is an American Cancer Society Scholar and Leukemia and Lymphoma Society Research Scholar. molecular mechanism for the negative regulation of MTSS1 by -TRCP in cancer cells. It further suggests that preventing MTSS1 degradation could be a possible novel strategy for clinical treatment of more aggressive breast and prostate cancers. Keywords:tumor suppressor, MTSS1, ubiquitination, phosphorylation, IAXO-102 migration == INTRODUCTION == Tumor metastasis is a major problem encountered during clinical anti-cancer treatments, which causes higher mortality in cancer patients [1]. Therefore, elucidating the underlying molecular mechanisms that cause tumor growth and metastasis will lead to the development of more effective therapies, in part by eradicating metastatic cancer cells. To this end, it has been established that for many types of human cancers, tumor cells could acquire the capability to metastasize to distant organs that ultimately results in organ failure and death [1,2]. Although the mechanisms remain largely unknown, overexpression of certain oncoproteins [3] or downregulation of tumor suppressor proteins [4] have been demonstrated to play important roles in the process of tumor growth and metastasis. In this regard, the metastasis suppressor Rabbit Polyclonal to ATP1alpha1 1 (MTSS1) protein, which is also known as MIM (missing in metastasis) has been recently characterized as a tumor suppressor protein [5]. Notably, MTSS1 is mostly expressed in normal tissues and in some non-metastatic cancer cell lines, however its expression is significantly decreased or mostly absent in many metastatic cancers including metastatic bladder cancer [6], prostate cancer [7], gastric cancer [8] and kidney cancer [9], suggesting that MTSS1 could function as an anti-metastatic protein. Furthermore, an inverse correlation IAXO-102 was also observed between MTSS1 expression and poor prognosis in breast cancer [10]. Specifically, findings from large cohort breast cancer clinical samples indicated that decreased MTSS1 expression was positively associated with poorer prognosis, whereas high levels of MTSS1 correlated IAXO-102 with an increased overall patient survival [10]. Interestingly, it was reported that all three MTSS1 splice variants were significantly reduced in prostate cancer, whereas overexpression of MTSS1 markedly reduced the proliferation of prostate cancer cells [7]. These findings indicate that MTSS1 might function as a tumor suppressor, and loss of MTSS1 facilitates the development of human cancers including breast and prostate cancers. However, in contrast to its reduced expression in many human cancers, overpression of MTSS1 was observed in hepatocellular carcinoma [11] although its physiological significance to liver cancer remains elusive. Functionally, MTSS1 acts as a cytoskeletal scaffold protein that regulates cytoskeletal dynamics through interacting with many different proteins such as Rac, actin and actin associated proteins [12-14]. By doing so, MTSS1 increases the formation of lamellipodia, membrane ruffles, and filopodia-like structures and is also involved in promoting the disassembly of actin stress fibers. Mechanistically, MTSS1 contains a WH2 domain in its c-terminal region that preferentially interacts with ATP-bound G-actin, an active form of actin involved in polymerization. Furthermore, it has been observed that MTSS1 possesses five-folds more affinity towards ATP-bound G-actin compared to ADP-bound G-actin associated with actin monomers in the cell. Recent studies have also suggested that MTSS1 competes with the WH2 domain-containing neuronal WiskottAldrich Syndrome protein (N-WASP) VCA protein for binding with G-actin [15]. Given the critical role of N-WASP in actin modeling and cytoskeleton formation [16], these findings reveal a critical role for MTSS1 in G-actin polymerization in part by inhibiting the physiological interaction between N-WASP and G-actin. In addition to interacting directly with G-actin, MTSS1 is also reported to interact with various other proteins such as the Rac GTPase, Cortactin and RPTP, all of which have been well-characterized as critical regulators of cell migration, invasion and cell-cell interaction [14,16,17]. Therefore, MTSS1 might govern various cellular processes including cellular migration or invasion.