Mathematical modeling of YAP and TAZ nuclear/cytoplasmic shuttling in liver cancer cells

Background YAP (yes-associated protein) and TAZ (WW domain containing transcription regulator 1, WWTR1) are oncogenic transcriptional co-activators, which facilitate biological functions of the Hippo pathway in many tumor cells. However, if and to which extent YAP and TAZ differentially respond to extracellular information such as cell-cell contact in the liver carcinogenesis is not well understood.

Methods Using lentiviruses, we stably transfected Hep3B (hepatocellular carcinoma; HCC) cells with Cerulean-tagged histon 2B (H2B), Venus-tagged YAP and mCherry-tagged TAZ reporter constructs that allowed us to study nuclear-cytoplasmic shuttling of both factors via time-lapse microscopy. Cells were cultured under varying cell density conditions and time-resolved data was analyzed in a quantitative manner with the software Fiji. To address the question, whether the differential YAP and TAZ shuttling can be explained with dynamic differences in nuclear transport kinetics, we created a partial differential equation (PDE)-based computational model using a Spatial Model Editor. The PDE model simulates the two-dimensional reaction-diffusion equation, which takes into account the concentration of species at a position in time and the diffusion constant of species, and the reaction term, which describes rate of change of species concentration at every point inside the compartment.

Results As expected, YAP and TAZ shuttled out of the nucleus upon increased cell density; however, dynamics between both factors differed with YAP already responding at lower densities than TAZ. The interactions between phosphorylated (cytoplasmic) as well as unphosphorylated (nuclear) YAP and TAZ, their degradation and how these processes impact the molecular shuttling over the nuclear membrane was mathematically modeled. With help of the PDE model we could describe experimental observations that the dynamics of TAZ is considerably different from YAP, moreover, we could pinpoint key parameters, which define these differences in subcellular localization.

Summary We developed an analysis tool and a novel PDE-based mathematical model that quantitatively describe the cellular YAP/TAZ response of HCC cells on varying cell density conditions. As several YAP and TAZ-specific inhibitors are currently being developed (AACR virtual annual meeting; June 22-24, 2020), there is a growing importance of exploring how strongly YAP and/or TAZ respond to nuclear exclusion on these substances.

Publication History

Article published online:
04 January 2021