The Tumor Microenvironment

The tumor microenvironment comprises the tumor and surrounding supportive tissue called the stroma. It includes an extracellular matrix (ECM)—a complex network of macromolecules like collagen and fibrin—along with cellular components such as cancer-associated fibroblasts, immune cells, and blood vessels. This complex environment enables tumor growth and metastasis through constant interaction between cancer cells and stromal cells.

In cancerous tissue, the ECM structure and composition are modified to promote tumor progression. For example, in breast cancer, increased protease and oxidase expression degrades and linearizes dense collagen fiber networks, increasing tissue stiffness. This altered ECM facilitates tumor cell migration and allows cancer cells to escape immune surveillance and metastasize to new locations.

Cancer-associated fibroblasts (CAFs) remain constitutively active in tumors, unlike in normal wound healing. They secrete growth factors and vascular endothelial growth factors that trigger new blood vessel formation, providing oxygen and nutrients to rapidly dividing cancer cells. CAFs also remodel the extracellular matrix, recruit inflammatory cells, and directly stimulate cancer cell proliferation.

Tumor cells acquire mutations through genetic instability that allow them to evade immune detection. They induce immunosuppressive T-cells, which produce reactive oxygen species that inhibit lymphocytes and prevent them from killing tumor cells. These reactive oxygen species also increase mutation rates in tumor cells, facilitating rapid tumor progression and immune evasion.

Hypoxia is acute oxygen limitation experienced by innermost cancer cells in solid tumors as they grow. Cancer-associated fibroblasts respond by secreting vascular endothelial growth factors (VEGF) that trigger formation of new blood vessels around the tumor. This angiogenesis provides essential oxygen and nutrients to support the rapidly dividing cancer cells.

Effective tumor management requires targeting the microenvironment to disrupt the supportive environment enabling cancer progression. Therapies targeting VEGF synthesis, activity, or VEGF receptors have significantly improved patient survival. By creating a hostile microenvironment, treatments can prevent angiogenesis, reduce inflammation, and eliminate the stromal support cancer cells depend on for growth.

Adipose cells in the tumor microenvironment secrete over 50 different cytokines and chemokines that increase tumor initiation chances. Hypoxia in adipose tissue is highly pro-inflammatory and promotes tumor formation and maintenance. Excess adipocytes also increase blood estrogen levels, causing chronic low-grade inflammation that facilitates cancer development.