular oscillator whose normally heterogeneous pulses can be experimentally phase-reset and synchronized across individual cells. Using time-lapse microscopy and controlled DNA-damage stimulation in MCF-7 cells, they showed that p53 oscillations could be brought into significantly greater temporal alignment by appropriately timed repeated stimuli. Two damage pulses spaced approximately 4.0 to 5.5 hours apart produced significantly increased synchronization compared with a single stimulus.

This is important because the synchronization was not merely cosmetic. Altering p53 pulse timing changed downstream gene-expression dynamics, and phase resetting affected cellular fate. The study showed that changes in p53 oscillatory frequency altered expression of p53 target genes and that greater synchronization reduced the probability that cells escaped cell-cycle arrest. In other words, p53 synchronization produced synchronized biological consequences.

This materially strengthens the hypothesis we have been developing. p53 should no longer be viewed only as a molecular switch whose biological significance is determined by how much p53 is present. Its timing, frequency and phase also contain biological information. The new work demonstrates experimentally that p53 dynamics can be reset, that cells can be brought into temporal alignment, and that changing those dynamics changes downstream molecular programmes and cell fate.

The implication for Codondex is potentially significant. A p53 HDNRF need not simply influence whether there is “more” or “less” p53. If repeat architecture has regulatory significance, it could conceivably affect p53 expression, RNA processing, response thresholds, pulse amplitude, oscillatory frequency or the persistence of downstream signals. PIN3 becomes particularly interesting in this context because Codondex independently converged on an intronic TP53 region already associated with regulation of TP53 processing. The question may therefore be broader than whether repeat fields alter p53 abundance. It may be whether they identify or influence the dynamic state of the p53 regulatory system.

This offers a new way of interpreting the repeated pNK result. Unstimulated pNK cytotoxicity may represent a relatively delicate basal relationship between the target cell and the Natural Killer cell. For killing to occur, the target must become sufficiently visible, activating signals must overcome inhibitory signals, the NK cell must be in a competent functional state, the two cells must successfully engage and the target must remain susceptible to the resulting cytotoxic attack. These conditions need not individually produce large experimental signals. Their biological importance may lie in whether they occur together.

The emerging hypothesis is therefore that the Codondex repeat signal is associated not simply with “NK activity,” but with the state in which the target and NK cell become appropriately coordinated. The target-side p53 programme can influence recognition ligands and apoptotic susceptibility, while the NK-side p53 programme can influence cellular homeostasis and activation pathways. The new Molecular Systems Biology work adds the crucial observation that p53 itself possesses a genuine temporal synchronization mechanism capable of coordinating downstream biological responses.

It is important not to claim more than the evidence demonstrates. The new study did not place an NK cell beside a tumour cell and show that p53 oscillations in the two cells phase-lock to one another. It synchronized p53 oscillations across MCF-7 cells using externally delivered DNA-damage pulses. It therefore proves that p53 is a synchronizable biological oscillator, but does not yet prove that NK and target-cell p53 oscillators synchronize during immune surveillance. That distinction defines the next experiment rather than weakening the hypothesis.

The research question can now be framed much more precisely. Tumour targets should be classified according to their relevant Codondex HDNRF state and then observed together with Natural Killer cells while p53 dynamics are measured in real time. Instead of measuring p53 only at a single time point, the experiment should measure p53 pulse timing, frequency, amplitude and phase in the target and, where technically possible, in the NK cell. These measurements should be followed simultaneously by target-cell ULBP1/2, PVR/CD155 and other recognition signals, NK activation and engagement, CD107a degranulation, IFN-γ production and ultimately target-cell killing.

The decisive observation would be whether these variables move together in time. Does a change in target-cell p53 state precede altered NK-recognition ligand expression? Does successful NK engagement occur preferentially during a particular target-cell p53 state? Does the NK cell itself undergo a corresponding p53-dependent state change? Do successful killing events show greater coordination between these trajectories than unsuccessful encounters? And most importantly for Codondex, does the strength or architecture of the relevant HDNRF predict any of these dynamic relationships?

