TiPED’s first two tiers are “target” based assays. That is, they specifically identify chemicals that interact with target proteins to modulate their activity. The first tier predicts activity using in silico modeling of protein-small molecule interactions. The second tier tests targeted activity of the chemicals on the activity of transcription factors such as nuclear hormone receptors and other modulators of endocrine signaling pathways.
In contrast, the assays in this tier are “process and function” based. That is, assays in the Cell-Process tier integrate cellular functions to reveal the activity of a test chemical on an endocrine signaling pathway that may be cell type specific.
For example, while a chemical with estrogenic activity may directly activate one of the nuclear estrogen receptors, it might also act through cell surface receptors that modulate different cellular pathways. Alternatively, the estrogen may behave as an activator of estrogen signaling in one cell type but an inhibitor in another cell type. A chemical might be too weak an activator to be considered positive in Cell-Target assays but could give strong results in Cell-Process testing. Cell-Process assays need not identify, a priori, through which molecular pathways a chemical acts. Rather Cell-Process assays only need to produce an effect of concern that can be measured. For example, an obesogenic chemical will induce cells to become fat cells, although, the mechanism through which this action is mediated may not be fully known in order for the assay to produce valuable information.