P53: The Endothelium Defender

P53 represents the paradigm of a multitalented transcription factor, responsible for the cellular defense against a plethora of potentially harmful stimuli. It exercises the ability to strongly oppose both cancer and inflammation, partially due to the fact that both conditions are highly interrelated. Endothelial hyperpermeability is considered the hallmark of severe lung inflammation, and the cardinal feature of the lethal acute respiratory distress syndrome. An emerging body of evidence suggests a strategic role of P53 towards vascular barrier integrity. The "endothelium defender" orchestrates meticulously devised responses; to counteract toxin-induced destructions of endothelial monolayers. The present effort seeks to further our understanding on the expanding P53 universe, discussing the most recent information regarding the involvement of that molecule in the pulmonary function.

The discovery of P53 initiated intense efforts to oppose cancers via the promotion of tumor suppressive activities, although at the onset of its discovery the “Guardian of The Genome” (Lane, 1992) was considered to be an oncogene that promoted malignancies(Levine, 2018). It was soon realized that the oncogenic activities of that molecule were inflicted to the mutant P53 counteracts, abundantly existent in a diverse variety of tumors(Labuschagne, Zani, & Vousden, 2018). More than 50% of human cancers express mutants P53, responsible for malignant transformations and metastasis(Levine, 2017a).

P53 is a transcription factor, devoted to sense a plethora of environmental and cellular stresses. It orchestrates carefully devised molecular responses, in order to counteract a diverse variety of stimuli and prevent cellular dysfunctions. P53 post-translational modifications, such as phosphorylation, acetylation and methylation, may trigger cell cycle arrest or apoptosis (Levine, 2017b). In the instances of less severe circumstances, it initiates mechanisms to repair molecular functions (Tiwari, Jones, & Abrams, 2018).

P53 have been long associated with the suppression of both cancer and inflammation(Barabutis, Schally, & Siejka, 2018). The advanced properties of this “multi-talented” transcription factor, are partially due to the tight association between cancer and inflammation(Bottazzi, Riboli, & Mantovani, 2018). Cancer has been shown to progress more aggressively under severe inflammatory conditions, while such circumstances are ideal for malignant transformations(El-Hajjar et al., 2018; Gupta, Kunnumakkara, Aggarwal, & Aggarwal, 2018). The anti-inflammatory properties of that protein have been extensively investigated by numerous groups, and laborious efforts on the investigation of the relevant mechanisms are in progress(Liu, Zeng, Wang, Zheng, & Luan, 2018; Sagiv et al., 2018; Wang et al., 2018; Xue & Li, 2018).

For more than a decade, we investigate the interrelated signaling pathways involved in the progression of both cancer and inflammation(Barabutis & Schally, 2010). We have reported in cancer cells that P53 is involved in the mediation of the anti – oxidant and anti - cancer activities of Growth Hormone Releasing Hormone (GHRH) antagonists(Barabutis & Schally, 2008a). Furthermore, P53 was found to be associated with the anti - inflammatory activities of those compounds in non - cancerous human tissues(Barabutis, Siejka, & Schally, 2011). GHRH antagonists represent one of the most promising therapies against a plethora of cancers. They “sabotage” the growth factor activity of GHRH, and corrupt major signaling cascades that lead to tumor prevalence(Popovics, Cai, Sha, Rick, & Schally, 2018). The in vitro “silencing” of the intrinsic GHRH in the most common human malignancies, virtually stopped their proliferation, which in turn it was restored after supplementation of exogenous GHRH(Barabutis & Schally, 2008b). Currently, the development of those compounds is advanced by prominent leaders on the field of endocrinology and oncology(Mendoza-Valdes, 2015).

