Fibroblasts in cancer: Unity in heterogeneity

Abstract

Tumor cells do not exist in isolation in vivo, and carcinogenesis depends on the surrounding tumor microenvironment (TME), composed of a myriad of cell types and biophysical and biochemical components. Fibroblasts are integral in maintaining tissue homeostasis. However, even before a tumor develops, pro-tumorigenic fibroblasts in close proximity can provide the fertile 'soil' to the cancer 'seed' and are known as cancer-associated fibroblasts (CAFs). In response to intrinsic and extrinsic stressors, CAFs reorganize the TME enabling metastasis, therapeutic resistance, dormancy and reactivation by secreting cellular and acellular factors. In this review, we summarize the recent discoveries on CAF-mediated cancer progression with a particular focus on fibroblast heterogeneity and plasticity.

Figure 1.. Heterogeneity in fibroblast function during homeostasis and cancer development

The heterogeneity principles are common to both normal fibroblasts and cancer-associated fibroblasts (CAFs) and occur at multiple levels; cellular level depending on the state/function of the fibroblast, microenvironment level depending on the communication with other cellular and acellular components, regional level depending on the residing tissue and its function and lastly at the organismal level encompassing the attributes of demographics, physiological status, and intersectionality. During normal homeostasis and healing after an acute injury, the quiescent fibroblasts become activated (activated fibroblasts) to mediate repair and eventually perish. In certain conditions such as desmoplasia, the activated fibroblasts can establish self-activating and feedback loop, which gives rise to their pro-tumorigenic capacities. Intrinsic or extrinsic stressors enables their transition into CAFs providing greater proliferative capacity. The principles above dictate if the quiescent fibroblast present in the normal tissues transition into activated fibroblasts, a phenomenon common to both wound repair in healthy tissues and transition to CAFs in a tumor microenvironment. Due to the inherent plasticity, the proliferative CAFs can quiesce and become tumor-restraining, but it remains unknown if they can completely revert back to their normal non-tumorigenic state. Within the tumor microenvironment (TME), CAFs communicate with each other (via autocrine signaling) and other stromal and tumor cells (in a paracrine manner) through growth factors, metabolites, extracellular vesicles (EVs) and chemokines while constantly shaping and reorganizing the extracellular matrix (ECM) to promote a tumor permissive environment.

Figure 2.. Heterogeneity in cancer-associated fibroblasts occurs at multiple levels

Cancer-associated fibroblasts (CAFs) comprise a complex and heterogeneous group of cells as a consequence of its diverse sources of origin. These cells respond to intrinsic and extrinsic cues in the tumor microenvironment (TME) that can be grouped into defined mechanisms permitting CAF activation. Multiple cell types within the TME can answer the cancer hijack call in initiating transition to CAFs through direct activation of resident fibroblast and stellate cells. Transdifferentiation of adipocytes, pericytes and smooth muscle cells; endothelial to mesenchymal transition (EndMT) of endothelial cells; epithelial to mesenchymal transition (EMT) of epithelial cells and recruitment and activation of mesenchymal stem cells are other mechanisms that can give rise to CAFs. CAFs show heterogeneity of molecular expression based on the differentiating markers of its progenitor state. Markers indicated in purple are shared by both CAF and the cell type of origin, whereas those indicated in green are unique to the cell type of origin. In addition, CAFs also show heterogeneity in their function and features based on their anatomical localization within the body.

Figure 3.. Functional heterogeneity in cancer-associated fibroblasts

Cancer-associated fibroblasts (CAFs) represent a heterogeneous cellular population within the tumor microenvironment. Single-cell RNA sequencing and multiplex imaging have revealed different CAF subtypes across multiple cancer types. These CAFs are divided into 4 main groups: iCAFs (inflammatory CAFs), myCAFs (myofibroblastic CAFs), apCAFs (antigen presenting CAFs) and vCAFs (vascular CAFs). The cellular function, plasticity and the heterogeneity in the molecular marker expression associated with these CAFs are indicated.

Figure 4.. Role of fibroblasts in carcinogenesis

The phenotype and functions of fibroblasts are in dynamic evolution along with tumor initiation and progression. During early stages of carcinogenesis, pro-tumorigenic fibroblasts can drive tumor initiation and promote proliferation of mutated cancer cells. These activated fibroblasts in close proximity to the tumor cells can then transition to cancer-associated fibroblasts (CAFs). During this initial phase of cancer development, the heterogeneous cancer-associated fibroblast (CAF) subpopulations, referred to as tumor-restraining CAFs, can potentially protect normal tissue against cancer invasiveness through the various indicated mechanisms. However, in advanced-stage disease, tumor cells and other stromal cells reprogram the CAFs via continuous crosstalk to promote a tumor permissive niche. This allows tumor cells to proliferate and grow through dynamic processes that includes ECM remodeling, growth factors, cytokines and extracellular vesicles secretion, metabolic reprogramming and creating an immunosuppressive tumor microenvironment (TME). The mechanical coupling between CAFs and cancer cells, permits local invasion and regional dissemination of tumor cells from the primary tumor site. As the tumor evolves, CAFs reorganize the TME by promoting angiogenesis and epithelial-to-mesenchymal transition of tumor cells enabling distal metastasis (shown here with lung as an example) following intra- and extravasation. These disseminated tumor cells also overcome immune surveillance by resident immune cells, and direct immune suppression. At the metastatic site, the disseminated tumor cells, in the face of contextual signals, either enter or exit dormancy. This is governed by a plethora of microenvironmental cues, encompassing the cellular, molecular, and physical factors converge to induce either stress-related or mitogenic signals to tumor cells. Upon reawakening, proliferating cancer cells then colonize the metastatic niche forming macrometastases, co-opting and recruiting local stromal cells to further support cell growth by indicated mechanisms. Subsequently, fibroblasts and CAFs continue to play important roles across the continuum of tumor progression and metastasis to visceral sites.

Figure 5.. Aging, cancer and fibroblasts

Fibroblasts have been shown to play tumor-promoting roles in cancer during aging. This can occur when fibroblasts become senescent, or prematurely aged, due to replicative stress or oxidative stress and DNA damage, or in response to therapeutic interventions or when fibroblasts age normally (epigenetic changes). Secreted changes that occur during senescence such as the acquisition of senescence-associated secretory phenotype (SASP) as well as the aged fibroblasts have been shown to promote tumor progression and therapy resistance by modulating the immune microenvironment, driving multiple changes including proangiogenesis, ECM remodeling, metabolic changes that also impact the outgrowth of tumor cells at metastatic sites. However, an important distinguishing factor between senescent fibroblasts and aged CAFs is that, whereas senescence promotes growth arrest, CAFs continue to proliferate. It remains unknown if aged CAFs resemble quiescent senescent CAFs or can proliferate via crosstalk with the surrounding tumor cells.