PTPN14 is required for the density-dependent control of YAP1

Inactivation of tumor suppressors and activation of oncogenes lead to uncontrolled proliferation and neoplastic transformation (Hanahan and Weinberg 2011). In recent years, as the remarkable prevalence of activated protein tyrosine kinases (TKs) as oncoproteins and their mutations are being identified in numerous cancers, the control of protein tyrosine phosphorylation has been considered to play a central role in ensuring the homeostasis of cellular physiology and thus preventing tumorigenesis (Lemmon and Schlessinger 2010). Another family of enzymes, protein tyrosine phosphatases (PTPs), can contribute to this equilibrium of protein tyrosine phosphorylation and thereby antagonize the oncogenic activities of TKs (Alonso et al. 2004; Tonks 2006). Therefore, PTPs are prominently considered to act as tumor suppressors. Indeed, numerous genetic alterations have been identified in genes encoding PTPs, which contribute to their functional loss and, consequently, cancer development (Tonks 2006; Julien et al. 2011). For example, the receptor tyrosine phosphatases PTPRA (Ardini et al. 2000) and PTPRG (Zheng et al. 2000) were found to be down-regulated in breast cancers. PTEN (Liaw et al. 1997; Garcia et al. 2004) and PTPRF (Levea et al. 2000) were shown to harbor genetic deletion or mutations in several tumor types. PTPN12 was recently discovered to be frequently inactivated in triple-negative breast cancers (Sun et al. 2011). PTPN13 is considered a suppressor for HER2-positive breast cancers, as it can antagonize HER2 activation (Zhu et al. 2008). PTPRK (Nakamura et al. 2003), PTPN7 (Fridberg et al. 2008), and PTPN13 (Ying et al. 2006) were found to be down-regulated or mutated in lymphoma. Loss of function of DUSP1 (Denkert et al. 2002) and DUPS6 (Chan et al. 2008) was detected in ovarian cancers. Although the enzyme properties and crystal structures of many PTPs have been studied extensively (Wadham et al. 2003; Julien et al. 2011), the detailed mechanisms of their tumor suppressor functions remain to be explored because of the slow progress in the identification of their physiological substrates. Remarkably, some PTPs regulate cellular processes in a manner that is independent of their phosphatase activities. For example, PTPRM can stabilize cell–cell adhesion and protect tissue homeostasis through its physical interaction with E-cadherin (Sallee et al. 2006) and protein kinase C (Hellberg et al. 2002). These findings suggest that different PTPs may use distinct mechanisms in regulating various cellular signaling events. To identify tumor suppressors involved in breast cancer development, we performed a loss-of-function screen of human PTPs and identified PTPN14 as a potential tumor suppressor. PTPN14 (also known as PTPD2) is a nonreceptor PTP whose cellular functions are not well characterized. PTPN14 was initially identified as a cytoskeleton-associated protein with an N-terminal FERM domain (Smith et al. 1995), which plays important roles in cell adhesion and proliferation (Ogata et al. 1999). In HUVEC cells, PTPN14 was shown to mediate β-catenin dephosphorylation at adhesion junctions (Wadham et al. 2003). Moreover, density-dependent nucleus-to-cytoplasm translocation of PTPN14 was reported to be important for cell proliferation (Wadham et al. 2000), and overexpression of PTPN14 in MDCK cells led to the epithelial–mesenchymal transition (Wyatt et al. 2007; Wyatt and Khew-Goodall 2008). Interestingly, several PTPN14 mutations have been shown to be associated with human breast and colorectal cancers, implying that PTPN14 may act as a tumor suppressor (Wang et al. 2004; Laczmanska and Sasiadek 2011). Loss-of-function mutation of PTPN14 was also found to be correlated with autosomal recessive lymphedema-choanal atresia syndrome (Au et al. 2010). More recently, PTPN14 was shown to regulate angiogenesis and be involved in a vascular dysplasia syndrome, hereditary hemorrhagic telangiectasia (Benzinou et al. 2012). However, the mechanisms by which PTPN14 acts in these disease processes remain unknown. In this study, we show that PTPN14 associates with and negatively regulates yes-associated protein 1 (YAP1) (Yagi et al. 1999), the key oncoprotein that acts downstream from the hippo pathway. PTPN14 protein level was elevated in cells at high density, which correlated with cytoplasmic location and inhibition of YAP1. The stability of PTPN14 was controlled by cell density via the CRL2LRR1 (cullin2 RING ubiquitin ligase complex/leucine-rich repeat protein 1) E3 ligase complex. Together our findings suggest that PTPN14 acts as a negative regulator for YAP1 in a cell density-dependent manner.