CXCR4 is a G-protein coupled receptor activated by a sole chemokine agonist, CXCL12, and functions to cause the migration of cells to the appropriate anatomical location during embryonic development and in response to stress in adults. CXCR4 is also involved in growth and/or metastasis of a number of cancers, is a co-receptor for HIV-1, and mutant CXCR4 causes an immunosuppressive condition known as WHIM syndrome. A chemokine released by herpesvirus-8, v-MIP-II, is an antagonist of CXCR4. CXCL12 and vMIP-II are cognate molecules because there are members of the chemokine superfamily as defined by their sequence and structure and would be expected to interact with chemokine receptors. More recently, a non-cognate protein, macrophage migration inhibitory protein (MIF), was reported to functionally activate CXCR4. We verified that MIF and CXCR4 interact with each other. It is interesting to note that some of the biological functions of CXCL12 and MIF overlap raising the possibility that this may be due to activation of CXCR4. In addition to CXCR4 binding by CXCL12, vMIP-II, and MIF, two allosteric peptide agonists were identified from a library of 160,000 mutants selected from a strain of yeast that was genetically modified to express a functional CXCR4. This application aims (1) to determine the three-dimensional structures of the N-terminal region of CXCR4 complexed to (a) CXCL12 (b) MIF, and (c) vMIP-II using either X-ray crystallography or NMR, (2) to use S. cerevisiae to study the sites of interactions between the ligands of CXCR4 and CXCR4, and (3) to identify and characterize CXCR4 agonists and antagonists, as well as to kinetically, crystallographically, and biologically characterize small molecules inhibitors already identified by high throughput screening (HTS) of the catalytic site of MIF. Our collaborator, Dr. Joshua Rubin (Washington University School of Medicine) found that the CXCR4 antagonist AMD3100 and its derivatives prevent growth of these human brain tumors in a murine model of the disease, but this molecule and its analogues are toxic when administered in humans for long-term use (for anti-HIV therapy). It was also found that CXCR4 antagonism in the glioblastoma mouse model required an increase on cAMP and that the cAMP phosphodiesterase inhibitor rolipram had similar therapeutic effects as CXCR4 antagonism due to an increase in cAMP. We have identified five phosphodiesterase inhibitors, (including rolipram) that inhibit MIF. It is possible that rolipram has dual effects in inhibiting glioblastoma: inhibiting the CXCR4 agonist MIF and inhibiting the cAMP phosphodiesterase activity. We will characterize another 18 MIF inhibitors identified by HTS with respect to Ki and specificity against a panel of phosphodiesterases and choose molecules with no inhibition of cAMP phosphodiesterase for studying the role of MIF in activating CXCR4 or in cAMP phosphodiesterase activity in response to CXCR4 activation in this tumor. These studies will not only enlighten our understanding of CXCR4 in glioblastoma, but provide a greater insight into the potential mechanisms by which this GPCR is regulated by different ligands, and provide reagents for further studies or for future drug development.
The G-protein coupled receptor CXCR4 is involved in the metastasis and growth of several cancers, serves as one of two major co-receptors for HIV-1, and, when mutated at the C-terminus, is partly responsible for the immunosuppressive disorder known as WHIM syndrome. This application seeks to understand the mechanism by which proteins function as agonists or antagonists of CXCR4 and the effects that small molecules inhibitors of these proteins have on these functions and on CXCR4.
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