3D cancer models are used as preclinical systems to mimic physiologic drug response. We provide evidence for robust changes of proliferation and metabolic capacity in 3D by systematically analyzing spheroids of colon cancer cell lines. Spheroids showed relative lower AKT/mTOR/S6K activities compared to cells cultured in 2D. We identified spatial alterations in signaling, as the level of phospho-rpS6 decreased from the spheroid surface to the center, closely recapitulating the tumor areas around vessels in vivo. These 3D-models displayed augmented anti-tumor response to AKT/mTOR/S6K- or MAPK-pathway inhibition compared to 2D. Inhibition of AKT/mTOR/S6K resulted in elevated ERK phosphorylation in 2D, whereas under these conditions MAPK-signaling was reduced in spheroids. Inhibition of the MAPK-pathway led to decreased AKT/mTOR/S6K-signaling in 3D but not in 2D. These data indicate a distinct rewiring of signaling in 3D and during treatment. Detached tumor cell clusters in vessels, in addition to circulating single tumor cells, play a putative role in metastasis in human cancers. Hence, the understanding of signaling in spheroids and the response in the 3D models upon drug treatment might be beneficial for anti-cancer therapies.

WNT2 acts as a pro-angiogenic factor in placental vascularization and increases angiogenesis in liver sinusoidal endothelial cells (ECs) and other ECs. Increased WNT2 expression is detectable in many carcinomas and participates in tumor progression. In human colorectal cancer (CRC), WNT2 is selectively elevated in cancer-associated fibroblasts (CAFs), leading to increased invasion and metastasis. However, if there is a role for WNT2 in colon cancer, angiogenesis was not addressed so far. We demonstrate that WNT2 enhances EC migration/invasion, while it induces canonical WNT signaling in a small subset of cells. Knockdown of WNT2 in CAFs significantly reduced angiogenesis in a physiologically relevant assay, which allows precise assessment of key angiogenic properties. In line with these results, expression of WNT2 in otherwise WNT2-devoid skin fibroblasts led to increased angiogenesis. In CRC xenografts, WNT2 overexpression resulted in enhanced vessel density and tumor volume. Moreover, WNT2 expression correlates with vessel markers in human CRC. Secretome profiling of CAFs by mass spectrometry and cytokine arrays revealed that proteins associated with pro-angiogenic functions are elevated by WNT2. These included extracellular matrix molecules, ANG-2, IL-6, G-CSF, and PGF. The latter three increased angiogenesis. Thus, stromal-derived WNT2 elevates angiogenesis in CRC by shifting the balance towards pro-angiogenic signals.

Spheroid-based cellular screening approaches represent a highly physiologic experimental setup to identify novel anticancer drugs and an innovative preclinical model to reduce the high failure rate of anticancer compounds in clinical trials. The resazurin reduction (RR) assay, known as the alamarBlue or CellTiter-Blue assay, is frequently used to determine cell viability/proliferation capacity in eukaryotic cells. Whether this assay is applicable to assess viability in multicellular spheroids has not been evaluated. We analyzed the RR assay to measure cytotoxic and/or cytostatic responses in tumor cell spheroids compared with conventional 2D cultures. We found that tight cell-cell interactions in compact spheroids hamper resazurin uptake and its subsequent reduction to resorufin, leading to lowered reduction activity in relation to the actual cellular health/cell number. Treatment with staurosporine disrupted close cell-cell contacts, which increased resazurin reduction compared with untreated controls. Loss of tight junctions by trypsinization or addition of EGTA or EDTA restored high resazurin reduction rates in untreated spheroids. In conclusion, the RR assay is unsuited to quantitatively measure cellular health/cell number in compact spheroids. However, it can be used to distinguish between cytotoxic versus cytostatic compounds in spheroids. Restoration of the correlation of cell viability/number to resazurin reduction capacity can be achieved by disruption of tight junctions.