The experimental design can then move from association to causality. Removing or inhibiting functional p53 should disturb the predicted relationship. Restoring p53 should restore at least part of it. Directly altering a candidate HDNRF using locus-specific sequence or epigenetic intervention could then determine whether the repeat field is merely a marker of the p53 state or participates in establishing it. If changing the HDNRF changes p53 dynamics and those changes propagate through NK recognition and cytotoxicity, the evidence would move substantially closer to a causal genomic mechanism.

PIN3, the recurring pNK association and the new p53 synchronization data now form three independent but potentially connected observations. PIN3 showed that Codondex could converge computationally on an unusual TP53 sequence region already recognized by conventional biology as regulatory. The tumour-section analysis repeatedly brought pNK to the surface despite weak absolute unstimulated NK signals. The new phase-resetting experiments demonstrate that p53 itself can operate as a synchronizable dynamic system whose temporal state changes downstream gene expression and cellular fate.

None of these observations alone proves that an HDNRF synchronizes a Natural Killer cell with its target. Together, however, they produce a considerably more specific and experimentally falsifiable hypothesis. A p53 repeat field may identify or influence a dynamic genomic state that helps coordinate p53-dependent programmes in the target and Natural Killer cell, increasing the probability that recognition, engagement, susceptibility and cytotoxic response occur together.

That hypothesis also provides a possible explanation for why pNK keeps appearing in the Codondex analysis. The algorithm may not be detecting the strength of an immune response. It may instead be detecting sequence architecture associated with the underlying state that permits natural NK surveillance to occur.

Codondex was designed to find relationships in genetic sequence before we necessarily know what those relationships mean. PIN3 was an early indication that concentrated sequence relationships could converge on known p53 regulatory biology. The recurring pNK evidence suggested a possible connection with Natural Killer surveillance. We now know experimentally that p53 itself is capable of phase resetting and synchronization, and that synchronized p53 dynamics can alter downstream biological outcomes.

The next question is therefore unusually clear: do p53 repeat fields participate in setting the dynamic conditions under which a target cell and a Natural Killer cell become synchronized for recognition and killing?

That is now an experiment worth doing.

Angiogenic Growth Factor Flood

A previous series, about p53 culminated with "Blastocyst Development - A Perfected Cancer Model" that focused on the parallels in angiogenesis, triggered by blastocyst implantation and progression of tumors beyond ~1mm. Now, a recent study has found that conventional Natural Killer cells (cNK) control vascular remodeling in the uterus during pregnancy by acidifying the extracellular matrix (ECM) with a2V-ATPase that activates MMP-9 that degrades the ECM. Ablation of a2V-ATPase decreases Bax and p53 expression in testis and leads to implantation failure in the female mouse. The degrading ECM releases bound pro-angiogenic growth factors that contribute to Uterine artery (UtA) remodeling characterized by the loss of vascular smooth muscle cells (VSMCs) and dilation of the vessels. Without cNK, the UtA never lose VSMCs and UtA resistance remains high often leading to implantation failure.

Its logical that a timely flood of angiogenic growth factors, previously stored in the ECM would provide instant availability, but whether this explains the maternal-embryonic immune paradox remains to be determined? In the immune paradox maternal NK cells invade and maternal blood vessels are remodeled just before the arrival of trophoblasts, the external cells of the blastocyst, that carry male antigens during formation of the fetal placenta. A sudden flood of angiogenic factors preceding invading trophoblasts could provide the perfect environment required for maternal arterial/vascular remodeling.

Lymphocytes in the uterine lining (decidua) are dominated by a unique decidual natural killer (dNK) cell population. The dNK cell surface phenotype CD56bright CD16− CD3− and macrophages CD14+ CD206+(dMac) support a model whereby dNK cells, capable of killing extra-villous cytotrophoblasts (CTB), are prevented from doing so by neighboring macrophages thus protecting the fetal cells from NK cell attack. Existing research has centered on the function of the abundant and diverse sets of dNK, but now that cNK cells have been identified to play a more significant role, our understanding of the remodeling are likely to change.

