Balance between autophagy and apoptosis determines anoikis resistance: a mathematical model on cell fate

Although several cancer cells are shed into the circulation, most die due to the stresses they encounter. Detachment from the underlying extracellular matrix is one major stress; cell death due to matrix detachment is known as anoikis. Few cancer cells overcome this stress, become anoikis-resistant, and survive to seed metastasis. Autophagy, a cell survival mechanism during stressful conditions that promotes cellular homeostasis by recycling cellular components, is activated upon matrix detachment. On the other hand, apoptosis is a cellular mechanism that is responsible for programmed cell death which is also activated upon matrix detachment. It is unclear how matrix-deprived cancer cells maintain a balance between autophagy and apoptosis to decide the cell fate: whether the cell dies due to anoikis or acquires anoikis-resistance and survives to seed metastasis. Though multiple pathways contribute to cell fate decisions, we have shown experimentally that autophagy and apoptosis are influenced by Akt-AMPK axis and AMPK-ERK axis in matrix-deprived cancer cells. Since Akt, AMPK and ERK are in turn linked to each other it is essential to understand how these proteins simultaneously affect the cell signaling and survival/death outcomes of matrix-deprived cancer cells. To study the cumulative effect of Akt-AMPK-ERK activities on survival/death decisions and to understand the fine balance between apoptosis and autophagy that facilitates the survival/death of matrix-deprived cancer cells, we formulated a deterministic ordinary differential equation (ODE)-based protein interaction model of anoikis resistance. Model stability analysis and 3D-nullcline analysis depicted that the system has a unique steady state in matrix-attached and matrix-deprived condition. Parameter sensitivity analysis depicted that the model is highly robust, and the model variables are sensitive to only a few model parameters. By simulating differential activity of pAkt, pAMPK and pERK, the model predicted a heterogeneity in pERK levels: high/low levels pERK along with high pAMPK enable survival as long as levels of pAkt are maintained low. Additionally, the model predicted a heterogeneity in pAMPK: high/low levels of pAMPK along with low pERK determines the shift from survival to death when levels of pAkt are high. Such high levels of pAkt are obtained at critically low levels of pERK. Molecular perturbation revealed a hierarchy among proteins while deciding the cell fate: pAkt dominates over pAMPK which further dominates over pERK and intermediate to high levels of pAkt were sufficient for apoptosis to surpass autophagy in matrix-detached cells. The model also predicted that Akt impacts apoptosis more than autophagy and classified the cell fate decision into survival and death zones in matrix-deprived condition. Overall, this work provided multiple insights on the molecular interplay among key kinases Akt, AMPK and ERK and their effects on apoptosis and autophagy. This model also depicted that autophagosome formation is rather robust as compared to apoptosis which is more sensitive to molecular perturbations. Hence, apoptosis emerged as a deciding factor that influences the decision of cell fate of a matrix-deprived cancer cell.