CKIP-1 (CK2-interacting protein-1) was initially described on the basis of its interactions with protein kinase CK2 (Bosc et al. 2000). In this respect, the first publication on CKIP-1 described isolation of a cDNA encoding this novel protein from a human B-cell cDNA library using a yeast two-hybrid screen to identify interaction partners of protein kinase CK2. Interactions between CK2 and CKIP-1 were confirmed by co-immunoprecipitation experiments and in vitro interaction assays using recombinant and in vitro translated proteins. Prior to that report, a partial cDNA encoding its C-terminal 72 amino acids had been isolated from a mouse embryo cDNA library, using a yeast two-hybrid screen to identify interaction partners for the leucine zipper region of the c-Jun transcription factor (Chevray and Nathans 1992). Collectively, these reports suggested that the primary functions of CKIP-1 were mediated by interactions with other proteins within cells. Since these initial reports, subsequent studies have reinforced that suggestion by demonstrating that CKIP-1 interacts with a number of other cellular proteins and that CKIP-1 could be involved in a variety of distinct biological processes.
Domain Structure and Architecture
Regulation of CKIP-1
While a detailed understanding of its regulation remains incomplete, a number of potential mechanisms for controlling the activity of CKIP-1 have been reported. These mechanisms include both changes in its level of expression and posttranslational mechanisms. In terms of changes in its level of expression, CKIP-1 has been shown to be upregulated during muscle differentiation (Safi et al. 2004) and to be induced by cytokines (Zhang et al. 2007). Posttranslational mechanisms include PI3-kinase-dependent interactions between its PH domain and phosphatidylinositol 3-phosphate in the plasma membrane (Safi et al. 2004). By comparison, independent studies have demonstrated that CKIP-1 is constitutively localized to the plasma membrane and exhibits relatively nonspecific interactions with membrane phospholipids raising questions about its precise interactions with phosphatidylinositols (Olsten et al. 2004). It does not appear that the PH domain of CKIP-1 is exclusively involved in interactions with membrane phospholipids. In this respect, the PH domain of CKIP-1 has also been shown to be important for interactions with some of its protein partners, including CK2 and Akt (Olsten et al. 2004; Tokuda et al. 2007). In addition to its localization to the plasma membrane, it has also been reported that CKIP-1 can be localized within the cytoplasm and the nucleus of cultured cells (Bosc et al. 2000; Litchfield et al. 2001; Xi et al. 2010). In one instance, CKIP-1 has been identified as a substrate for caspase 3 following the induction of apoptosis (Zhang et al. 2005). In this situation, a C-terminal fragment of CKIP-1 is translocated to the nucleus where it represses AP-1 activity, presumably through interactions with c-Jun. An additional mechanism that may contribute to its regulation in cells is posttranslational modification including phosphorylation (Zhang et al. 2005, 2006) and potentially ubiquitination. From this perspective, a number of distinct phosphorylation sites and one additional ubiquitination site have been identified for CKIP-1 as summarized in the PhosphoSitePlus database (Hornbeck et al. 2014) that includes an analysis of data from the published literature and from global proteomic studies.
Biological Functions of CKIP-1
In view of its relatively diverse collection of binding partners, it is perhaps not surprising that CKIP-1 appears to participate in a variety of biological events (Fig. 2). In this respect, when it is expressed in cells under the control of an inducible promoter, increased expression of CKIP-1 promotes changes in cell morphology and the actin cytoskeleton, presumably through its interactions with the actin-capping protein (Canton et al. 2005). Through its induction by cytokines and interactions with proteins such as IFP35 and Nmi, CKIP-1 has also been implicated in cytokine signaling (Zhang et al. 2007) as well as other aspects of inflammation and immune cell signaling (Sakamoto et al. 2014; Zhang et al. 2014). Since interfering with activation of the PI3-kinase pathway or the expression of CKIP-1 in C2C12 cells blocks differentiation, it appears that CKIP-1 is a component of PI3-Kinase-regulated muscle differentiation (Safi et al. 2004). Additional involvement with the PI3-kinase pathway comes from the demonstration that increased CKIP-1 expression results in impaired growth of tumor cells in vitro and in mouse Xenografts that is accompanied by inactivation of Akt (Tokuda et al. 2007). In addition to its involvement in these events, there is mounting evidence demonstrating that CKIP-1 has an important role in bone homeostasis (Lu et al. 2008), a function that may relate to its designation as an osteoclast maturation-associated gene (i.e., OC120). Through its interactions with Smurf1, CKIP-1 promotes an activation of the ubiquitin ligase activity of Smurf1 (Wang et al. 2012). Furthermore, in CKIP-1-deficient mice, a decrease in Smurf1 activity is accompanied by an age-dependent increase in bone mass. Confirmation of its role in bone formation comes from studies where bone mass was enhanced in rats in response to targeted delivery of CKIP-1-specific siRNA to osteogenic cells in rats (Zhang et al. 2012). In addition to these roles related to immune function, muscle, and bone, CKIP-1 has also been implicated in cardiac hypertrophy (Ling et al. 2012), tumor suppression (Nie et al. 2014), and adipogenesis (Li et al. 2014).
