Protein engineering has been a useful tool in the fight against human diseases. Human insulin was the first recombinant DNA-derived therapeutic protein (Humulin®) approved by the US FDA in 1982. However, many of the early protein drugs were only recombinant versions of natural proteins with no modification of their primary amino acid sequence and most of them did not make optimal drug products mainly due to their short half-life or suboptimal affinity, leading to poor therapeutic efficacy. The difficulty in the large-scale production of some therapeutic proteins was another important issue. In the past three decades, different protein engineering platforms have been developed to overcome the obstacles seen in the first generations of these treatments. With the help of these new techniques, proteins have been purposefully modified to improve their clinical potential. The focus of my dissertation is the engineering of potential protein drugs to make them therapeutically useful and more valuable. Previously, our research group has developed cocaine hydrolases (CocHs) from human butyrylcholinesterase (BChE) for treatment of cocaine addiction and prevention of acute cocaine intoxication. In the first project, CocHs were further engineered to improve their performance, e.g., Fc-fused CocHs with an extended serum half-life. Then, I investigated the potential application of a long-lasting CocH for protection against the acute toxic and stimulant effects of cocaine. In the second project, I investigated the potential inhibition of CocH-mediated cocaine hydrolysis by heroin (3,6-diacetylmorphine) or its initial host metabolite, 6-monoacetylmorphine (6-MAM). The investigation of this possible inhibition was important to determine the in vivo efficacy of CocHs, as heroin is one of the most commonly co-abused drugs by cocaine-dependent individuals, as well as a possible metabolite of CocHs. In the third project, I expressed and characterized the recombinant human UDP-glucuronosyltransferase 1A10 (UGT1A10) enzyme, which can inactivate many therapeutically valuable substances. In the fourth and final project, prostate apoptosis response-4 (Par-4), a tumor suppressor protein, was engineered to have a prolonged duration of action so that it may be more therapeutically valuable for cancer treatment.

This work describes the fascinating interplay between two tumor suppressors Prostate apoptosis response-4 (Par-4) and p53. The guardian of the genome, p53, is frequently mutated in human cancers, and may contribute to therapeutic resistance. However, p53 is intact and functional in normal tissues, and we observed that specific activation of p53 in normal fibroblasts could induce apoptosis selectively in p53-deficient cancer cells. This paracrine apoptotic effect was executed by Par-4 secreted in response to p53 activation. Accordingly, activation of p53 in wild-type mice, but not in p53-/- or Par-4-/- mice, caused systemic elevation of Par-4 that induced apoptosis of p53-deficient tumor cells. Mechanistically, p53 induced Par-4 secretion by suppressing the expression of UACA, a binding partner of Par-4, and thereby releasing Par-4 from sequestration by UACA. Thus, normal cells can be empowered by p53 activation to induce Par-4 secretion for inhibition of therapy-resistant tumors. Conversely, our studies have also revealed a definite role for Par-4 in regulating p53 expression. The pro-apoptotic tumor suppressor Par-4 is lost, down-regulated, inactivated or mutated in a number of cancers. Loss of Par-4 is associated with therapeutic resistance and poor disease prognosis, yet the mechanism for resistance is not clearly understood. Using genetically matched cells, we show that Par-4 expression is required for stabilization and function of the tumor suppressor p53, which constitutes the hub of signaling networks controlling important cellular and organismal phenotypes. In particular, the expression of p53 protein and its stabilization in response to genotoxic stress were remarkably attenuated in response to Par-4 loss. Accordingly, Par-4-null or -knockdown cells demonstrated increased resistance to apoptosis induced by genotoxic stress. Par-4 loss resulted in elevated Mdm2 activity, which is known to cause p53 degradation. Our findings suggest that Par-4 stabilizes p53 by inhibiting Akt-mediated phosphorylation of Mdm2 that is known to prevent translocation of Mdm2 into the nucleus for p53 ubiquitination and degradation. These studies identify a novel regulatory relationship between two tumor suppressors and may provide a better understanding of therapeutic resistance in tumors with p53 wild type status.

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Le gène par-4 code pour une kinase à sérine/thréonine très conservée qui régule la polarisation précoce et la division cellulaire asymétrique de l’embryon de C. elegans. Une mutation de par-4 entraîne la létalité embryonnaire en perturbant trois processus: la ségrégation asymétrique des déterminants cellulaires, la régulation asynchrone de la progression du cycle cellulaire et la contractilité du réseau d’actomyosine. Pour identifier des régulateurs des voies de signalisation de PAR-4, nous avons procédé à un criblage pour des suppresseurs de la létalité embryonnaire associée à une mutation de par-4. Nous avons identifié 6 gènes qui codent pour des homologues conservés avec des activités définies telles que la phosphorylation, l’ubiquitination, la protéolyse et l’échafaudage. En employant l’imagerie quantitative pour suivre des événements cellulaires dépendants de PAR-4, nous avons déterminé quels processus sont contrôlés par chaque suppresseur durant le développement embryonnaire de C. elegans. Des analyses moléculaires de ces suppresseurs ont révélé des détails sur le mécanisme par lequel PAR-4 régule la polarisation cellulaire et promeut la division cellulaire asymétrique. / The gene lkb1 codes for a highly conserved serine/threonine kinase. The orthologue of lkb1 in the nematode Caeonorhabditis elegans, termed par-4, regulates early polarization and asymmetric cell division in the embryo. A mutation in par-4 causes embryonic lethality by perturbing three main cellular processes: asymmetric segregation of cell fate determinants, asynchronic regulation of cell cycle progression and contractility of the actomyosin network. To identify regulators of the PAR-4/LKB1-dependent pathways, we performed a screen for suppressors of the embryonic lethality associated with a mutation in par-4. We identified 6 genes that have conserved homologs with defined activities including protein phosphorylation, ubiquitination, proteolysis and scaffolding. We used quantitative imaging of specific PAR-4-dependent cellular events to determine which of these are controlled by each suppressor during early C. elegans embryonic development. Molecular analysis of these suppressors revealed details on the mechanism through which PAR-4 regulates cell polarization and promotes asymmetric cell division.