The Warburg effect describes a pro-oncogenic metabolism switch such that cancer cells take up more glucose than normal tissue and favor incomplete oxidation of glucose even in the presence of oxygen. To better understand how tyrosine kinase signaling, which is commonly increased in tumors, regulates the Warburg effect, we performed phosphoproteomic studies. We found that oncogenic forms of fibroblast growth factor receptor type 1 inhibit the pyruvate kinase M2 (PKM2) isoform by direct phosphorylation of PKM2 tyrosine residue 105 (Y 105 ). This inhibits the formation of active, tetrameric PKM2 by disrupting binding of the PKM2 cofactor fructose-1,6-bisphosphate. Furthermore, we found that phosphorylation of PKM2 Y 105 is common in human cancers. The presence of a PKM2 mutant in which phenylalanine is substituted for Y 105 (Y105F) in cancer cells leads to decreased cell proliferation under hypoxic conditions, increased oxidative phosphorylation with reduced lactate production, and reduced tumor growth in xenografts in nude mice. Our findings suggest that tyrosine phosphorylation regulates PKM2 to provide a metabolic advantage to tumor cells, thereby promoting tumor growth.

SUMMARY Many tumor cells rely on aerobic glycolysis instead of oxidative phosphorylation for their continued proliferation and survival. Myc and HIF-1 are believed to promote such a metabolic switch by, in part, upregulating gene expression of pyruvate dehydrogenase (PDH) kinase 1 (PDHK1), which phosphorylates and inactivates mitochondrial PDH and consequently pyruvate dehydrogenase complex (PDC). Here we report that tyrosine phosphorylation enhances PDHK1 kinase activity by promoting ATP and PDC binding. Functional PDC can form in mitochondria outside of matrix in some cancer cells and PDHK1 is commonly tyrosine phosphorylated in human cancers by diverse oncogenic tyrosine kinases localized to different mitochondrial compartments. Expression of phosphorylation-deficient, catalytic hypomorph PDHK1 mutants in cancer cells leads to decreased cell proliferation under hypoxia and increased oxidative phosphorylation with enhanced mitochondrial utilization of pyruvate, and reduced tumor growth in xenograft nude mice. Together, tyrosine phosphorylation activates PDHK1 to promote the Warburg effect and tumor growth.

Head and neck squamous cell carcinoma (HNSCC) is one of the most common types of human cancer and frequently metastasizes to LNs. Identifying metastasis-promoting factors is of immense clinical interest, as the prognosis for patients with even a single unilateral LN metastasis is extremely poor. Here, we report that p90 ribosomal S6 kinase 2 (RSK2) promotes human HNSCC cell invasion and metastasis. We determined that RSK2 was overexpressed and activated in highly

Dysregulation of the receptor tyrosine kinase fibroblast growth factor receptor 3 (FGFR3) plays a pathogenic role in a number of human hematopoietic malignancies and solid tumors. These include t(4;14) multiple myeloma associated with ectopic expression of FGFR3 and t(4;12)(p16;p13) acute myeloid leukemia associated with expression of a constitutively activated fusion tyrosine kinase, TEL-FGFR3. We recently reported that FGFR3 directly tyrosine phosphorylates RSK2 at Y529, which consequently regulates RSK2 activation. Here we identified Y707 as an additional tyrosine in RSK2 that is phosphorylated by FGFR3. Phosphorylation at Y707 contributes to RSK2 activation, through a putative disruption of the autoinhibitory alphaL-helix on the C terminus of RSK2, unlike Y529 phosphorylation, which facilitates ERK binding. Moreover, we found that FGFR3 interacts with RSK2 through residue W332 in the linker region of RSK2 and that this association is required for FGFR3-dependent phosphorylation of RSK2 at Y529 and Y707, as well as the subsequent RSK2 activation. Furthermore, in a murine bone marrow transplant assay, genetic deficiency in RSK2 resulted in a significantly delayed and attenuated myeloproliferative syndrome induced by TEL-FGFR3 as compared with wild-type cells, suggesting a critical role of RSK2 in FGFR3-induced hematopoietic transformation. Our current and previous findings represent a paradigm for tyrosine phosphorylation-dependent regulation of serine-threonine kinases.