Vascular hyperpermeability is associated with carcinogenetic and inflammatory processes. Moreover, it is the hallmark of the most lethal cardiovascular complications, such as the Acute Lung Injury (ALI) and its most severe form, namely the Acute Respiratory Distress Syndrome (ARDS)(Barabutis, Verin, & Catravas, 2016). Unfortunately, ARDS remains one of the main cause of morbidity and mortality in hospitalized patients. The deteriorating consequences of that syndrome are irreversible, since the respiratory function of those patients in need virtually collapses(Nanchal & Truwit, 2018). The pulmonary endothelium becomes dysfunctional, and these patients must be supported by mechanical ventilators due to severe hypoxemia. Due to all those events, the exploration of new therapeutic avenues to fight this devastating pathology, it is of the highest priority in the medical field(Thompson, Chambers, & Liu, 2017).

Hsp90 inhibitors have been long associated with anti-cancer properties, and ongoing trials will evaluate the therapeutic value of those drugs towards malignancies(Barabutis & Catravas, 2015). Those compounds stop the maturation of the plethora of Hsp90 client proteins (kinases, transcription factors), which are key players in the aggression of harmful cellular events(Barabutis et al., 2013). Further, Hsp90 inhibitors exercise the ability to recruit P53, which in turn orchestrates a battery of protective and anti–inflammatory responses against toxin-induced violations of the cellular homeostasis. To the best of our knowledge, on 2015 we were the first to report that P53 expression levels are crucial for the maintenance of the vascular barrier integrity. P53 silencing by siRNA that specifically targets this transcription factor resulted to the severe weakening of the human vascular barrier function.

The transendothelial resistance of those transfected monolayers was dramatically reduced. A similar result was exerted by using the P53 inhibitor Pifithrin. Those human lung microvascular endothelial cells became much more susceptible to LPS than the relevant control groups. The nutlin - induced P53 induction delivered the opposite effect, thus supported the vascular structure and protected those cells against the barrier destructive and LPS-induced inflammatory events(Barabutis et al., 2015). Indeed, both P53 inhibitors (MDM4 and MDM2) were affected by this toxin, in a manner that resulted to reduced intracellular P53 levels. Similarly to nutlin, Hsp90 inhibitors increased P53 abundance, probably due to the elevated expression of the Hsp90/P53 complexes in the intracellular niche(Barabutis et al., 2015).

In continuation of those studies, we explored the effects of P53 in major regulators of the actomyosin cytoskeleton, namely the Rac1 and RhoA GTPases. The former is responsible for the deactivation of the actin severing activity of cofilin, and the latter is the major transducer of robust inflammatory processes that enacts the formation of F actin fibers. Our observations suggested that P53 can direct the activities of both GTPases, to fight inflammation and support the pulmonary endothelium. It enhances the Rac1 activity to increase the barrier integrity; and/or suppress the RhoA activity to prevent the formation of stress actin fibers, as reflected in the levels of MLC2 phosphorylation(Barabutis, Dimitropoulou, Gregory, & Catravas, 2018).

Since Hsp90 inhibitors induce P53 expression, we decided to test whether those compounds can affect the phosphorylation levels of P53, and whether those post translational modifications have effects on the LPS – induced mice inflammation. Our results indicate that Hsp90 inhibitors suppress inflammation via the prevention of the LPS- inflicted P53 phosphorylation(Barabutis, Uddin, & Catravas, 2018). Those events, in combination with our previous observations in “Super P53” mice”(Barabutis, Dimitropoulou, et al., 2018), conclude that P53 may serve as the “Fire Fighter” of the endothelium. It enhances microvascular barrier integrity; and suppresses the prevalence of inflammation in the pulmonary microvasculature(Barabutis, Uddin, et al., 2018).

In our opinion, P53 represents an underappreciated factor in the field of vascular biology and pharmacology. Further exploration of the activities of that molecule in the vasculature (i.e. use of advanced models of genetically modified mice, exploration of P53 modulators in vascular permeability), will reveal important properties of that transcription factor towards the regulation of the vascular hemostasis and the protection of the lungs against harmful and toxic insults. Most importantly, the discovery of those new regulatory networks will provide new information that may lead to the development of improved pharmacological agents towards inflammatory pathologies that are currently endanger the lives of multifarious individuals worldwide.