Melanoma: How does the skin environment influence its progression?

Melanoma is a skin cancer that develops from melanocytes, the cells that produce melanin. Although it is not the most common form of skin cancer, it is the leading cause of skin cancer-related mortality due to its high metastatic potential. Its incidence continues to increase, linked to exposure to UV rays and the aging population. Despite major advances in medicine over recent decades, melanoma remains difficult to control, particularly due to the development of treatment resistance.

These phenomena are not explained solely by the emergence of new mutations; they also rely on substantial cellular plasticity, enabling tumor cells to alter their phenotype in response to their environment.

Tumor cells are in constant interaction with their environment, which consists of stromal cells, immune cells, and the extracellular matrix, composed of collagen, fibronectin, and other structural proteins. The latter does not simply serve as passive support but actively influences cell behavior, proliferation, migration, and survival. In melanoma, the extracellular matrix undergoes profound remodeling during tumor progression and in response to treatments. An increase in the amount of collagen and a rearrangement of its fibers result in a well-documented phenomenon of tumor tissue stiffening.

This increased rigidity is now recognized as a major biophysical characteristic of aggressive tumors.

It is in this context that the recent work of DECKERT and his team on melanoma is set. They build on a simple yet long-overlooked hypothesis: the stiffness of tumor tissue may not affect all melanoma cells in the same way. More precisely, the ability of a cancer cell to interpret the mechanical constraints of its environment could depend on its phenotypic state. To test this idea, the researchers chose to experimentally recreate environments of controlled stiffness by culturing different populations of melanoma cells on collagen matrices of varying compliance. This approach revealed an initial finding: when faced with the rigidity of the extracellular matrix, not all melanoma cells respond equivalently.

To test the hypothesis that extracellular matrix stiffness does not affect all melanoma cells in the same way, DECKERT and his team studied human melanoma cell lines exhibiting either a melanocytic phenotype or a dedifferentiated phenotype. In order to isolate the effect of matrix stiffness, the cells were cultured on collagen matrices that were either very soft (less than 1 kPa), comparable to soft tissues, or very stiff (greater than 16 kPa), mimicking fibrotic tumor tissue.

The initial morphological observations revealed a striking difference between the two cellular states. When cultured on a rigid matrix, dedifferentiated cells exhibit a pronounced cellular spreading and an increase in their adhesion area, signs of an active interaction with their environment. In contrast, melanocytic cells maintain a relatively similar morphology whether cultured on a soft or rigid matrix.

This behavioral difference was confirmed during the functional analysis of cell proliferation. Using real-time monitoring, the researchers demonstrated that matrix stiffness strongly stimulates the proliferation of dedifferentiated cells. In contrast, a soft matrix induces a cell cycle arrest in these cells, accompanied by a reduction in the expression of key cell cycle progression proteins, such as phosphorylated retinoblastoma protein (P-Rb) or the transcription factor E2F1. This effect is far less pronounced, or even absent, in differentiated melanocytic cells.

To assess their invasive potential, cells were cultured for several days on soft or stiff matrices and then tested in Boyden chamber invasion assays coated with a “standard” extracellular matrix (Matrigel). The results show that prior exposure to a stiff matrix markedly increases the ability of dedifferentiated cells to invade Matrigel, while having little effect on melanocytic cells. These data suggest that matrix stiffness does not act merely transiently, but induces a bona fide pro-invasive functional program in dedifferentiated cells.

Another important point: these response differences are observed regardless of the cells' mutational status.

Whether they harbor mutations in the BRAF or MEK genes, which are common in melanoma cells, dedifferentiated cells consistently remain more sensitive to mechanical cues than melanocytic cells, demonstrating that the response to extracellular matrix stiffness is governed not by genetic alterations but by the cells’ phenotypic state.

Finally, the researchers investigated the consequences of this mechanosensitivity on the response to targeted therapies. By culturing cells on soft or stiff matrices and treating them with a combination of BRAF and MEK inhibitors (Dabrafenib and Trametinib), they showed that rigidity confers marked protection to dedifferentiated cells, which become less sensitive to apoptosis induction. Conversely, a soft matrix resensitizes these cells to treatment, with a significant increase in caspase-3 activation and cell death. Again, this effect is specific to dedifferentiated cells and is not observed in melanocytic cells.

The researchers concluded that the ability of melanoma cells to exploit the stiffness of their environment depends closely on their phenotypic state.

