For decades, oncological research represented a complex combination of numerous, fragmented genetic mutations and heterogeneous cellular anomalies. Scientists attempted to find fundamental patterns within these extensive databases and to establish a unified hierarchy of the pathological process. The situation changed in principle in the year 2000, when a paper published by Douglas Hanahan and Robert Weinberg grouped the previously existing informational dispersion into clear conceptual principles. The authors presented the theory of six “hallmarks,” which described those acquired functional capabilities that allowed a tumor to gain the means for proliferation, invasion, and therapeutic resistance. This systemic view made it clear how a tumor achieves full control over the organism’s homeostatic mechanisms and what multi-step evolutionary path it traverses to gain biological autonomy.
Twenty-six years later, this concept has been further enriched and strengthened from a scientific standpoint. The modern multi-dimensional model, along with nine primary characteristics, integrates five enabling factors, the complex structure of the tumor microenvironment (TME), and systemic interactions with the entire organism. Such a conceptual evolution is of fundamental importance, since cancer remains one of the primary causes of mortality globally, while the effectiveness of treatment often decreases due to adaptive resistance.
The updated model demonstrates the universal pathophysiological mechanisms characteristic of various forms of tumors, which pushes medicine toward the introduction of multi-targeted combination therapies.
The Initial Six Hallmarks
The six traits identified by Hanahan and Weinberg unite those biological strategies through which a cancer cell differs in principle from a normal one. These signs appear step-by-step in the process of tumor formation, as the cell “accumulates” mutations and adapts to the environment.
This evolutionary process, first of all, begins with the loss of growth control. If a normal cell obeys external stimuli, a cancer cell maintains proliferative signaling constantly and by itself. This autonomy is often determined by the activation of oncogenes such as KRAS (in 30% of cases) and MYC (in 40% of cases), which gives the tumor the possibility of immediate multiplication. However, only the stimulation of growth is not enough; in parallel, the tumor must also turn off those natural “brakes” that regulate the cellular cycle. This function is performed by the inactivation of growth suppressors, including the TP53 and RB genes, which allows the cell to divide without restraint.
When a cell reaches such a pathological state, the organism attempts to destroy it through programmed death, or apoptosis. It is critical for the tumor’s survival to gain resistance to this mechanism, which it carries out by increasing the level of anti-apoptotic proteins (for example, BCL-2) and by blocking the protective function of TP53. In this way, the cell not only divides constantly but also gains the ability to defend itself from death.
This process is accompanied by an increase in replicative potential; normal cells have a clear limit of division due to the shortening of telomeres, after which they enter the phase of senescence (aging) or crisis. Most tumors overcome this barrier through the reactivation of telomerase and the mutation of the TERT promoter. Others choose an alternative way of lengthening telomeres (ALT) through genetic recombination.
As the tumor mass increases, cells face a deficiency of oxygen and nutrients. The tumor responds to this crisis by stimulating angiogenesis—it forces the organism, through the secretion of VEGFA and ANGPT2, to create new blood vessels that will directly supply the tumor focus.
Finally, the process moves into the most aggressive phase—invasion and metastasis. Cells break through tissue barriers, reach the blood circulation system, and colonize distant organs. Despite the fact that one specific “metastasis gene” does not exist, TP53 mutations and chromosomal instability significantly accelerate this process. This is also facilitated by pre-metastatic niches, which “prepare” distant tissues in advance to receive cancer cells.
Metabolic Reprogramming and Immune Tolerance
In the 2011 update, Hanahan and Weinberg demonstrated even deeper layers of cancer biology. Technological progress allowed scientists to establish that a cancer cell not only grows independently, but also radically changes its own energetic balance.
In this context, the seventh hallmark implies the dysregulation of cellular metabolism. Normal cells produce energy effectively through oxidative phosphorylation, while proliferative cancer cells choose hybrid strategies. They consume glucose through aerobic glycolysis, the so-called “Warburg effect,” in order to use energy and biosynthetic resources for rapid growth. At the same time, cells carry out the importation of glutamine and lactate from the tumor microenvironment (TME) on the principle of metabolic symbiosis. During nutrient stress, they turn to the mechanism of autophagy for the purpose of recycling components. Such metabolic plasticity is determined by oncogenes and signals received from the microenvironment (including hypoxia).
The eighth hallmark is the ability of cancer to avoid immune destruction. In physiological conditions, T-cells carry out the selective elimination of the tumor by identifying neoantigens, to which is added the cytotoxic destruction of targets having low MHC-expression by NK-cells. Despite this, with long-term and continuous exposure to the antigen, the tumor determines the functional decline, or exhaustion, of T-cells. In parallel, the tumor microenvironment suppresses the immune response through specific cytokines, regulatory T-cells, and myeloid suppressors. The importance of this mechanism was confirmed by immune checkpoint inhibitors (anti-PD1/PD-L1, anti-CTLA4), which brought a real revolution in the treatment of melanoma and lung cancer.
