Haissi Cui

The proteasome is an intracellular protease central to protein homeostasis as it degrades ubiquitin-tagged and misfolded proteins. As such, inhibition of the major isotype, the constitutive proteasome, leads to the induction of apoptosis. This cytotoxicity is explored in the clinic by the application of proteasome inhibitors for treatment of mantle cell lymphoma and multiple myeloma. Already during my PhD in the department of Experimental Oncology and Molecular Immunology at the Klinikum Rechts der Isar der TUM, we started several collaboration projects to evaluate the potential of new proteasome inhibitors in cell culture, which continued into my postdoctoral work in the group of Michael Groll.

Here, I could contribute to the general understanding of proteasome inhibitor specificity and potency by systemically characterizing the induction of cell death by different electrophilic warheads coupled to the same peptidic backbone. Thus, we were able to compare proteasome inhibition, cell toxicity, and the reversibility of compound action in human cell lines solely in dependency of the electrophile. However, as the constitutive proteasome is found in all cells, the half-life of proteasome inhibitors is short and side-effects limit both dose and application length. We explored the possibility to specifically deliver proteasome inhibitors to tumor cells by conjugation to a vehicle, which targets specific cell surface receptors. Indeed, this resulted in the efficient and specific induction of tumor cell death.

While inhibition regardless of the proteasome isotype can be used to induce cytotoxicity in cancer, other applications demand a fine-tuning of inhibitor specificity. Inhibition of the proteasome isoform predominantly found in immune cells, the immunoproteasome, is currently under investigation as a treatment for inflammatory diseases. The development of immunoproteasome inhibitors is limited by the highly similar active sites and the usage of strongly reactive electrophiles. We were able to generate proteasome inhibitors, which act selectively on immune cells with high levels of immunoproteasome, by exploring the unique mechanism of sulfonyl fluorides as warheads. In a second approach, we could downregulate cytokine production in a model cell line by structure-guided targeting of a unique cysteine in the substrate channel of the immunoproteasome.

Additionally, I could contribute to the development of new methods for the investigation of proteasome function and structure. Fluorescent probes based on peptidic sulfonate esters can be used to simply and sensitively quantify proteasome activity. Establishment of large scale fermentation of human cells in a bioreactor in collaboration with the Technikum of the TUM enabled access to sufficient quantities of protein, which are otherwise difficult to achieve.