Dedifferentiation not only confers invasive properties and treatment resistance, it is also accompanied by a dependence on mechanical signals from the extracellular matrix. One can then ask the following question: by what molecular mechanisms do these cells sense collagen stiffness and convert this physical information into biological signals that promote their aggressiveness? This is precisely the point scientists sought to elucidate in their subsequent work.

Cells detect the mechanical properties of their environment through transmembrane receptors that interact with components of the extracellular matrix. While integrins are the most extensively studied matrix receptors in this context, attention here has focused on another family of collagen receptors: the discoidin domain receptors DDR1 and DDR2. These receptors are unique in that they are activated by fibrillar collagen and are involved in transmitting mechanical signals inside cells.

Researchers first analyzed the expression and activation of these receptors across the various cellular states of melanoma. They observed that, although several collagen receptors are expressed in dedifferentiated cells, only DDR1 and DDR2 exhibit a significant increase in activation when cells are cultured on a rigid matrix. This activation is much weaker, or even absent, in differentiated melanocytic cells, suggesting that DDR1 and DDR2 may play a role in the mechanosensitivity of dedifferentiated cells.

To address this question, the researchers blocked DDR1 and DDR2 in two different ways: by reducing their expression or by inhibiting their activity. In both cases, they observed that the cells lose their ability to respond to rigidity. They generate less internal force, their cytoskeleton becomes disorganized, and their matrix-adhesion structures are less developed. These results demonstrate that DDR1 and DDR2 are essential for activating the cell’s contractile machinery, which is required for the activation of mechanosensitive signaling pathways, particularly those involving the transcriptional coactivator YAP. When cells are subjected to high mechanical stress, YAP accumulates in the nucleus and triggers the expression of genes involved in proliferation, migration, and cell survival.

The researchers thus examined YAP transcriptional activity in melanoma cells cultured on matrices of differing stiffness. The results show that in dedifferentiated cells, a rigid matrix induces a marked nuclear translocation of YAP, accompanied by an increase in its gene expression. Conversely, on a soft matrix, YAP remains predominantly cytoplasmic and its transcriptional activity is strongly reduced. This response is much less pronounced in melanocytic cells, confirming that YAP activation by matrix stiffness is closely tied to the cells’ phenotypic state. Furthermore, when DDR1 and DDR2 are inhibited, this YAP activation almost completely disappears, even in the presence of a rigid matrix.

These studies thus allow the reconstruction of the following transmission chain: collagen stiffness is sensed by DDR1 and DDR2, relayed by cellular contractility, and translated into YAP activation.

However, one question remained unanswered: in melanoma cells, how does YAP remain active when it is normally rapidly degraded? This question led researchers to explore another level of regulation: protein stability and ubiquitination mechanisms.

Under normal conditions, YAP activation is transient: after fulfilling its role as a regulator of gene expression, this protein is rapidly targeted for degradation by the ubiquitin–proteasome system. This mechanism limits the duration of mechanosensitive signaling. However, in the context of a rigid extracellular matrix, such as that observed in melanoma, YAP remains persistently active. To understand this abnormal persistence, researchers moved beyond traditional signaling pathway analysis and investigated the mechanisms that control protein stability. They thus explored the role of deubiquitinases (DUBs), enzymes capable of removing the ubiquitin chains attached to proteins and protecting them from proteasomal degradation.

Specifically, dedifferentiated melanoma cells were cultured on soft or stiff collagen matrices and then lysed to analyze their enzymatic activity. The cell extracts were incubated with a modified ubiquitin probe designed to bind specifically to the active site of functional deubiquitinases. The labeled enzymes were then isolated and identified. This approach revealed that the activity of several DUBs depends on the stiffness of the extracellular matrix. Among them, the enzyme USP9X stands out: its activity increases significantly when cells are exposed to a rigid matrix, while it decreases in a softer environment.

Researchers then sought to understand what triggers the activation of the enzyme USP9X in a rigid environment. To do this, they blocked cytoskeletal contractility, meaning the cell’s ability to generate internal forces through actin–myosin coupling. When they inhibit myosin II, USP9X is no longer activated. This demonstrates that the activation of USP9X depends directly on the mechanical tension generated within the cell. Similarly, when DDR1 and DDR2 receptors are blocked, USP9X fails to activate, even on a stiff matrix.

Other tests carried out by scientists have shown that USP9X acts directly on YAP. When USP9X activity is blocked, YAP is rapidly tagged with ubiquitin and destroyed by the proteasome, even in the presence of a rigid matrix. Conversely, when USP9X is active, it removes these degradation marks, allowing YAP to accumulate within the cell.