During this same period, two enabling characteristics were also added to the concept. The first of them implies the increase of genomic instability, which represents the basis for acquiring new traits through the accumulation of mutations and chromosomal anomalies. The second factor is tumor-promoting inflammation, which creates the favorable biological conditions necessary for cancer progression.
The Ninth Hallmark: Phenotypic Plasticity
In 2022, single-cell profiling methods revealed such scales of intra-tumor heterogeneity that are not explained only by genetic mutations. This discovery laid the foundation for the recognition of phenotypic plasticity as the ninth independent hallmark. For its part, this ninth sign implies the dynamic change of their own biological state by cancer cells.
This process is divided into two main directions. During “intra-lineage” plasticity, cells undergo de-differentiation and return to the initial, progenitor, or cancer stem cell (CSC) state. In parallel, there exists trans-lineage plasticity, which includes hybrid forms of epithelial-mesenchymal transition (EMT). Such transformations add the ability of invasion to cells and significantly strengthen resistance toward drugs.
Structural shifts of this kind are facilitated by the loss of TP53 function in the genome, while epigenetic changes and transcriptional factors direct the process itself. It is noteworthy that such plasticity is not exhibited only by tumor units; other components of the microenvironment, including tumor-associated macrophages (TAMs), actively participate in it. Precisely this biological flexibility represents that fundamental difficulty which underlies therapeutic resistance in all types of cancer.
The Four-Dimensional Framework
In conditions of an increasing informational flow, to maintain conceptual clarity, the modern scientific view groups the accumulated knowledge into four hierarchical dimensions. This structure demonstrates the biological complexity of the tumor in a multi-layered context:
First Dimension: Unites the nine fundamental hallmarks.
Second Dimension: This level includes the five enabling characteristics. The mentioned factors help the tumor in acquiring those genetic and epigenetic changes that determine the formation of the aforementioned fundamental signs.
Third Dimension: Describes the dynamic composition of the tumor microenvironment (TME). Here, emphasis is placed on those non-tumor cells and structures that serve the practical realization of pathological processes.
Fourth Dimension: On the highest step of the hierarchy, systemic interactions occurring with the entire organism are considered. This dimension studies the influence of the tumor on distant organs, metabolism, and the overall homeostasis of the organism.
Enabling Characteristics: Catalysts for the Formation of Tumor Hallmarks
In the process of tumor progression, there exist five fundamental traits that significantly simplify the acquisition of hallmarks and their persistence:
Loss of Genomic Integrity: Point mutations, amplifications, deletions, and extrachromosomal DNA (ecDNA) generate genetic chaos in the cell. This process is also strengthened by chromothripsis (simultaneous shattering of chromosomes) and full genome doubling. According to the COSMIC database, today more than 750 genes are classified as oncogenes or suppressors. The role of the telomeric crisis is also noteworthy, which increases genetic diversity until the “immortality mechanism” adds fixed stability to it.
Non-mutational Epigenetic Reprogramming: Without direct structural change of the DNA, the modification and methylation of histones fundamentally change the expression of genes. This mechanism directs phenotypic plasticity and gives rise to some pediatric tumors that lack classical driver mutations.
Tumor-Promoting Inflammation: Tumors often mimic “wounds that do not heal,” where macrophages and neutrophils are mobilized through damage signals (DAMPs). This inflammatory environment simultaneously stimulates five different signs, including proliferation and angiogenesis. At the same time, KRAS and TP53 mutations facilitate the active mobilization of myeloid helper cells.
Innervation: Modern research confirms that nerve fibers penetrate deeply into the tumor and form direct synapses with cancer cells. Secreted neurotransmitters facilitate the survival, invasion, and avoidance of the immune system by the tumor. To prove these connections, cancer neuroscience uses the method of optogenetics. For their part, tumors stimulate the intensive growth of axons and direct the functional reprogramming of neurons.
Polymorphic Microbiome: The microbiome of the intestine, skin, or directly the internal microbiome of the tumor differs sharply among individuals. Certain bacteria facilitate the suppression of the immune system, which leads us to resistance toward ICI-therapy. In contrast, fecal transplantation significantly improves the treatment outcome. Dysbiosis, for its part, further increases the intensity of mutational processes and inflammation.