In CTB exogenous p53 is able to down-regulate MMP-9 promoter activity, but endogenous p53 is not able to regulate MMP-9 expression in first trimester CTB cells. Inactivation of p53 through mutation is the most common trait in cancer. By loosing its onco-suppressive activity, p53 becomes oncogenic in almost all malignant tumors (Soussi and Lozano, 2005). Although p53 is not mutated in the human placenta, it has become functionally incompetent. Understanding why and how p53 is functionally incompetent in CTB might well be the key to understanding trophoblast invasion.

Downregulation of EMMPRIN (BSG,CD147) by p53 leads to a decrease in the activity of MMP-9 and an inhibition of tumor cell invasion. Upregulation of EMMPRIN seen in many cancers can be attributed to, at least in part, to the dysfunction of p53 and thus provides new evidence for the roles of p53 in tumor development and progression. Epithelial derived MMP-9 exhibits a novel defensive role of tumor suppressor in colitis associated cancer by activating MMP9-Notch1-ARF-p53 axis. MMP-9 mediates Notch1 signaling via p53 to regulate apoptosis, cell cycle arrest, and inflammation. 

The inter-activity of p53, cNK and MMP-9 are complexed, but this novel research may lead to the mechanisms by which arterial remodeling occurs after release of angiogenic factors from ECM. If that shares characteristics of NK invasion into developing tumor micro environment's a new therapeutic approach may arise.

 

Chemo vs. Mecho

Data strongly suggests interaction between plasma membrane and submembrane at the endothelial surface controls the inflammatory response. 

A meta-analysis from six studies of global gene expression profiles of Blood Pressure (BP) and hypertension was performed in 7017 individuals. 34 genes were differentially expressed. Of these, 6 genes were linked including MYADM, which was the only gene, of 34 discovered across diastolic, systolic BP and hypertension. Knockdown of MYADM (19q13), a component of endothelial surface rafts induced an inflammatory phenotype altering barrier function through the increase of the adhesion receptor ICAM-1 (19p13). This is mediated by MYADM activation of ERM actin cytoskeleton proteins. 

Mechanical forces, without a definitive direction e.g., disturbed flow and relatively undirected stretch at branch points and other complex regions cause sustained molecular signaling of pro-inflammatory and proliferative pathways that include mechanical stretch tied to p53. 

ERM proteins also facilitate Sphingosine-1-phosphate (S1P) dependent egress for T-cells to migrate from lymphoid organs. Their directional migration, by blebbing is contained at the T-cell’s leading edge. This fundamentally different mode of migration is characterized by intracellular pressurization. Of the five S1P receptors S1P2 (19p13) is critical in the immune, nervous, metabolic, cardiovascular, musculoskeletal, and renal systems. Results suggest that the ratio between S1P1 and S1P2 (19p13) governs the migratory behavior of different T cell subsets. 

Human NK cells express S1P1 mRNA. Activation with IL-2 increases S1P1, promotes S1P4 (19p13) and S1P5 (19p13) but not S1P2 (19p13) expression. Unlike S1P1, S1P2 (19p13) signals through several different G-alpha subunits, Gi, G12/13, and Gq. S1P5 (19p13) is also expressed in human and mouse NK cells and was required for mobilization to inflamed organs. S1P5-deficient mice had aberrant NK cell homing during steady-state conditions. NK cell trafficking in vivo requires a dedicated sphingosine 1-phosphate receptor. 

Virus-infected mast cells selectively recruit NK cells and positively modulate their functions through mechanisms dependent on soluble mediators, such as interferons. Skin mast cells protect mice against vaccinia virus by triggering mast cell receptor S1P2 (19p13) and releasing antimicrobial peptides. S1P2 (19p13),  a negative regulator of platelet derived growth factor (PDGF) induced migration and proliferation as well as SphK1 expression. 

