Our life starts from fertilization—the fusion of the sperm and oocyte. After this, a series of developmental processes take a single fertilized egg, via birth, to an adult organism consisting of many cells with various functions. Once this process is initiated, the name changes to an embryo, a fetus, a newborn, a kid and an adolescent. There is no point in pausing, but it keeps changing in morphology, size and functionality. After the first cue, no more cue is required but just nutrients to support growth. Once it reaches a point where there is no more change, we call it an adult and maturation.
Why do we NOT keep changing like an embryo or a kid? How do we know the form created is the final form? How is the end of developmental processes controlled?
I want to bring two examples of the end of product-producing processes as an analogy. The first one is manufacturing a car. What we need is a clear, detailed blueprint, all parts consisting of a car and all tools assembling the parts. The endpoint is super straightforward - when all components are assembled. The second one is a painting by an artist. A canvas, blushes and paints are probably needed. Some vision or idea before starting? How does an artist know or recognize when to end? Is there any difference between paintings of objects and abstracts?
How about ourselves? Are we like a car or a painting? How much detail is encoded in our genome? Why do we stop changing in shape and size (of course, with some exceptions…)?
Our bodies are not built like a car. If you look at your hand, it might look like the product of assembled pieces. However, that is not the case. The formation of hands starts as a bud, a group of cells forming a bump on the frank surface of an embryo. The bump gets bigger and sculpts itself. However, it is not like a sculpture trimmed from an original mass material but like pottery shaping a form. The final form is not assembled, but a balancing point stabilizes the form.
I capture the development of multicellular organisms as not a continuous single process but a series of discontinued events. Each event has a beginning and end. Two successive events sometimes occur simultaneously in a single embryo at the whole embryo level. However, at each local compartment level, the previous event properly ends before the subsequent event begins. The developmental process is stepwise progressive, not continuous and gradual.
Epithelial-mesenchymal interactions are essential for various organ/tissue development. Most organs, if not all, have multiple steps of epithelial-mesenchymal interactions, and they play a crucial role in organogenesis. Usually, their interaction is initiated as epithelial-mesenchymal transition or mesenchymal-epithelial transition, which is the process of making a new cell population from the original uniform population. Then, the two populations interact. This is the beginning. How does this end?
By physically separating them. Their interaction is biochemical communications between two cell types, epithelial and mesenchymal cells. They can be secretory and cell-surface molecules - long and short-range communications and direct contacts. By placing a barrier preventing communication, their interaction will end – like ECM deposition forming the basal lamina. This is a negative feedback loop. The initial activation of the interaction activates the production of the barrier. Then, the barrier terminates the interaction.
This mechanism will stabilize the process at the stable equilibrated state but not the static, fixed state. In some cases, cells in the equilibrated state could progress to the next stage and lose the original competency to interact with the other cell types. However, it is also possible that cells retain this ability and actively keep the boundary. In other words, the boundary actively suppresses the interaction. Because of this equilibrated state, the system can restore the original state when a perturbation like an injury occurs. We call this a repair.
The structure of tissues represents the equilibrated state. Healthy tissue structures look static in normal conditions but are in a dynamic equilibrated state. If the structure is destroyed, the epithelial-mesenchymal interaction can occur again. Then, it progresses until the barrier is set again.
Scientists use this property to culture cells isolated from tissues/organs. The organoid culture from various epithelial organs is releasing the cells from the structural equilibration. Then, the isolated cells are placed in an in vitro equilibrated condition. Passaging organoids are repeating the equilibration process.
After birth, multicellular organisms have an interesting communication system, inflammation. Inflammation is a communication system crossing the barrier, the tissue boundary. Inflammation is activated when damage occurs, or the pathogen invades. Usually, this happens in the surface layer, epithelial cells. Then, the emergency is sensed by tissue residential macrophages (TRMs) nearby. TRMs clear the environment together with the barrier. Because of the cleared barrier, the local epithelial-mesenchymal interaction is initiated and restores the normal environment.
How is inflammation initiated? Sensing damage like cellular debris from a dead cell or pathogens by pathogen pattern recognition motif receptors. These are the common causes of inflammation.
Then, how about viral infection? Viral infections like flu and COVID can cause inflammation. But they are hidden in a cell. How does a cell sense viral infection? There are two types of viruses: RNA retrovirus and DNA virus. A somatic cell can sense it through viral sensing mechanisms when infected. In our cells, DNA is stored in the nucleus and mitochondria, which are the organelles surrounded by their own membranes. DNA is not directly exposed in the cytoplasm except during the mitotic phase. DNA in the cytoplasm is recognized as foreign DNA. In the case of RNA retrovirus, RNA is reverse-transcribed to form proviral DNA in the cytoplasm. Cytoplasmic DNA is shattered by cytoplasmic DNA exonucleases and sensed by cGAS/STING dependent type1 IFN response. This initiates inflammation. The cell has two choices before the amplified virus lyses the cell to spread to other surrounding cells. Kill itself by apoptosis to be engulfed by macrophages. Or get killed by T-cells through presenting viral peptides on MHCs.
