G proteins and GPCRs transforming potential
Our laboratory pioneered the study of the oncogenic activity of G proteins and G protein coupled receptors (GPCRs), including virally-encoded GPCRs, and how their signaling circuitry regulates normal and aberrant cell growth and metastasis. We demonstrated the potent transforming activity of G proteins and GPCRs, including Gαq (GNAQ), Gα12 (GNA12) and their coupled GPCRs, and showed that this remarkable biological activity requires a novel signaling network involving mitogen activated kinases (MAPK), ERK, JNK, p38, and their regulation by small GTPases of the Ras and Rho family. The latter had broad implications in the signal transduction field, as we showed that Rho GTPases, which were known for their cytoskeletal effects, stimulate JNK, thereby providing the first demonstration that Rho proteins are integral components of signaling routes linking the cell surface to the nucleus. We also made the unexpected discovery that rather than G protein α subunits (Gα), Gβγ subunits initiate the activate ERK through Ras, thus providing the first demonstration that cell growth promotion by GPCRs involves a complex protein-protein interaction network rather than relying solely on diffusible second messengers. We also reported the first studies supporting that non-receptor tyrosine kinases and tyrosine kinase growth factor receptors converge with GPCRs in the activation of Rho-regulated MAPK signaling networks to control the expression of growth promoting genes.
GPCR initiates Kaposi's sarcomagenesis
Our team revealed the transforming potential of a GPCR encoded by the open reading frame 74 of the Kaposi’s sarcoma (KS)-associated herpesvirus (KSHV) genome, thus providing a direct link between GPCRs and human viral-associated malignancies. For these studies, we developed a novel high throughput animal model system enabling the tissue-specific viral gene delivery and expression in mouse endothelial cells in vivo. This approach revealed that among all candidate KSHV oncogenes, only the viral GPCR was able to recapitulate KS sarcoma pathogenesis. Dissection of the underlying mechanism led to the discovery that the stimulation of AKT and mTOR by the viral GPCR were strictly necessary for sarcomagenesis. Our basic studies provided a rationale for the clinical evaluation by other groups of rapamycin, an mTOR inhibitor, in renal transplanted patients developing KS, which represented the first successful use of mTOR inhibiting agents to treat human malignancies. We also showed that the GPCR signaling specificity can be exploited by targeting selectively the PI3Kγ isoform for KS treatment, thereby circumventing the potential immunosuppressive activity of mTOR inhibitors in AIDS patients, who are at high risk of developing KS.
G proteins and GPCRs in human malignancies
G protein-coupled receptors (GPCRs) represent the largest family of cell surface proteins involved in signal transmission. These receptors play key physiological roles, and their dysfunction contributes to some of the most prevalent human diseases. However, the contribution of GPCRs, their linked heterotrimeric G proteins, and their regulated signaling circuitries in cancer initiation, progression, metastatic spread, and cancer therapy resistance has not been thoroughly investigated, Emerging evidence from our laboratories at UCSD has revealed that malignant cells often hijack the normal physiological functions of GPCRs to proliferate autonomously, evade immune detection, enhance their nutrient and oxygen supply, invade their surrounding tissues, and disseminate to other organs. Strikingly, our recent analysis of human cancer genomes revealed an unanticipated high frequency of mutations in G proteins and GPCRs in most tumor types, which we refer to as the onco-GPCRome. Indeed, nearly 30% of
human cancers harbor mutations in GPCRs or G proteins. For example, mutually exclusive activating mutations in GNAQ or GNA11 (encoding Gαq family members) occur in ~90% and ~4% of melanomas arising in the eye and skin, respectively, where they act as driver oncogenes. Remarkably, approximately 5% of all sequenced tumors harbor activated GNAS mutants, encoding a constitutively active Gas. Oncogenic GNAS mutations were initially discovered in multiple endocrine tumors, such as growth hormone-secreting pituitary (28%) and thyroid adenomas (5%). However, we have recently found that GNAS is mutated in many human cancers, including colorectal carcinoma (CRC) (4%), pancreatic tumors (12%), hepatocellular carcinoma (2%) and, strikingly, appendix cancers (70%). In this regard, there is substantial epidemiological evidence supporting the efficacy of aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) that block COX2 for the prevention of CRC. Our team discovered that prostaglandin E2 (PGE2), which accumulates due to COX2 overexpression, can act on CRC cells directly by stimulating the β-catenin pathway through Gs-linked PGE2 receptors, supporting the direct relevance of GNAS and its activated pathways for CRC initiation and progression. Overexpression of GPCRs and their activation by locally released and circulating agonists in an autocrine and paracrine fashion is also a frequent event in cancer. Overall, as GPCRs represent the target of >25% of all drugs in the market, we hypothesize that interfering with the human onco-GPCRome by targeting GPCRs, G proteins, or their downstream signaling pathways may provide a unique opportunity for the development of novel, mechanism-based strategies for cancer diagnosis, prevention and treatment.
