km23: A novel TGFβ receptor-interacting protein important in the trafficking of TGFβ signaling components


km23 is a novel "motor receptor" involved in TGFβ signaling (Ding and Mulder, CTR 119:315-27, 2004; Jin et al, 2007b). We have constructed the model shown below to depict the intracellular functions of km23. Upon phosphorylation of km23 by the TGFβ receptors (TβRs), km23 helps to recruit endosomal TGFβ signaling components to the dynein motor complex through the intermediate chain (DIC). This km23-containing motor complex facilitates the trafficking of endosomal complexes containing TGF&beta signaling components along the microtubules toward the nucleus. After trafficking through multiple vesicular compartments, TGFβ signaling complexes can be translocated to the nucleus for transcriptional regulation of target genes, at which time they are no longer co-localized with km23. Smad2 is shown as an example of a relevant TGFβ signaling component in the model below (Jin et al, JBC 282, PIP, Apr 9 2007). We are currently identifying other cargoes (ie, endosomal signaling complexes) that km23 helps transport along the microtubules. In addition, we are identifying the precise serine residues on km23 that are phosphorylated by TGFβ as well as other upstream kinases involved in km23 phosphorylation.


A light chain of the motor protein dynein frequently altered in human ovarian cancer


We have cloned a novel TGFβ receptor-interacting protein, termed km23 (Tang et al, MBC 13:4484-96, 2002). This protein is the mammalian homologue of the km23/LC7/robl/DYNLRB/Dnlc2 family of dynein light chains (DLCs). TGFβ stimulates not only the phosphorylation of km23, but also the recruitment of km23 to the dynein intermediate chain (DIC). Kinase-active TGFβ receptors are required for km23 phosphorylation and interaction with DIC (Tang et al, MBC 13:4484-96, 2002). Moreover, km23 can mediate specific TGFβ responses, including activation of Jun N-terminal kinases (JNKs), phosphorylation of c-Jun, and growth inhibition. Blockade of km23-1 results in reduced Smad2-dependent TGFβ signaling. Furthermore, we have identified altered forms of km23 both in TGFβ-resistant human ovarian cancer cell lines and in cancer tissues from ovarian cancer patients (Ding and Mulder, CTR 119:315-27, 2004; Ding et al, CR 65:6526-33, 2005). Our data suggest that these alterations in km23 modify km23 functions in TGFβ signaling and tumorigenesis. We have also shown that km23 may be a tumor suppressor for ovarian cancer. Novel functions of km23, in addition to its role in TGFβ signaling, have also been identified and indicate that km23 plays an important role during mitosis. Ongoing studies are addressing the mechanisms underlying this additional critical function of km23. Further, the effects of knocking out km23 are being investigated to identify other novel functions of km23.


A novel anti-cancer target for the development of diagnostics and therapeutics


We have identified km23-1 as a novel TGFβ signaling component, which is also a light chain of the motor protein dynein. We have also identified alterations of km23 in 42% of epithelial ovarian cancers from patients; these mutations do not occur in normal tissues from the same patient (Ding et al, CR 65:6526-33, 2005). In order to develop km23-based therapeutic agents, we have determined the precise three-dimensional structure of km23 (Ilangovan et al, JMB 352:338-54, 2005), and plan to compare this structure to that resulting from the mutations in km23 we have identified in the ovarian cancer patients. These studies are part of an ongoing collaboration with investigators at the University of Texas Health Sciences Center at San Antonio. We have also developed a novel screening method to detect km23 alterations in the circulating nucleic acids in the plasma/serum (CNAPS) of ovarian cancer patients. A U.S. patent was recently issued to KMM related to this method. The overall goal is to use this screening method to identify the class of patients that will respond to the km23-based therapeutics we are developing.


Biological Significance of TGFβ activation of Ras/MAPKs/JNKs


We have also shown that TGFβ activation of both the extracellular signal-regulated kinase (ERK) and JNK/SAPK MAPK cascades is required for the ability of TGFβ to induce its own production. Further, we have identified the precise AP-1 proteins that mediate this biological response to TGFβ (Yue and Mulder, JBC 275:30765-73, 2000; Yue et al, JCP 199:294-92, 2004; Liu et al, JBC 281:29479-90, 2006), and are investigating other relevant transcription factors. Understanding the precise mechanisms underlying TGFβ production/secretion into the tumor microenvironment are important, since blockade of this effect in late-stage solid cancers should reverse the paracrine, tumor enhancing effects of TGFβ (Liu et al, MC 45:582-93, 2006). We are currently determining which of the signaling components mediating TGFβ production/secretion can be selectively targeted to block tumor progression by TGFβ in vivo.


Overall Goals


The major objective of the research in the Mulder Lab is to identify alterations in TGFβ signaling pathways that contribute to tumor formation or progression in ovarian, colon, and breast cancer models. Specific TGFβ signaling components are being investigated as critical therapeutic targets for the restoration of negative growth control by TGFβto solid tumors. In addition, we are defining which TGFβ signaling pathways lead to the growth inhibitory effects of TGFβ in epithelial cells and which lead to the tumor-enhancing effects of TGFβ in vivo. Selective targeting of these signaling pathways is being evaluated to determine the effects of TGFβ-based therapeutics on tumor formation or progression in vivo.