Tissue stiffness therefore does more than activate YAP: it also prevents its degradation. USP9X stabilizes YAP and prolongs the mechanical response of dedifferentiated cells. This stabilization explains why these cells remain migratory, invasive, and treatment-resistant as long as their environment stays rigid.

To verify that this mechanism is not limited to observations in vitro, the researchers then evaluated the role of USP9X in an animal model of melanoma. They used bioluminescent melanoma cells, enabling real-time tracking of their behavior in vivo. These cells, either control or genetically depleted of USP9X, were injected intravenously into immunodeficient mice. This model allows the study of the earliest stages of metastatic dissemination, particularly the ability of tumor cells to exit the bloodstream and colonize the lungs.

Just hours after injection, bioluminescence imaging revealed a clear difference between the two groups. Cells lacking USP9X exhibited a dramatically reduced ability to extravasate from blood vessels and establish themselves in lung tissue. Longitudinal monitoring of the animals over nearly two months confirmed this initial finding: while mice injected with control cells gradually developed lung metastases, no detectable metastases were observed in animals receiving USP9X-deficient cells.

These results demonstrate that USP9X is indispensable for the early stages of migration and invasion necessary for metastasis formation, consistent with its role in stabilizing YAP.

The research team then sought to determine whether targeting USP9X could also interfere with the mechanically driven reprogramming induced by targeted therapies. Indeed, it is known that inhibition of the BRAF-MEK pathway, although initially effective, promotes extracellular matrix remodeling, increased tumor stiffness, and sustained YAP activation, contributing to relapse. To investigate this, BRAF-mutant murine melanoma cells were injected into immunocompetent mice, which were subsequently treated with either targeted therapy alone, the USP9X inhibitor alone (referred to as G9 in the study), or the combination of both.

As expected, administering the USP9X inhibitor alone only modestly slows tumor growth without inducing marked regression. In contrast, when USP9X inhibition is combined with targeted therapy, tumor relapse is significantly delayed and animal survival is improved.

Tumor analysis reveals that this combination prevents the fibrotic remodeling typically induced by BRAF and MEK inhibitors. Collagen networks are less dense, deubiquitinase activity is reduced, and the activation of YAP along with its target genes—implicated in tumor cell migration, invasion, and resistance—is markedly diminished. These findings demonstrate that blocking USP9X disrupts the self-sustaining mechanical feedback loop established by treatment by preventing YAP stabilization and the mechanical adaptation of tumor cells.

Above all, this work offers a new perspective on our understanding of melanoma.

They show that tumor cell adaptation does not rely solely on genetic or transcriptional mechanisms, but also on their ability to read their environment and to stabilize over time certain proteins such as YAP. By identifying USP9X as an intermediary between extracellular matrix stiffness, mechanosignaling, and therapeutic resistance, these studies pave the way for new strategies targeting the mechanisms that enable melanoma cells to adapt and persist in the body.

These findings also suggest that targeting the mechanical response of melanoma could complement existing therapeutic approaches. Inhibiting USP9X, in particular, appears to be an indirect means of limiting sustained YAP activation and restraining the mechanically driven reprogramming induced by targeted therapies. More broadly, this strategy could be relevant to other cancers developing in stiffened tissues, in which YAP plays a central role, such as lung cancer.

That said, this research presents certain limitations pointed out by the researchers themselves. The experiments are based primarily on simplified collagen matrices that do not capture the full complexity of the extracellular matrices produced in vivo by tumor and stromal cells. Moreover, the precise mechanisms linking tissue stiffness, collagen receptor activation, and the upregulation of USP9X activity remain only partially understood. Therefore, these findings represent an initial step that warrants future studies to investigate these mechanisms in more physiological settings and to determine the extent to which they can be translated into clinical practice.

• LAVERSANNE M. & al. Global burden of cutaneous melanoma in 2020 and projections to 2040. JAMA Dermatology (2022).

• OLIVIA A. G. & al. Skin-on-a-chip technology: Microengineering physiologically relevant in vitro skin models. Pharmaceutics (2022).

• DECKERT M. & al. USP9X is a mechanosensitive deubiquitinase that controls tumor cell invasiveness and drug response through YAP stabilization. Cell Reports (2025).

• DECKERT M. & al. Extracellular matrix stiffness determines the phenotypic behavior of dedifferentiated melanoma cells through a DDR1/2-dependent YAP mechanotransduction pathway. Research Square (2025).