PI3K/AKT/mTOR pathway activation in oral cancer
PI3K/AKT/mTOR pathway activation in oral cancer
Based on our significant contributions to the dissection of growth promoting signaling networks, Dr. Gutkind was given the opportunity to develop and lead a new NIDCR and trans-NIH head and neck cancer (HNSCC) research program. Examples of the early success of our team science approach included the “Oral Cancer Genome Anatomy Project” a decade ago, and the “International Oral Cancer Tissue Array Initiative”, which provided a wealth of resources to the community and a platform for breakthrough discoveries moving the field forward. Among them, we focused on the molecular dissection of aberrant signaling networks in HNSCC. This led to our discovery that the persistent activation of the PI3K/mTOR signaling pathway is one of the most frequently dysregulated molecular mechanisms in HNSCC, including in HPV-associated lesions, and that mTOR inhibition causes tumor regression in a series of genetically-defined and chemically-induced experimental HNSCC models. These findings and the discovery that most HNSCCs exhibit genomic alterations leading to PI3K/mTOR activation, provided a strong rationale for launching a multi-institutional clinical trial (NCT01195922), in which Dr. Gutkind serve as the PI, inhibiting mTOR with rapamycin in newly diagnosed HNSCC patients. The trial has been recently completed with encouraging results in terms of objective responses and limited toxicities.
Targeting mTOR in oral premalignant leisions
There is an urgent need to develop prevention options to halt the progression of oral premalignant lesions (OPL) into OSCC. Our lab discovered that activation of mTOR is an early event in OPL, and we showed that metformin, which is currently used by millions of Americans for type 2 diabetes, inhibits mTOR indirectly in OPL and their potential cancer initiating cells, thereby halting tumor progression. Following our passion for translating lab’s findings into effective cancer therapies, we have recently relocated our research team to the UCSD Moores Cancer Center. Based on our discoveries, and recent epidemiological data showing a significant reduction in OSCC incidence in diabetic patients on metformin, our multidisciplinary team has recently launched a multi institutional phase IIa clinical trial (NCT02581137) to explore the potential use of metformin for OSCC prevention. This ongoing trial may provide a precision prevention strategy for OSCC, and reveal that the use of metformin may represent a potential safe and low cost chemopreventive approach for OSCC in at risk patients.
Harnessing the full potential of novel immune therapies for oral cancer treatment
For patients with recurrent or metastatic HNSCC, standard of care combination chemotherapy yields a median overall survival of only 10 months and long-term survival is rare. Most tumors deploy multiple mechanisms to avoid immune recognition and an anti-tumor immune response. A key emerging mechanism of tumor immunosuppression involves T cell exhaustion, whereby T cell reactivity is impaired due to activation of T cell checkpoints, including PD-1, by its ligand, PD-L1 that is expressed by multiple immune cells and some cancer cells, including HNSCC. Recent revolutionary therapeutic strategies restoring T cell mediated anti-tumor immunity by immune oncology (IO) agents targeting immune checkpoints,
including CTLA-4, PD-1, and PDL-1, have achieved remarkable clinical responses.In HNSCC, recent studies of the anti-PD-1 antibodies nivolumab and pembrolizumab demonstrated immune modulation and durable remissions, but the overall response rate was only 20% and only ~30% of patients were alive one year after the initiation of therapy. In this regard, we still do not know when and how an immune suppressive tumor microenvironment is established during HNSCC progression, in part due to the dearth of experimental animal models to study HNSCC development in immunocompetent mice. This gap in knowledge has limited achieving the full clinical potential of IO therapies in HNSCC patients. We have recently developed the first murine HNSCC cell line collection from a relevant carcinogen-induced tongue cancer mouse model. In preliminary studies, we have sequenced 5 carcinogen (4NQO)-induced mouse HNSCC lesions, and confirmed that they recapitulate typical human HNSCC genomic alterations (e.g., TP53, PIK3CA, FAT1, MLL2, NOTCH1, and CDKN2A mutations). From these cells, we have developed syngeneic (C57BL/6) orthotopic and flank-implantation models, which, importantly, demonstrate a tumor growth/size-dependent switch from a PD-1 responsive to non-responsive phenotype. This model affords us the opportunity to investigate this phenotypic conversion and test hypotheses as to the critical cell types and immune evasive pathways underlying the progression to the PD-1 nonresponsive state.
GNAQ activation in uveal melanoma
The most frequent mutations observed in GNAQ and GNA11 render them GTPase defective and constitutively active, leading to prolonged signaling. Of interest, ~83% of ocular melanomas harbor mutations in GNAQ or GNA11, where they are now considered to represent the driver oncogenes. This provides a clear example of a human malignancy that is initiated by gain of function mutations in Gaq and Ga11 proteins. The best-known downstream signaling event initiated by Gαq involves its ability is to activate phospholipase C (PLC) b and the consequent increased hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to produce two second messengers: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 raises cytoplasmic Ca2+ levels, which stimulates multiple calcium-regulated pathways and, together with DAG, activates classic protein kinase C (PKC) isoforms. However, the molecular events underlying
GNAQ-driven malignancies are not yet defined, thus limiting the ability to develop novel anticancer-targeted therapies. In our recent studies, we found that activating mutation of Gαq can trigger YAP translocation into the nucleus and stimulates YAP-dependent transcription, and that this process is independent from PLCb stimulation but requires the activation of a Gαq-regulated guanine nucleotide exchange factor, Trio, and the subsequent activation of the small GTPases RhoA and Rac1 and their associated signaling networks. Gαq activation is sufficient to stimulate YAP in animal cancer models, and that knock down of Gαq in uveal melanomas harboring GNAQ mutations reduces the nuclear location of YAP, and YAP-dependent gene transcription. Gαq-Trio-Rho/Rac signaling circuitry contributes to YAP-dependent growth in uveal melanoma, which is the first described GNAQ/GNA11-initiated human malignancy, and, thus, that YAP may represent a novel therapeutic target for uveal melanoma treatment.