Microcystins (MCs) are hepatotoxins produced by cyanobacteria responsible for toxicity in humans and animals. Here, we investigate unexplored molecular pathways by which microcystin-LR (MC-LR) acts on hepatocytes to elucidate unknown modes of action. We focus on the endoplasmatic reticulum (ER) stress response or unfolded protein response (UPR), and on mechanisms that may contribute to the tumor-promoting effect of MCs in animals, including the activation of NFκB, the expression of interferon alpha (IFN-α) and the induction of interferon stimulated genes (ISGs), as well as the expression of tumor necrosis factor alpha (TNF-α). To this end, we exposed human hepatoma cells (Huh7) to 0.5 μM (nontoxic concentration), 5 μM (EC50 concentration), 25 μM and 50 μM (cytotoxic concentrations) MC-LR for 6, 24, 48, and 72 h. The expression of phosphatase 2A (PP2A) mRNA and protein was induced at 5 μM MC-LR. Phosphorylated P-CREB, a transcription factor for PP2A, leads to elevated expression of PP2A. Furthermore, all of the three ER stress pathways, the UPR and the endoplasmic reticulum-associated degradation were activated after exposure to 5, 25, and 50 μM MC-LR. Additionally, the expression of NFκB, IFN-α, and several INF-α-stimulated genes was strongly activated. The proinflammatory cytokine TNF-α was also induced. Our data demonstrate that MC-LR induces all ER stress response pathways. Consequently NFκB is activated, which in turn induces the expression of IFN-α and TNF-α. All of these activated pathways, which are analyzed here for the first time in detail, may contribute to the hepatotoxic, inflammatory, and tumorigenic action of MC-LR.