The Tumor Microenvironment
In the modern view, a tumor functions as a complex pathological organ that uses normal cells for its own interests. In this environment, each component serves the progression of the disease in its own way:
Tumor Cells: They represent the genetic core of the disease, carry driver mutations, and contain reservoirs of cancer stem cells (CSC). The latter oppose therapeutic intervention with a state of dormancy: while chemotherapy destroys actively multiplying cells, these “sleeping” cells temporarily stop dividing, maintain viability, and determine the regrowth of the tumor after the completion of treatment.
Endothelial Cells and Pericytes: They create structurally defective blood vessels that supply the tumor with growth factors. At the same time, this vascular network restricts the infiltration of T-cells and significantly simplifies metastatic processes.
Cancer-Associated Fibroblasts (CAFs): They are formed from local fibroblasts, endothelial-mesenchymal transition (EndMT), or bone marrow stem cells. Among them, myofibroblastic CAFs thicken the extracellular matrix, while inflammatory CAFs secrete specific cytokines and stimulate metabolic activity.
TAMs and TANs: Tumor-associated macrophages and neutrophils are characterized by high plasticity; they are transformed from an anti-tumor state to its enabling phase. For example, M2-type macrophages direct angiogenesis and invasion, while myeloid-derived suppressor cells (MDSCs) actively suppress the function of T-lymphocytes.
Immune Cells: Regulatory T-cells (Tregs), dysfunctional dendritic cells, and NK cells lose their original purpose in the tumor microenvironment. Instead of a defensive reaction, they ensure the formation of an immunosuppressive environment and the inviolability of the tumor.
Neurons: Neurons form direct synapses and adhesive connections with tumor cells, after which, by secreting neurotransmitters, they give out signals causing traits characteristic of cancer.
Senescent Cells: Aged cells of any origin secrete SASP-factors, which paradoxically kindle proliferation, angiogenesis, and inflammation in neighboring units.
The destructive contribution of the mentioned cells is also strengthened by such physical factors as the high density of the matrix, hypoxia, and the acidity of the environment.
Systemic Interactions — Cancer as a Disease of the Whole Organism
An oncological pathology is not limited only to a local tumor. Modern medicine considers it as a systemic process that interacts closely with the overall biological background of the organism:
Aging and the Immune Background: The aging process causes the accumulation of mutations in the organism and a universal decline of immunity. At the same time, the number of senescent cells increases, which secrete pro-tumor SASP-factors. These changes represent a kind of biological basis for the hallmarks of cancer and prepare a favorable ground for the development of the disease.
Metabolic Influence and Obesity: Obesity increases the level of leptin in the organism and suppresses the synthesis of adiponectin. The mentioned hormonal imbalance directly stimulates proliferation, angiogenesis, chronic inflammation, and metabolic shifts.
Distant Communication: A dysregulated microbiome spreads a distant influence on various systems of the organism, while innervation and vascularization create a unified informational and transport network between the tumor and distant organs.
Clinical Manifestations: The mentioned complex interactions are often manifested in the sharp wasting of muscle mass and in the general weakening of the immune system. These destructive processes are further exacerbated by environmental toxins (including tobacco smoke) through the strengthening of inflammatory reactions and the escalation of processes.
Achievements of the 2020s
This latest stage unites those transformative discoveries that raised the perception of a tumor from a “genetic error” to a complex, multi-layered ecosystem. The main achievement of the 2020s turned out to be the unification of these isolated signs into a single, interactive network. If previously we considered each characteristic (for example, innervation or the microbiome) as an individual phenomenon, today science distinguishes the close hierarchical connections and functional intersections existing between them. However, the most important remains the confirmed strategies of “co-targeting” (simultaneous purposeful therapy), which create a real possibility for maintaining the therapeutic effect and long-term remission of the disease.
Against this background, the future of oncology is based on the synthesis of advanced technologies and personalized approaches. The management of the disease is now moving to predictive models, where single-cell multi-omics, spatial transcriptomics, and artificial intelligence-based digital pathology determine the behavior of the tumor and the expected response to treatment with high accuracy.
In this technological ecosystem, liquid biopsy gives a unique means for constant and non-invasive monitoring of processes. Future therapeutic strategies imply simultaneous purposeful impact on several independent signs of the tumor, which significantly reduces the probability of the formation of resistance toward treatment. A visible example of this approach is the combination of VEGF and immune checkpoint inhibitors (ICI), which ensures the normalization of blood vessels and the activation of T-cells in parallel mode. Future strategies envision the complex integration of immunotherapy and microenvironment reprogramming agents, while the use of bispecific antibodies significantly strengthens the effectiveness of dual molecular blockade.
Source: cell
