Chemotherapy has been shown to enrich cancer stem cells in tumors. Recently, we demonstrated that administration of chemotherapy to human bladder cancer xenografts could trigger a wound-healing response that mobilizes quiescent tumor stem cells into active proliferation. This phenomenon leads to a loss of sensitivity to chemotherapy partly due to an increase in the number of tumor stem cells, which typically respond to chemotherapy-induced cell death less than more differentiated cells. Different bladder cancer xenografts, however, demonstrate differential sensitivities to chemotherapy, the basis of which is not understood. Using mathematical models, we show that characteristics of the tumor cell hierarchy can be crucial for determining the sensitivity of tumors to drug therapy, under the assumption that stem cell enrichment is the primary basis for drug resistance. Intriguingly, our model predicts a weaker response to therapy if there is negative feedback from differentiated tumor cells that inhibits the rate of tumor stem cell division. If this negative feedback is less pronounced, the treatment response is predicted to be enhanced. The reason is that negative feedback on the rate of tumor cell division promotes a permanent rise of the tumor stem cell population over time, both in the absence of treatment and even more so during drug therapy. Model application to data from chemotherapy-treated patient-derived xenografts indicates support for model predictions. These findings call for further research into feedback mechanisms that might remain active in cancers and potentially highlight the presence of feedback as an indication to combine chemotherapy with approaches that limit the process of tumor stem cell enrichment. Cancer Res; 77(9); 2231-41. ©2017 AACR.

A major metastasis suppressing mechanism is the rapid apoptotic death of cancer cells upon detachment from extracellular matrix, a process called anoikis. Focal adhesion kinase (PTK2/FAK) is a key enzyme involved in evasion of anoikis. We show that loss of the Cub-domain containing protein-1 (CDCP1), paradoxically stimulates FAK activation in the detached state of prostate cancer cells. In CDCP1^low DU145 and PC3 prostate cancer cells, detachment-activation of FAK occurs through local production of PI(4,5)P₂. PI(4,5)P₂ is generated by the PIP5K1c-201 splicing isoform of PIP5K1c, which contains a unique SRC phosphorylation site. In the detached state, reduced expression of CDCP1 and an alternative CDCP1-independent SRC activation mechanism triggers PIP5K1c-pY644 phosphorylation by SRC. This causes a switch of Talin binding from β1-integrin to PIP5K1c-pY644 and leads to activation of PIP5K1c-FAK. Reduced CDCP1 expression also inactivates CDK5, a negative regulator of PIP5K1c. Furthermore, immersion of prostate cancer cells in 10% human plasma or fetal bovine serum is required for activation of PIP5K1c-FAK. The PIP5K1c induced detachment-activation of FAK in preclinical models sensitizes CDCP1^low prostate cancer cells to FAK inhibitors. In patients, CDCP1^High versus CDCP1^low circulating tumor cells differ in expression of AR-v7, ONECUT2 and HOXB13 oncogenes and TMPRSS2 and display intra-patient heterogeneity of FAK-pY397 expression. Taken together, CDCP1^low and CDCP1^high detached prostate cancer cells activate distinct cytoplasmic kinase complexes and targetable transcription factors, which has important therapeutic implications.