Our Science – Blumberg Website
Peter M. Blumberg, Ph.D.
Mechanism of Action of Phorbol Esters and Related Derivatives
The core focus of my group is on the network of proteins which recognize and respond to sn-1,2-diacylglycerol (DAG), the ubiquitous second messenger downstream of phosphoinositide turnover. C1 domains, functioning as the DAG recognition module, represent the common structural motif in protein kinase C (PKC), RasGRP, and the other members of this signaling network. We seek to understand the factors which determine ligand interaction with these signaling proteins, the mechanisms by which ligands can distinguish between branches of these signaling pathways, and the importance of such subpathways for the cancer cell. While my program is directed at the basic science underlying cellular signaling through DAG, PKC has emerged as a validated and druggable therapeutic target for cancer and RasGRP represents a potential target.
A secondary research focus is on the capsaicin receptor TRPV1, an exciting new therapeutic target for pain, a critical quality of life issue for cancer patients. My group made key contributions to the initial identification of the capsaicin receptor and to its pharmacology, and conceptually this area is closely linked to that of PKC, reflecting similarities of ligands and marked cross-modulation. Our core goal is to understand the nature of ligand interactions with TRPV1 and the basis for the diverse outcomes, ranging from activation to antagonism and desensitization, as a consequence of ligand interaction and the cellular environment.
Both research projects provide models for translational research, developing the basic science underlying rational drug development for proven therapeutic targets, generating lead structures, and exploiting current structures to provide insights into optimization of their applications and structural refinement. Team science underlies much of our effort. For the more structural aspects, we combine our strength in biology with the expertise of synthetic/medicinal chemistry groups to deploy a full methodological toolbox ranging from computational and synthetic chemistry through biochemical pharmacology, cell biology, and molecular biology. For the more biological aspects, we benefit from the profound expertise in whole animal biology within LCBG. Finally, although cancer represents the lead therapeutic target for both projects, the breadth of medical conditions impacted by the DAG signaling pathways or by TRPV1 provides a wealth of additional potential therapeutic applications. Our mechanism-based approach is particularly appropriate for an institution such as the NIH, which includes within its portfolio the full breadth of human health issues, and provides a paradigm of Dr. Niederhuber’s concept that “progress against cancer is truly progress against all disease”.
A seminal contribution of my group in the mid-70’s was the demonstration of a receptor for the phorbol esters. The field has greatly broadened since then, with several thousand papers published yearly. It is now clear that the phorbol esters provide a window into a central second messenger signaling pathway, that of DAG. The phorbol ester receptor has grown to encompass PKC and six other families of signaling proteins, sharing the C1 domain as the DAG recognition motif and further intertwined through cross-regulation. Within this ferment of activity, our research focus is on the mechanisms which underlie the diversity of responses consequent to interaction of DAG and other ligands at the C1 domain. Bryostatin 1, a structurally complex natural product in multiple clinical trials for cancer, uniquely functions as an antagonist of many but not all PKC mediated responses but paradoxically behaves as an ultrapotent activator of PKC in vitro. In collaboration with the chemistry group of Gary Keck, a breakthrough has been the preparation of structural derivatives of bryostatin 1 which retain full binding potency but lack antagonistic activity, retain antagonism, or show intermediate behavior depending on the specific cell type and cellular response. These emerging bryostatin analogs should allow us to dissect the links between structure, antagonism and response for this class of anticancer agents. Definition of the contribution of receptor structure to ligand recognition complements structural studies with ligands. For example, we have shown that the C1 domains of the “atypical” PKCs are largely unresponsive to DAG because of charged residues lining the rim of the C1 domain. Current efforts include analysis of the C1 domains of Vav, a protooncogene which activates Rac. The Vav C1 domain is closely homologous to the C1 domains of PKC but has been described as phorbol ester unresponsive. Can Vav be the newest DAG signaling protein? RasGRP family members represent another of the DAG-responsive signaling proteins, functioning as guanyl exchange factors (GEFs) for Ras and Rap. RasGRP1/3 play critical roles in B- and T-cell signaling. We find their involvement in a broader range of systems. Both in human prostate cancer and melanoma cell lines, suppression of endogenous RasGRP3 leads to inhibition of cell proliferation and anchorage independent growth as well as to inhibition of growth in mouse xenografts. Ongoing work seeks to define the role of RasGRP3 within the signaling pathways of cells and its role in cancer development.
The mission of the NCI is to “eliminate the suffering and death from cancer”. A critical contribution of my group was the identification of resiniferatoxin (RTX), a phorbol related diterpene, as an ultrapotent capsaicin analog with a unique pharmacological profile – while being only modestly more pungent, RTX was orders of magnitude more potent for desensitization of the capsaicin pain pathway (C-fiber pathway), which mediates chronic and inflammatory pain. Using RTX, we identified and characterized the capsaicin receptor. These findings, together with cloning of the capsaicin receptor by D. Julius and M. Caterina, have fueled intense and very promising efforts by the pharmaceutical industry to develop capsaicin antagonists. Our extensive collaborative work with the medicinal chemistry group of Jeewoo Lee has revealed a diversity of patterns of ligand behavior, such as partial agonism. We have shown that signaling pathways can differentially modify both relative potencies and extents of stimulation. An important implication is that it may be possible to selectively target the capsaicin pain pathway in a particular signaling context, such as at a site of inflammation. We have begun to define the involvement of specific residues in ligand recognition and response.
Our collaborators include Victor Marquez, NIH; Gary Keck, University of Utah; Jeewoo Lee, Seoul National University; Alan Kozikowski, University of Illinois at Chicago; Chaya Brodie, Henry Ford Hospital;; James Stone, University of Alberta; Patricia Lorenzo, University of Hawaii; Stuart Yuspa, NIH; Attila Toth, University of Debrecen; Tamas Biro, University of Debrecen.
This page was last updated on 6/7/2013.