S1P inhibits macropinocytosis (internalizing extracellular materials) and phosphorylation of Akt via S1P2 (19p13) stimulation resulting in diminished antigen capture.

S1P1, S1P2 (19p13) and S1P3 receptors have redundant or cooperative functions for the development of a stable and mature vascular system during embryonic development. S1P2 (19p13)  and S1P3 are involved in regulation of endothelial barrier function, fibrosis, and vasoconstriction. 

Adipogenic differentiation is inhibited by S1P2 (19p13) as mediated by C/EBPα and PPARγ, which induces PEPCK, a more recent gene of interest in cancer that acts at the junction between glycolysis and the Krebs cycle.

Mecho or chemo, chicken or egg, what first?

Systolic Blood Pressure and Innate Immunity vs. the Cancer Brain

Participants with a valid heart disease phenotype (atherosclerosis) were identified in a MESA blood pressure analysis conducted over 10 years. The valid group varied from 770 to 1113 patients from whom further blood analysis queried a primary and exploratory hypothesis of immune cell subsets. Four statistically significant innate cell subsets were discovered to be associated with Systolic blood pressure (SBP); Natural Killer (NK) cells, gamma delta T cells and classical monocytes.

Separately, an analysis of 7017 individuals from 6 international studies of gene expression signatures for SBP, diastolic blood pressure (DBP) and hypertension (HTN) found 7717 genes of which 34 were most differentialy expressed. Enrichment analysis for the systolic and diastolic gene group's associated to NK cell mediated cytotoxicity and 13 other pathways including antigen processing and inflammatory response, pointing strongly to innate and adaptive immunity. MYADM was the only gene identified for all groups SBP, DBP and HTN.

MYADM controls endothelial barrier function through ezrin, radixin, and moesin (ERM)-dependent regulation of ICAM-1 expression. ERM expression is required for ICAM-1 expression in response to MYADM suppression or TNF-α. ICAM-1 is a paradigmatic adhesion receptor that regulates leukocyte adhesion together with integrin LFA-1. This connection between endothelial membrane and cortical actin cytoskeleton appears to modulate the inflammatory response at the blood tissue barrier. 

Pressure overload activates the sympathetic nervous system (SNS) and up-regulates p53 expression in the cardiac endothelium and in bone marrow (BM) cells. Increased p53 expression promotes endothelial-leukocyte cell adhesion and initiates inflammation in cardiac tissue, which exacerbates systolic dysfunction. SNS activates, at least by significant increase of circulating norepinephrine (NE), which up-regulates p53 expressions, while forced expression of p53 increased ICAM-1 expression. 

On endothelial cells SNS is mediated via catecholamine-β2-adrenergic signaling, which up-regulates the production of reactive oxygen species (ROS), activates p53 and induces cellular senescence. Immune cells, including macrophages, monocytes, NK cells, B and T cells express the β2-adrenergic receptor and catecholamine. During pressure overload, NE cultured macrophages up-regulated p53 expression, whereas introduction of p53 increased Itgal (LFA-1) expression (which binds ICAM-1). Treatment with NE increased ROS, which was attenuated after inhibition of β2- adrenergic signaling in macrophages. Endothelial cell–macrophage interaction via NE-ROS-p53 signaling induces up-regulation of adhesion molecules, thus contributing to cardiac inflammation and systolic dysfunction.

During hypertension the vascular endothelium activates monocytes, in part through ROS by a loss of nitric oxide (NO) signaling, increased release of IL-6, hydrogen peroxide and a parallel increase in STAT activation in adjacent monocytes. NO inhibits formation of intermediate monocytes and STAT3 activation. Humans with hypertension have increased intermediate and non-classical monocytes and  intermediate monocytes demonstrate evidence of STAT3 activation. Mice with experimental hypertension exhibit increased aortic and renal infiltration of monocytes, dendritic cells, and macrophages with activated STAT3.

