Spontaneous Reversion of the Angiogenic Phenotype to a Nonangiogenic and Dormant State in Human Tumors

Abstract

The angiogenic switch, a rate-limiting step in tumor progression, has already occurred by the time most human tumors are detectable. However, despite significant study of the mechanisms controlling this switch, the kinetics and reversibility of the process have not been explored. The stability of the angiogenic phenotype was examined using an established human liposarcoma xenograft model. Non-angiogenic cells inoculated into immunocompromised mice formed microscopic tumors that remained dormant for ~125 days (vs. <40 days for angiogenic cells) whereupon the vast majority (>95%) initiated angiogenic growth with second-order kinetics. These original, clonally-derived angiogenic tumor cells were passaged through four in vivo cycles. At each cycle, a new set of single-cell clones was established from the most angiogenic clone and characterized for in vivo for tumorigenic activity. A total of 132 single-cell clones were tested in the 2nd, 3rd and 4th in vivo passage. Strikingly, at each passage, a portion of the single-cell clones formed microscopic, dormant tumors. Following dormancy, like the original cell line, these revertant tumors spontaneously switched to the angiogenic phenotype. Finally, revertant clones were transcriptionally profiles and their angiogenic output determined. Collectively, these data demonstrate that the angiogenic phenotype in tumors is malleable and can spontaneously revert to the non-angiogenic phenotype in a population of human tumor cells.

Tumor dormancy in bone

“…SW872 liposarcoma tumors found that revertant clones that remained dormant and did not grow into large tumors when implanted subcutaneously had much higher expression of CXCR4 than the angiogenic clones, 139 supporting a potential pro-dormancy role for CXCR4 that may be dependent upon the vasculature. In contrast, cells isolated from breast cancer patient-derived xenografts (either in the orthotopic tumor or from lung metastases) that exhibited undetectable CXCR4 expression were more frequently non-proliferative.…”

“…138 Interestingly, SW872 liposarcoma tumor cells that had an angiogenic phenotype in vivo, meaning they have the capacity to promote angiogenesis and tumor progression, can transition into a non-angiogenic phenotype when injected into a different mouse, and these dormant non-angiogenic cells were able to then spontaneously switch back to an angiogenic phenotype, suggesting that angiogenic dormancy may be a fluid state. 139 "Revertant clones" that were non-angiogenic but had been derived from an angiogenic clone were found to have decreased VEGF expression but increased TSP1 expression, supporting the fact that they are not able to induce angiogenesis. TSP1 is an angiogenesis inhibitor 140 and matricellular protein known to interact with dozens of factors, leading to a wide variety of effects on cellular activity by aggregating, sequestering, or activating many proteins, and modifying the extracellular matrix.…”

“…Abundant CXCR4 expression on prostate cancer cells also mediates interactions with α 5 and β 3 integrins, which may make CXCR4 another attractive therapeutic target to mobilize cells and increase their chemosensitivity. Comparison of “angiogenic” and “revertant” SW872 liposarcoma tumors found that revertant clones that remained dormant and did not grow into large tumors when implanted subcutaneously had much higher expression of CXCR4 than the angiogenic clones, supporting a potential pro‐dormancy role for CXCR4 that may be dependent upon the vasculature. In contrast, cells isolated from breast cancer patient‐derived xenografts (either in the orthotopic tumor or from lung metastases) that exhibited undetectable CXCR4 expression were more frequently non‐proliferative .…”

“…A set of 19 microRNAs that govern the phenotypic switch of dormant MDA‐MB‐436 breast cancer cells, T98G glioblastoma cells, KHOS‐24OS osteosarcoma cells, and SW872 liposarcoma cells to fast‐growing angiogenic tumors has also been identified by testing for the expression of 378 microRNAs in dormant or angiogenic/fast‐growing tumors of each cell line with high‐throughput real time qRT‐PCR based miR expression analysis . Interestingly, SW872 liposarcoma tumor cells that had an angiogenic phenotype in vivo , meaning they have the capacity to promote angiogenesis and tumor progression, can transition into a non‐angiogenic phenotype when injected into a different mouse, and these dormant non‐angiogenic cells were able to then spontaneously switch back to an angiogenic phenotype, suggesting that angiogenic dormancy may be a fluid state . “Revertant clones” that were non‐angiogenic but had been derived from an angiogenic clone were found to have decreased VEGF expression but increased TSP1 expression, supporting the fact that they are not able to induce angiogenesis.…”

Toward understanding of the role of reversibility of phenotypic switching in the evolution of resistance to therapy

“…selection unit) in the evolution of cancer cells, nevertheless with non-genetic heterogeneity of its eventual clone being the crucial adaptive trait at cancer-relevant, instead of proximate timescale. Phenotypic plasticity confers to cellular tissues important properties, such as the ability of cancer cells to escape targeted therapy by switching to an alternative phenotype [25,26,27,28,29,30]. It motivates the effort to stimulate (or prevent) specific phenotype switching purposefully as a therapeutic strategy [31], which requires deep understanding of the phenotype switching causation.…”