Sensing the Cellular Environment Through Integrins

January 4, 2017

Integrins are integral membrane proteins found on nearly every cell in multicellular animals. They provide the means by which cells interact with components of the extracellular matrix (ECM). In humans, integrins are heterodimers composed of one of 18 alpha subunits and one of 8 beta subunits, with 24 combinations found in cells. Much of what we know about integrin function comes from studies of the αIIbβ3 protein from platelets. This integrin appears to exist in a default “inactive” form, meaning that it does not bind to its ECM target, fibrinogen. Platelet activation leads to binding of an intracellular protein, talin, to the C-terminus of the integrin, destabilizing two structural “clasps” that bind the alpha and beta subunits to each other and freeing the protein for fibrinogen binding. Despite this detailed knowledge about αIIbβ3, it is unclear to what degree its activation mechanism applies to other integrins. To answer this question, Vanderbilt Basic Sciences researcher Chuck Sanders, his collaborator (Roy Zent) and their laboratories carried out extensive structural and functional studies of α1β1 and α2β1, the major integrins that bind to collagen. For this work, they focused on a particular lysine residue in the β1 subunit, as the homologous residue in the β3 subunit αIIbβ3 plays a key role in the activation of that protein. The researchers discovered that the α1β1 and α2β1 integrins likely exist in a default “active” state, able to bind collagen, and that structural alterations to the key lysine residue that activate αIIbβ3 have the opposite effect on these proteins. Their findings demonstrate that the integrins are structurally and functionally diverse, and that no single mechanism of activation can be generalized to the entire protein class. Clearly further work is needed to fully understand how cells interact with their environment through integrins. The work is published in the journal eLife [Z. Lu, S. Mathew, J. Chen, et al., eLife, 2016:10.7554/eLife.18633].

Boosting the Immune System to Fight Breast Cancer

January 4, 2017

Despite major strides in the early detection and treatment of breast cancer, metastatic disease remains a therapeutic challenge, resulting in over 40,000 deaths per year in the United States. Most breast cancers are the result of genetic mutations that lead to abnormal growth and invasive behavior of the tumor cells. Thus, therapies that reverse the effects of such mutations should be highly efficacious. However, this approach is often thwarted by the acquisition of drug resistance by the cancer. Genes encoding the PI3K (phosphatidylinositol 3-kinase) class of enzymes are frequently mutated in breast cancer, and drugs are now available that target these proteins. PI3Ks, however, are also important in regulating the immune response, leading to the concern that drugs that alter their activity may suppress the patient’s own anti-tumor defenses. These considerations led Vanderbilt Basic Sciences investigator Ann Richmond and her laboratory to embark on a careful study of the effects of a PI3K inhibitor on the growth of and immune response to breast cancer. Using mouse models of both murine and human breast cancers, they showed that PI3K inhibition led to a marked reduction in the rate of growth and metastasis of the tumors. In addition, they obtained the somewhat surprising result that PI3K inhibition augmented rather than suppressed the immune response to the cancers. In fact, genetic deletion of one PI3K isoform in the host mouse increased the immune response to a tumor that expressed the enzyme. Further studies showed that combining PI3K inhibition with an anti-PD-1 antibody (a therapy that blocks the ability of cancer cells to evade the immune system) led to a stronger response than could be obtained with either therapeutic approach alone. If these findings can be translated to human patients, they offer a promising new approach to the treatment of breast cancer. The work is published in the journal Clinical Cancer Research [J. Sai, et al. Clin. Cancer Res., published online December 21, DOI: 10.1158/1078-0432.CCR-16-2142].

Tapping a Wealth of Information in Tumor Tissue

December 12, 2016

Accumulating evidence indicates that cancer is the result of multiple genetic mutations that lead to dysregulated cell signaling, growth, and death. This knowledge has led to the discovery of an exciting new armamentarium of therapies intended to disrupt the abnormal cellular responses that result from cancer-associated gene mutations. However, these therapies have not lived up to expectations in the clinic, often because of the ability of cancer cells to develop resistance or due to the fact that cancers comprise heterogeneous populations of cells that may have vary genetically and physiologically. Consequently, a better approach to treating cancer must rely on a more thorough understanding of the exact cellular composition of the target tumor. To address this problem, Vanderbilt Basic Sciences investigators Ken Lau and Jonathan Irish along with their Vanderbilt Collaborators Kay Washington and Bob Coffey have devised an exciting new method to analyze cell signaling pathways in formalin-fixed, paraffin-embedded (FFPE) tumor tissue that is typically acquired from patients in community hospitals. The method includes removing the paraffin and reversing formalin-dependent protein-protein crosslinks in the samples, followed by labeling of key signaling molecules with antibodies, and finally dispersion of the cells. The researchers then subject the resulting cell suspensions to multiplexed analysis by single-cell mass cytometry. Pilot studies of samples of normal colonic mucosa revealed well organized patterns of cell signaling networks associated with cells of distinct type, state of differentiation, and location within the intestinal epithelium. These patterns were consistent between specimens from different patients. In contrast, analysis of samples of colorectal cancer revealed loss of organized signaling pathways and heterogeneity both within a tumor and between tumors from different patients.  In addition, the results provided interesting new insights into specific signaling pathway dysregulation in the tumor tissue. This approach promises an exciting opportunity to mine new information from samples acquired routinely from patients every day and to substantially increase our understanding of the scope of heterogeneity within and between cancers. The work is published in the journal Science Signaling [A. J. Simmons, et al. (2016), Sci. Signal., 9, rs11].