A senescence-associated secretory phenotype (SASP) was induced in epithelial cells after DNA damage of sufficient magnitude. In premalignant epithelial cells SASPs induced an epithelial–mesenchyme transition and invasiveness, hallmarks of malignancy by a paracrine mechanism that largely depended interleukin (IL)-6 and IL-8. Strikingly, loss of p53 and gain of oncogenic RAS exacerbated the pro-malignant activities. This suggests a cell-non-autonomous mechanism by which p53 can restrain and oncogenic RAS can promote the development of age-related cancer by altering the tissue microenvironment. Oncogenic signaling pathways inhibit the p53 gene transcription rate through a mechanism involving Stat3, which binds to the p53 promoter in vitro and in vivo. Blocking Stat3 in cancer cells up-regulates expression of p53, leading to p53-mediated tumor cell apoptosis. 

Induced stretch or stretch from pressure overload may engage a non-autonomous, p53 centric micro-mechanical mechanism that escalates or deescalates innate responses against cells functioning outside the mechanical ranges that macrophages or NK cells permit. Thus, the neuro-immune extension through SNS signaling, may begin with circulating blood pressure or stretch promoted through inflammation. 

p53 vasoregulation and NK cell depletion in SARS-CoV2

p53 has earned first prize in the academic stakes. It is also the most mutated gene in cancer and elephant's have 20 copies, which probably explains their surprisingly low rate of cancer. Its associations to innate immunity, particularly Natural Killer (NK) cells through the mechanics of vasoconstriction-dilation have become a point of interest in COVID19 patients.

Remarkably COVID19 has inspired the global scientific community to focus a significant portion of its aggregate research toward the impact of  SARS-CoV2 (CoV2). For the first time in history global research is singularly focused because a large number of other protein's and gene's are affected by CoV2 binding Ace2. The Ace2 receptor is important in systems of vasoconstriction-dilation and has wide ranging impact.

CoV2 binding Ace2 reduces its availability to convert Angiotensin1 to Angiotensin 1-7 (Ang1-7) or Angiotensin 1-9 (Ang1-9), which primarily interact via MAS and Angiotensin2 Receptor (AT2R) respectively. These have been linked to signaling and stretch caused by vasoconstriction-dilation, mitochondrial dysfunction, mitochondrial fission as well as cardiac and vascular remodeling.

Ang1-7 and Ang1-9 interactions with MAS or AT2R cell surface receptors have been linked to signaling events that drive p53 binding DNA and transcription. Myocyte stretching activates p53 and p53-dependent genes, leading to the formation of Angiotensin II (Ang II) and apoptosis. AngII, stimulates phosphorylation of p53 (on serine 15) and CREB (on serine 133) and signaling converges on the p53-CRE enhancer to stimulate Bradykinin receptor 2 (BK2) gene transcription. BK2 is a key element in the p53 related kallikrein-kinin system (KKS) of vasodilation that counters the Renin-Angiotensin-Aldosterone-System (RAAS) of vasoconstriction. 

Aldosterone was shown to induce mitochondrial dysfunction and podocyte injury mediated by p53/Drp1-dependent mitochondrial fission. In neuronal cells p53 dependent declines in Drp1 and parkin contribute to altered mitochondrial morphology and cell death. Parkin, via Pink1 activity binds depolarized mitochondria to induce autophagy of mitochondria. Mutations in both Drp1 and Pink1 were fatal in Drosophila models. These events also implicate a direct functional link to chronic inflammation in ageing between p53 and expression levels of ICAM1 on endothelial and NK cells required to bind targets. The p53 mediated negative regulation of autophagy is Pink1 dependent and experiments have shown that mitochondrial antigens, recognized by NK cells presented on MHC's are Pink1 and parkin dependent. 

Severe COVID-19 patients have highly elevated Bradykinin and AngII, perhaps an indication of elevated p53 trends that have been discovered in these patients. Under normal circumstances, on endothelial cells Bradykinin would act as a potent vasodilator via its BK2 receptor. However, since Ang (1-7) potentiates Bradykinin action on BK2 receptors its near absence may reduce KKS vasodilation. On the other hand RAAS, also via p53 and elevated AngII primarily interacts with AT1R to promote vasoconstriction.  