Since its original characterization as a CK2-interacting protein and as a putative partner for the leucine zipper of the c-Jun transcription factor, additional partners for CKIP-1 have been characterized. In conjunction with the identification of its partners, CKIP-1 has been implicated in a number of distinct biological events. Although it would appear that a comprehensive understanding of CKIP-1 and its regulation and functions is far from complete, its involvement in processes such as muscle differentiation, tumor growth, bone homeostasis, as well as cardiac hypertrophy and adipogenesis suggests that a detailed understanding of CKIP-1 could ultimately yield novel approaches for preventing or treating disease. It can also be envisaged that the involvement of CKIP-1 in processes that underlie human health and disease will spur more attention on its regulation and functional properties. Challenges for the future include efforts to elucidate its high-resolution structure, comprehensive identification of its cellular partners, and thorough evaluation of its expression levels at both gene and protein levels during development and in response to different stimuli. Collectively, this information will yield a more precise understanding of CKIP-1 and its role in biological processes that could ultimately instruct efforts to target its activity for the prevention or treatment of diseases such as cancer, musculoskeletal disorders, and heart disease.
- Barrios-Rodiles M, Brown KR, Ozdamar B, Bose R, Liu Z, Donovan RS, Shinjo F, Liu Y, Dembowy J, Taylor IW, Luga V, Przulj N, Robinson M, Suzuki H, Hayashizaki Y, Jurisica I, Wrana JL. High-throughput mapping of a dynamic signaling network in mammalian cells. Science. 2005;307:1621–5.CrossRefPubMedGoogle Scholar
- Canton DA, Olsten ME, Kim K, Doherty-Kirby A, Lajoie G, Cooper JA, Litchfield DW. The pleckstrin homology domain-containing protein CKIP-1 is involved in regulation of cell morphology and the actin cytoskeleton and interaction with actin capping protein. Mol Cell Biol. 2005;25:3519–34.CrossRefPubMedPubMedCentralGoogle Scholar
- Ling S, Sun Q, Li Y, Zhang L, Zhang P, Wang X, Tian C, Li Q, Song J, Liu H, Kan G, Cao H, Huang Z, Nie J, Bai Y, Chen S, Li Y, He F, Zhang L, Li Y. CKIP-1 inhibits cardiac hypertrophy by regulating class II histone deacetylase phosphorylation through recruiting PP2A. Circulation. 2012;126:3028–40.CrossRefPubMedGoogle Scholar
- Safi A, Vandromme M, Caussanel S, Valdacci L, Baas D, Vidal M, Brun G, Schaeffer L, Goillot E. Role for the pleckstrin homology domain-containing protein CKIP-1 in phosphatidylinositol 3-kinase-regulated muscle differentiation. Mol Cell Biol. 2004;24:1245–55.CrossRefPubMedPubMedCentralGoogle Scholar
- Sakamoto T, Kobayashi M, Taka K, Shinohara M, Io K, Nagata K, Iwai F, Takiuchi Y, Arai Y, Yamashita K, Shindo K, Kadowaki N, Koyanagi Y, Takaori-Kondo A. CKIP-1 is an intrinsic regulator of T-cell activation through an interaction with CARMA1. PLoS One. 2014;9:e85762.CrossRefPubMedPubMedCentralGoogle Scholar
- Tokuda E, Fujita N, Oh-hara T, Sato S, Kurata A, Katayama R, Itoh T, Takenawa T, Miyazono K, Tsuruo T. Casein kinase 2-interacting protein-1, a novel Akt pleckstrin homology domain-interacting protein, down-regulates PI3K/Akt signaling and suppresses tumor growth in vivo. Cancer Res. 2007;67:9666–76.CrossRefPubMedGoogle Scholar
- Zhang G, Guo B, Wu H, Tang T, Zhang BT, Zheng L, He Y, Yang Z, Pan X, Chow H, To K, Li Y, Li D, Wang X, Wang Y, Lee K, Hou Z, Dong N, Li G, Leung K, Hung L, He F, Zhang L, Qin L. A delivery system targeting bone formation surfaces to facilitate RNAi-based anabolic therapy. Nat Med. 2012;18:307–14.CrossRefPubMedGoogle Scholar