What is Cancer? That is the disease condition created by a false activation of viral sensing mechanisms in somatic cells. Because of compromised organelle functions due to high cell stress, DNA is leaked out in the cytoplasm from the nucleus and mitochondria. The cells sense their own DNA as a viral infection and cause inflammation. Inflammation recruits TRMs to destroy the barrier between epithelial and mesenchymal cells. This initiates epithelial-mesenchymal interaction, like wound healing. The fundamental problem is that the cells causing inflammation cannot be eliminated unless they kill themselves by apoptosis. If the cells cannot die by themselves, the inflammation continues. TRMs engage the inflammation to restore the environment. However, the cells initiating the inflammation cannot be cleared, and no foreign pathogenic antigen is presented. TRMs digest ECMs to facilitate remodelling. Thus, the barrier is ruined, and epithelial-mesenchymal interaction continues. Contrasting to normal wound healing, this will be an unhealed wound.
How do the cells get high stress? How does the normal environment start diversifying from normal? Mutations? Inflammation? Both can be the causes, and both can be the consequence of the other. But either alone is not cancer. Importantly, mutations do not directly cause inflammation. Inflammation does not directly cause mutations.
DNA replication and chromosomal segregation during cell division are the highest mutation risk conditions. The most critical mutations are ‘do not die’ mutations. If the process of apoptosis is compromised, the cell cannot die by itself in homeostatic turnover. But this is not cancer.
Disruption of the structural equilibrated state and the reactivation of epithelial and mesenchymal interaction by inflammation is sufficient for the cells to enter cell cycle. But this is not cancer.
Oncogenic mutations alone cannot override the structural equilibrated state. Clonal growth can be observed by modulating the frequency of entering cell cycle or cell survival, but this is not cancer.
The frequency of entering cell cycle contributes to the increased risk of errors in DNA replication and chromosome segregation. The cause of this could be a part of normal homeostasis, surrounding inflammation or intrinsic mutations. The consequence of this is further increased intrinsic mutations. However, none can induce intrinsic inflammation directly in the cell.
As I mentioned, getting malignant results from false activation of viral sensing mechanisms. How does this happen? The formation of lagged chromosomes can lead to micronuclei formation. The chromosome in the micronucleus might get exposed to the cytoplasm. When this happens, the chromosome will be highly rearranged, called chromothripsis, and activate the viral sensing mechanisms. The conflict between transcription and replication is also problematic. Resolving the stalled DNA replication fork can create R-loops RNA-DNA complex fragments. They will be released to the cytoplasm. Increased ROS production and mitochondria stress by specific mutations, accelerated proliferation or hypoxia can release mitochondria DNA into the cytoplasm.
Unlike proliferation, which can be facilitated by mutations and selection, intrinsic inflammation cannot be selected because of no proliferative and survival advantage. All stress-related challenges potentially slow proliferation and may reduce survival, but further errors and selection enable adaptation eventually. This suggests that recurrent mutations and CNAs are involved in the early clonal expansion or later adaptation to high stress. But not directly related to the cause of intrinsic inflammation, thus malignancy.
The identical cellular phenotypic changes can be caused by epigenetic, genetic (SNPs) or genomic (CNAs) changes. In this sense, malignant cells must deal with high stress and are highly opportunistic. They do not care which strategy is used but phenotypically adapt to the stressful environment. This means that all recurrent genetic/genomic events are involved in proliferation, survival and adaptation, but none directly causes malignancy.
Many current cancer drugs are aiming to kill malignant cells with high stress. Our current scientific scopes often focuses on how to apply high-stress to malignant cells specifically. However, killing cells with high stress means expecting the activation of intrinsic apoptosis cascades, often the first targets compromised in carcinogenesis. In addition, killing malignant cells with high stress is a double-edged sword. Although aiming to kill all malignant cells uniquely, high stress simultaneously facilitates adaptation in a proliferative population. Unfortunately, resistance is inevitable if eradication is not achieved.
Inflammation is the communication mechanism to recruit TRMs for clearance and reactivating the developmental epithelial-mesenchymal interaction for remodelling. This would be the reason some chronic inflammation progresses to cancer after long latency. After all, the beneficial effects of low-dose aspirin in epidemiological studies might have been pointing to these phenomena. Disengaging the epithelial-mesenchymal populations by suppressing the inflammation is the best reasonable strategy.