Emerging Roles for SSeCKS/Gravin/AKAP12 in the Control of Cell Proliferation, Cancer Malignancy, and Barriergenesis

Abstract

Emerging data suggest that SSeCKS/Gravin/AKAP12 ("AKAP12"), originally identified as an autoantigen in cases of myasthenia gravis, controls multiple biological processes through its ability to scaffold key signaling proteins such as protein kinase (PK) C and A, calmodulin, cyclins, phosphoinositides, "long" β-1,4 galactosyltransferase (GalTase) isoform, Src, as well as the actin cytoskeleton in a spatiotemporal manner. Specialized functions attributed to AKAP12 include the suppression of cancer malignancy, especially aspects of metastatic progression, regulation of blood-brain and blood-retina barrier formation, and resensitization of β2-adrenergic pain receptors. Recent data identify a direct role for AKAP12 in cytokinesis completion, further suggesting a function as a negative regulator of cell senescence. The current review will discuss the emerging knowledge base of AKAP12-related biological roles and how the factors that affect AKAP12 expression or that interact with AKAP12 at the protein level control cancer progression and blood-tissue barrier formation.

Keywords: G1→S progression; PKA; PKC; Ras; SSeCKS/Gravin/AKAP12; Src; VEGF; cell motility; cell-cell barriers; cyclin D; cytokinesis; metastasis; neovascularization; prostate cancer; tumor invasiveness.

Figure 1.

AKAP12 scaffolding domains. Known PKC phosphorylation sites^, are identified at top as Ser507/515, 599, and 748 (based on the rat protein sequence: NP_476444.2) as well as an implied EGF- or PDGF-induced, FAK-dependent tyrosine phosphorylation site at Y835. A Src SH3 binding site (P¹⁵xxP). AKAP12 also encodes at least 4 nuclear localization signals (NLS) homologous to those from SV40 Tag, a cyclin binding domain containing 2 CY motifs, an acidic/basic region (KR- and E/D-rich), an A kinase anchoring protein (AKAP) domain for PKA RII isoform binding, a nuclear exclusion domain, 3 polybasic domains involved in membrane association, at least 4 calmodulin binding domains of the so-called 1-5-10 motif, a phosphatidylserine (PS) binding domain that facilitates association with PKC isoforms, 2 distinct binding domains for β1,4-galactosyltransferase (B4GALT1), and an F-actin binding domain.

Figure 2.

Control of cell shape and cytoskeletal dynamics by AKAP12. The downregulation of AKAP12 in v-Src–transformed NIH3T3 murine fibroblasts correlates with a transition from the polygonal morphology of parental NIH3T3 (top panel, middle) to a so-called fusiform morphology (top panel, left). In contrast, overexpression of AKAP12 via a Tet-OFF vector system causes cell flattening, the thinning of stress fibers, and the production of long filamentous (F) projections (top panel, right). The staining intensities in the top panel reflect relative levels of AKAP12. AKAP12 loss in MEF (bottom left) or in stellate mesangial cells due to treatment with antisense (ASN) AKAP12 oligonucleotides (bottom right) results in cell flattening marked by thickened, longitudinal stress fibers (“F-actin”) and increased numbers of focal adhesion plaques (“vinculin”).

Figure 3.

Decreased AKAP12 expression in metastatic progression. Oncomine study data showing statistically lower AKAP12 RNA expression levels in metastases compared to primary-site colon, endocrine, or breast cancers, as well as lower AKAP12 RNA levels correlating with incidence of metastasis formation at 1 year after primary diagnosis (left).

Figure 4.

Pathways controlled by AKAP12 in cancer. AKAP12 suppresses cancer progression by disengaging adhesion- and growth factor–induced activation of Src-FAK complexes from transducing Raf/MEK/ERK- and JNK-mediated proliferation, angiogenesis, and cytoskeletal remodeling signals, possibly through a direct binding of AKAP12 to the Src-SH3 domain, resulting in the altering of Src-FAK signaling complexes. AKAP12 also directly inhibits PKC activation by a direct scaffolding function. In contrast, AKAP12 scaffolding of PKA facilitates cAMP-induced cytoskeletal remodeling.