3142 The Warburg effect describes a pro-oncogenic metabolic switch in which cancer cells, including leukemia cells, take up more glucose than normal tissue, yet use less glucose for oxidative phosphorylation and favor glycolysis even in the presence of oxygen (aerobic glycolysis). However, the molecular mechanisms underlying the Warburg effect remain unclear. Growth factor (GF) receptors are believed to play a key role in programming cancer cell metabolism. These GF receptors are expressed in many hematopoietic malignancies as constitutively activated tyrosine kinase mutants. Thus, we examinined whether tyrosine kinase signaling — commonly upregulated in hematopoietic malignancies — regulates the Warburg effect to contribute to leukemogenesis and disease progression. We performed phospho-proteomics studies and found that pyruvate kinase M2 isoform (PKM2), which is a rate-limiting enzyme of glycolysis, is tyrosine phosphorylated in leukemia cells expressing FGFR1 fusion tyrosine kinases, which are associated with 8p11 leukemia/lymphoma syndrome. We also found that 8p11 leukemogenic FGFR1 directly phosphorylates and inhibits PKM2. Recent seminal studies from Dr. Lew Cantley's group demonstrated that the enzymatic activity of PKM2 is inhibited by phosphotyrosine binding; PKM2 expression is important for aerobic glycolysis and provides a growth advantage to tumors. However, it remains unclear which dedicated tyrosine kinase pathways are physiologically responsible for this regulation and whether PKM2 itself is tyrosine phosphorylated to achieve inhibition of PKM2 in cancer cells. Here we report that FGFR1 inhibits PKM2 by direct phosphorylation at Y105. This consequently inhibits the formation of tetrameric, active PKM2 by disrupting cofactor fructose-1,6-bisphosphate (FBP) binding in a putative “inter-molecule manner”, where one molecule in an active PKM2 tetramer, when phosphorylated, may function as an inhibitory binding partner to the other sister molecules. In addition, phosphorylation of PKM2 at Y105 is common in many human leukemia cell lines expressing oncogenic tyrosine kinases such as BCR-ABL, FLT3-ITD, and JAK2V617F. Furthermore, expression of the PKM2 Y105F mutant in cancer cells following RNAi-mediated knockdown of endogenous PKM2 leads to decreased cell proliferation under hypoxia, increased oxidative phosphorylation with reduced lactate production, and reduced tumor growth in xenograft nude mice. Our findings suggest that tyrosine phosphorylation regulates PKM2 to program cancer cell metabolism and promote tumor growth. This may represent a common, acute molecular mechanism to regulate the Warburg effect, in addition to the chronic changes that are believed to be regulated by hypoxia inducible factor 1 and Myc. Disclosures: No relevant conflicts of interest to declare.

Head and neck squamous cell carcinoma (HNSCC) is one of the most common types of human cancers and frequently metastasizes to lymph nodes. We found that the protein expression pattern of p90 ribosomal S6 kinase 2 (RSK2) correlates with the invasive ability of diverse HNSCC cell lines, and with human head and neck cancer progression. Continued RSK2 expression is vital for maintenance of the invasive and metastatic potential of HNSCC cells in vitro and in vivo, respectively. In a phospho-antibody microarray-based study, we identified pro-metastatic protein factors that are phosphorylated and activated by RSK2, including the previously identified CREB and a newly identified RSK2 substrate Hsp27, which are important for the RSK2-mediated pro-invasive ability of HNSCC cells. Hsp27 has been found to be overexpressed in many types of cancers, and its overexpression is associated aggressive tumor behavior. Here we found that RSK2 promotes stabilization of actin filaments in HNSCC cells through phosphorylation and activation of Hsp27 RNAi-mediated knockdown of Hsp27 significantly attenuated HNSCC Tu212 cell invasion conferred by exogenous expression of RSK2. In an in vitro kinase assay using purified recombinant Hsp27 WT and individual S15A, S78A or S82A mutant proteins incubated with active recombinant RSK2, we found that RSK2 directly phosphorylates Hsp27 at S78 and S82, but not S15. Moreover, stable expression of phospho-mimetic Hsp27 S78D/S82D double mutant led to further significantly enhanced cell invasion of poorly invasive HNSCC Tu212 and 686LN cells, compared to exogenous expression of Hsp27 WT. Stable expression of Hsp27 S78D/S82D mutant, but not WT or S78A/S82A mutant, rescued the cell invasion and tumor metastasis attenuated by stable knockdown of RSK2 in highly invasive HNSCC M4e cells in vitro and in vivo using xenograft mice, respectively. Hsp27 regulates actin dynamics and apoptosis. Actin immunofluorescent staining showed that RNAi-mediated stable knockdown of RSK2 resulted in disruption of actin filaments in 212LN and M4e cells. However, stable expression of the Hsp27 phospho-mimetic mutant S78D/S82D, but not WT or the phospho-deficient S78A/S82A mutant, rescued the formation of actin filaments. Our findings demonstrate that RSK2 signals through Hsp27 to regulate actin filaments and cell invasion via direct phosphorylation of Hsp27 at Ser78 and Ser82. This may represent a transcription-independent mechanism underlying RSK2-mediated pro-metastatic signaling in HNSCC cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2367.