1. Introduction
Various progeroid syndromes have been linked to genome instability, indicating an important causal role for DNA damage accumulation in the aging process and in the development of age-related pathologies. Aging is a characteristic of practically all biological organisms and is characterized by a steady deterioration in the function of various cells and organs. According to a World Health Organization (WHO) report, by 2030, one in every six people on the planet will be 60 years old or older [
1]. Cellular aging is a dynamic and versatile process. This response is initiated by various intrinsic and extrinsic triggers, including oncogenic activation, oxidative and genotoxic stress, telomeres, DNA damage, mitochondrial dysfunction, irradiation, and chemotherapeutic agents. This response is linked to various simple and complicated processes, such as embryogenesis, tumor suppression, organismal aging, and tissue repair [
2,
3], indicating the importance of DNA damage accumulation during the aging process.
An influential discovery in cellular senescence was that it is a symbol of aging. In 1961, Hayflick and colleagues stated that human fibroblasts demonstrate a finite capacity for cell division before irreversible growth arrest, known as replicative senescence, leading to the hypothesis that aging is caused by cells losing their proliferative ability progressively, which is important to replace damaged or aging cells that naturally accumulate over time. Several studies have been conducted over the last decade to demonstrate the role of excessive or accumulated aging cells in promoting organismal aging and related disabilities [
3]. Accumulation of aging cells may disturb the normal cellular environment, ultimately generating abnormal responses and resulting in various aging-related diseases. These diseases include cancer, cardiovascular diseases, and neurodegeneration. Multiple factors are associated with anti-aging, which encourage aging and aging-related diseases [
2,
3].
However, studying the factors involved in cellular aging, either directly or indirectly, may provide a pathway to develop anti-aging therapies. The theory of this review is based on an in-depth study of the pathological processes involved in identifying targets. When a target is indicated or marked, it is relatively easy to identify substantial therapies for such pathological processes or conditions.
DNA is a complex molecule that contains all the information necessary to develop and maintain an organism. Defects in DNA repair accelerate human aging and, in return, these aging cells produce the senescence-associated secretory phenotype (SASP), which has been considered a focal driver of age-related dysfunction [
4]. Therefore, mediating SASP may promote the repair of damaged tissues by correcting the DNA damage response, either to delay aging or to provide protection to aging cells for further damage. Saying good bye to this world or to loved ones is a painful and heart-breaking process. This is the point at which researchers and scientists are unable to bring their loved ones back.
In this review, we explain and highlight natural products and synthetic compounds that are connected either directly or indirectly to the DNA damage response (DDR)or SASP in relation to aging.
Bioactive compounds from natural or synthetic sources have advantages such as diverse structures, affordability, availability, and effectiveness with multiple targeted deliveries to mediate different signaling pathways of aging with the ability to correct the underlying signaling pathway or pathological condition. Therefore, the main aim of this paper is to highlight the hidden factors involved in the prevention, delay, elimination, and replacement of aging cells, or encouraging the production of young cells. Studying these factors and their underlying mechanisms may energize researchers to develop or explore novel anti-aging approaches that can bring more happy moments or add healthier and more valuable years to human life.
4. Discussion
The steady deterioration of the functions of various cells and organs leads to early aging. Reports from the World Health Organization have shown that by 2030, one out of every six people will reach 60 years or older. Cellular aging is a versatile process initiated in response to various intrinsic and extrinsic triggers [
168]. These responses are interconnected and involve several simple and complex processes. Reversing aging is impossible; however, the aging process can be delayed or corrected by adopting certain measures. For example, a better understanding of the aging signaling pathway or the identification of aging triggers and how aging can be delayed [
39]. The main purpose of this paper is to provide a clear and in-depth review of the possible signaling pathways involved in aging to identify aging targets. Once ageing targets have been explored or studied in depth, researchers can readily develop anti-aging measures.
From the above studies, we now know the most important target, which is the correction of DNA damage in aged cells or modification of the programming of aged cells, such as SASP regulation and modification. Based on this concept, we sought to explore possible means and anti-aging compounds that might overcome the aging process to live a long time along with a healthy lifestyle.
By assessing various recent studies regarding aging signaling networks via this review, we have certain suggestions and opinions for other researchers and scientists, and we hope that our opinions and conclusions will be very helpful for identifying and exploring potential therapeutic anti-aging approaches.
DDR can be controlled or overcome by regulating p53, p16, and ARF [
17]. By monitoring these factors via control of corrupted information, transmission of corrupted information into next-generation cells could be avoided, thus a delay in aging can be achieved. This ultimately leads to the production of newly generated healthy cells without any dysfunction and the production of young cells. Therefore, p53, p16, and ARF may act as targeted factors in the development of anti-aging approaches.