NK cells through their Renin Angiotensin System may counter-regulate target cells in response. However, in COVID19 patients depletion of NK cells, invasion of Neutrophils and endothelial cell damage, in part through elevated p53 autophagy and apoptosis is the overwhelming nasty work of CoV2 against the backdrop of dysregulated blood pressure in tissue.

 

 

Impotent Natural Killers by Cancer Stem Cells and Ageing

Cancer stem cells have been found, through various mechanisms to alter the sentinel function and innate, immune surveillance of Natural Killer cells (NK). In senescent cells that have stopped cell division, including in cancer stem cell niches and NK induced vascular remodeling (as found in the developing placenta) NK's sentinel vigilance is also reduced.

Senescence-associated mitochondrial dysfunction, a significant trigger of multiple dimensions of the senescent phenotype is caused by disruption of normal mitochondrial autophagy (mitophagy). Mitophagy increases with aging and this age-dependent rise is abrogated by PINK1 or parkin deficiency. Deletion of a p53 response element on PINK1 promoter impacts p53-mediated PINK1 transcriptional repression. This p53-mediated negative regulation of autophagy has been found to be PINK1-dependent and constitutes a p53-PINK1 loop in nucleus and cytoplasm.

Further, mitophagy controls the activities of tumor suppressor p53 to regulate, at least hepatic cancer stem cells via Nanog. Prostate cancer cells escape NK attack by Nanog down-regulating ICAM1 (LFA1), to which NK would normally bind its target. In lung cancer NK have been found to limit the efficient clearance of senescent tumor cells from the mouse lung after p53 restoration. This indicated p53 may promote conditions for cellular survival and NK induced vascular remodeling or angiogenesis, necessary for the growth of tumors.

When under stress and inner mitochondrial membrane pressure gradient moves toward depolarization, Pink1 slots into the membrane, binds and phosphorylates p53 at Serine 392 (p53s392) and aids phagophore formation to enhance mitophagy. Mitophagy traps cytoplasmic p53s392, which reduces its transport to the nucleus where it would otherwise disrupt transcription of Nanog. (As illustrated below). 

Activated p53s392 nucleoside concentrations are effected by mitophagy

On the other hand, the sentinel function of NK may be subject to this PINK1 mediated mitochondrial switch. In prostate cancer cells Nanog promoted ICAM1 transcription required for NK binding target and cell killing. In prostate cancer cells Nanog over-expression restricts ICAM1, which promotes tumor formation. (As illustrated below). Investigating further, the direct functional link between p53 and ICAM-1 (CD54) in senescence and age-related disorders appears to be deeply integrated in mitophagy, senescence and immunity.

Nanog over-expression appears to be deterministic 

In stem cells where normal expression of Nanog transcribes ICAM1 and cancer stem cells where over-expression of Nanog restricts ICAM1, the variable PINK1-p53 switch may represent a "canary" that signals the state of  mitochondrial health to sentinel NK. However in some cancer cells where normal mitophagy is impaired and Nanog expression is restricted by p53s392, other p53 isoforms may directly promote the transcription of ICAM1.

In  two manipulation experiments using five different fibroblast cell lines that accelerated development of senescent associated secretory phenotypes a striking result was observed: oncogenic RAS expression, which causes genotoxic stress and senescence in normal cells, and functional loss of the p53 tumor suppressor protein. Both loss of p53 and gain of oncogenic RAS also exacerbated pro-malignant paracrine signaling activities. Experiments show that PINK1 and Parkin, which are regulated by p53 specifically regulate mitochondrial antigen presentation of both MHC classes.

So, the question is whether the p53-PINK1 mitochondrial switch acts as cell-health "canary" for sentinel NK, where its inherent variables and regulatory loop may be fertile ground for the challenges of developing cancers?