Winfried Elis

TBC1D7 is a third subunit of the TSC1-TSC2 complex upstream of mTORC1.

The tuberous sclerosis complex (TSC) tumor suppressors form the TSC1-TSC2 complex, which limits cell growth in response to poor growth conditions. Through its GTPase-activating protein (GAP) activity toward Rheb, this complex inhibits the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1), a key promoter of cell growth. Here, we identify and biochemically characterize TBC1D7 as a stably associated and ubiquitous third core subunit of the TSC1-TSC2 complex. We demonstrate that the TSC1-TSC2-TBC1D7 (TSC-TBC) complex is the functional complex that senses specific cellular growth conditions and possesses Rheb-GAP activity. Sequencing analyses of samples from TSC patients suggest that TBC1D7 is unlikely to represent TSC3. TBC1D7 knockdown decreases the association of TSC1 and TSC2 leading to decreased Rheb-GAP activity, without effects on the localization of TSC2 to the lysosome. Like the other TSC-TBC components, TBC1D7 knockdown results in increased mTORC1 signaling, delayed induction of autophagy, and enhanced cell growth under poor growth conditions.

Down-Regulation of Class II Phosphoinositide 3-Kinase α Expression below a Critical Threshold Induces Apoptotic Cell Death

Members of the phosphoinositide 3-kinase (PI3K) family collectively control multiple cellular responses, including proliferation, growth, chemotaxis, and survival. These diverse effects can partly be attributed to the broad range of downstream effectors being regulated by the products of these lipid kinases, the 3′-phosphoinositides. However, an additional layer of complexity is introduced by the existence of multiple PI3K enzyme isoforms. Much has been learned over the last years on the roles of the classes I and III PI3K members in cellular signaling, but little is known about the isoform-specific tasks done by the class II PI3Ks (C2α, β, and γ). In this study, we used quantitative reverse transcription–PCR and RNA interference in mammalian cells to gain further insight into the function of these lesser studied PI3K enzymes. We find that PI3K-C2α, but not PI3K-C2β, has an important role in controlling cell survival and by using a panel of RNA interference reagents, we were able to determine a critical threshold of PI3K-C2α mRNA levels, below which the apoptotic program is switched on, via the intrinsic cell death pathway. In addition, knockdown of PI3K-C2α to levels that by themselves do not induce apoptosis sensitize cells to the anticancer agent Taxol (paclitaxel). Lastly, we report that lowering the levels of PI3K-C2α in a number of cancer cell lines reduces their proliferation and cell viability, arguing that PI3K inhibitors targeting not only the class Iα isoform but also class IIα may contribute to an effective anticancer strategy. (Mol Cancer Res 2008;6(4):614–23)

Abstract 2733: LJM716: an anti-HER3 antibody that inhibits both HER2 and NRG driven tumor growth by trapping HER3 in the inactive conformation.

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL HER3 (ErbB3) is a member of the ErbB family of receptor tyrosine kinases (RTK). Inappropriate HER2/ HER3 dimerization as a result of HER2 or neuregulin (NRG) over-expression in cancer results in HER3 mediated activation of PI3K signaling. Consequently, HER3 is a mediator of oncogenic transformation. Although, ligand blocking HER3 antibodies inhibit growth of neuregulin driven xenograft models they are ineffective in models of HER2 amplified cancer as HER2 mediated activation of HER3 occurs in a ligand- independent manner. LJM716 is a high affinity HER3- targeted antibody selected from a Human Combinatorial Antibody Library (HuCAL) specifically for its ability to neutralize multiple modes of HER3 activation. LJM716 is a potent inhibitor of HER3/ AKT phosphorylation and proliferation in a range of HER2 amplified and NRG expressing cell lines in vitro. LJM716 induced tumor regression in Fadu (NRG expressing, HNSCC) tumor xenografts and significant tumor growth inhibition (>80%) in a variety of xenograft models including BT474 (HER2 amplified breast). Furthermore, the combination of LJM716 with trastuzumab, cetuximab or PI3K- targeted agents was synergistic in a panel of in vitro cell lines while the in vivo combination of LJM716 with trastuzumab or erlotinib was efficacious in BT474 and L3.3 (pancreatic) tumor xenografts respectively. To further understand the mechanism by which LJM716 inhibits multiple modes of HER3 activation we solved the crystal structure of LJM716 bound to the HER3 extra-cellular domain. This data revealed that LJM716 binds to a novel conformational epitope contained within domains 2 and 4 and appears to trap HER3 in the inactive conformation. Interestingly, LJM716 does not block NRG binding to HER3 nor does it affect the binding affinity of the HER3/ NRG interaction. LJM716 therefore possesses a novel mechanism of action; it prevents the structural rearrangements required for HER3 activation induced by either HER2 or NRG. Thus LJM716 is the first HER3 antibody to display efficacy in both HER2 and ligand driven xenograft models. Based on preclinical data, combining LJM716 with either trastuzumab, cetuximab or PI3K- targeted agents may lead to greater and more sustained clinical efficacy in ErbB driven cancers. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2733. doi:1538-7445.AM2012-2733