To find anti-aging therapeutic approaches at the cellular level by studying signaling networks deeply, DDR can be controlled by modulating telomere shortening via telomerase activation [
169]. Therefore, identifying potential agents that act on telomeres shortening, novel anti-aging approaches can be introduced [
170]. Introducing agents that can boost NAD+ levels may boost surtuin activity, which functions as an anti-aging gene and novel anti-aging therapy. As an oncogene,
PTEN loss induces aging; therefore, by introducing approaches to reduce the loss of, or damage to,
PTEN, aging can be delayed or controlled. Another approach to prevent or delay aging is the identification of potential histone deacetylase inhibitors that can act as senolytic agents [
171]. The current review reveals that no single compound has been detected targeting the DDR, suggesting that a massive field of aging research targeting the DDR signaling pathway for identifying anti-aging therapeutics in nature needs to be explored.
Activation of the SASP is a self-motivated process that causes aging; therefore, research can be moved towards the exploration of SASP inhibitors [
172]. DDR activation is a rapid response, whereas SASP establishment is gradual; therefore, continuous treatment is required to control SASP activation. In addition, SASP and DDR signaling are associated with the promotion of aging. Studies have also indicated that GATA4 interacts with DDR signaling and is involved in aging pathology [
17]. Targeting GATA4 has been predicted to deliver novel approaches against aging. This review also reveals that exploring JAK/STAT inhibitors could provide new insights into aging targets. Inhibition of BRD4 reduces SASP activation and suppresses senescence. HMGB1 and HMGB2 are key regulators of SASP expression [
173]. Due to the complex nature of SASP modulation, irregular stimulation of the cGAS-STING mechanism has been found to be interconnected during aging.
This review highlights the importance of therapeutic agents from natural and synthetic sources in lifespan extension, as shown in the figures targeting SASP and DDR. Combinations of drugs and use alone may bring some potential effects in delaying the ageing process via different signaling pathways targeting various pharmacological targets. For example, A-1331852, A1155463, and EF24 regulate SASP signaling by targeting BCL-2, BCL-X, BCC-W, and quabain. Azithromycin, SB203580, simvastatin, cortisol, canakinumab, siltuximab, ankinra, rilonacept, and tocilizumab act on SASP signaling pathways via targeting IL-1α,IL-6, IL-8, and IL-1β. RD001 acts on mTOR and mTORCI in the SASP signaling pathway. Another interesting factor highlighted in this review is that no structure-activity relationship has been studied against the aforementioned compounds for anti-aging drugs. Notably, once a potential pharmacological compound is identified against any pathological conditions, scientists always try to find their derivatives either from natural resources or via synthetic pathways. For example, EF24 is a novel derivative of curcumin with significant identified roles such as anti-tumor, anti-bacterial, and anti-inflammatory effects [
174]. Thus, curcumin and its derivatives may have greater therapeutic potential. A-1331852 and A1155463, BCL-XL inhibitors, indicate that other already identified BCL-XL inhibitors can be explored and investigated to identify hidden potential compounds for anti-aging. In addition, already identified compounds provide clues and direct methods for searching for new compounds sourced from natural resources or synthetic pathways based on structure-activity relationships.
One major drawback in the development of anti-aging drugs [
175] is the complex and continuous signaling pathways in ageing, which cannot be arrested. Also, ageing is not a disease that needs to be cured, but a long, continuous, natural, and irreversible process that is difficult to measure and identify as an anti-aging target. Experimental validation is a major challenge in the development of anti-aging drugs. Negative thoughts and emotions during stress can lead to poor health with rapid ageing that ultimately represent a considerable challenge to developing anti-aging measures. Ethical concerns that could arise from cultural and economic perspectives are another obstacle to the development of anti-aging medicines [
175]. To overcome these challenges, in-depth studies are required to identify potential targets for delaying the aging process. As the target is approached, exploring new therapies becomes easy. To eliminate negative thoughts and emotions, one should have a healthy diet and keep engaged in precious moments with family members, which can bring about a happy change in life. Strong social support and good friends’ networks can relieve stress and ultimately delay aging. Economic impacts and social changes bring about drastic changes in delayed aging. Better socioeconomic circumstances delay the aging process, as better living conditions improve health standards and ultimately lead to less spending on healthcare expenditure towards the terminal stages of an individual’s life. Therefore, it is recommended to focus on a healthy lifestyle without stressing on reducing the financial burden during the peak years of life.
The evidence from the above literature review revealed that almost all studies identified were conducted to find anti-aging targets and their detailed signaling networks, such as the DDR and SASP signaling networks, as shown in
Figure 3. However, there is also a need to explore bioactive agents from different resources, such as natural and synthetic products, and already recognized potential drugs for different diseases that need to be tested for anti-aging purposes. Recognizing that any potential anti-aging compound will not bring happiness to the world belonging to all age groups, but also boosts the pharmaceutical industries, biotech companies, and investors in terms of revenue. Aging is a very complex mechanism and identifying aging models that match human models is quite challenging for researchers. Therefore, future studies should focus on the identification of anti-aging agents. This highlights the importance of our literature review, as it provides updated information and analysis surrounding the development of anti-aging products.