Membrane Biophysics & Drug Delivery
Specific delivery of therapeutic peptides to cancer cells using pHLIP
The development of targeted delivery systems is the current trend in cancer therapies. Peptides that interfere with protein interactions have great potential as anticancer agents, owing to their target specificity and ease of rational design and synthesis. However, their low stability and poor tumor penetration limit their use. Therefore, successful development of delivery technologies could open the door for effective peptide therapy, thus making an entirely new class of molecules useful as anticancer drugs. To that end, we use the pH (Low) Insertion Peptide (pHLIP) to specifically target and deliver peptides to cancer cells. pHLIP can target acidic tissues in mice (such as tumors and inflammation sites), and be used to localize molecules at cell surfaces or to translocate otherwise cell-impermeable cargo molecules in the cytoplasms of cancer cells at low pH (Thévenin et al. Chemistry & Biology, 2009). Our group has recently shown that Monomethyl auristatin derivatives (MMAE) when conjugated to pHLIPinduce a potent cytotoxic effect (>90% inhibition of cell growth) in a concentration- and pH-dependent manner. pHLIP−MMAE conjugates exhibit between an 11- and 144-fold higher antiproliferative effect at low pH than that at physiological pH and a pronounced pH-dependent cytotoxicity as compared to that of free drug. Furthermore, we demonstrate that a pHLIP−MMAE drug conjugate effectively targets triple-negative breast tumor xenografts in mice.
Role of the transmembrane region in the activity and regulation of Receptor Protein-Tyrosine Phosphatases
Many cell-signaling events are regulated through reversible tyrosine phosphorylation of proteins. This phosphorylation cycle is controlled by the counterbalanced actions of two enzyme families: protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). While the regulation and actions of many receptor protein tyrosine kinases are well characterized, the mechanisms controlling the enzymatic activity of receptor protein-tyrosine phosphatases (RPTPs) and how these receptors transduce signals across the plasma membrane remain to be discovered. Thus, a clear need for structure/function studies exists. Homo-dimerization has been shown to regulate the activity of several RPTPs either negatively or positively. Even though the TM region has been proposed to be involved in this process there is no clear structure-based proposal for the role of the TM region. Understanding how these receptors function and interact will give insights on how tyrosine phosphorylation is finely tuned in cells, and how it can be modulated.
Determining the structural features of the signaling by the thrombopoietin receptor and of its modulation by small molecule
The thrombopoietin receptor (TpoR) is a type 1 cytokine receptor that regulates, in part, the production of platelets in response to its ligand thrombopoietin. The single transmembrane (TM) domain is thought to be involved in the process of receptor dimerization and activation, but its exact role is poorly defined. We have recently shown that the TM region of TpoR dimerizes strongly and can adopt three different stable, rotationally related conformations involving distinct sets of TM residues, which may correspond to specific states of the full-length receptor (active, inactive and partially active). We have also proposed a novel allosteric mechanism of action for a small molecule TpoR agonist that does not mimic the natural ligand: the equilibrium existing between the inactive and active states allows the binding of the small drug-like compound to an essential TM histidine (which is exposed in the active state), leading to a shift of the equilibrium toward the active state and consequently, to signaling (see figure). However, several important questions remain to be answered to obtain a clearer understanding of TpoR dynamics and modulation of cytokine receptor by small molecules.