ReviewPhosphorylation of connexin43 induced by Src: Regulation of gap junctional communication between transformed cells
Section snippets
Src, Cx43, and cancer
Intercellular communication plays a crucial role in regulating cell growth, differentiation, and migration. Communication between cells can be achieved either directly by cell–cell contact through cell junctions or indirectly by the release of diffusible signaling factors that are transmitted between cells. This review focuses on communication mediated by gap junctions.
Gap junctions are composed of transmembrane proteins called connexins. Connexins have evolved into a family of about 20 members
Src phosphorylation of Cx43
Decreased gap junctional communication between Src transformed cells was first reported over 40 years ago [23]. Later use of a temperature sensitive Src construct found that Src exhibits a dynamic ability to block gap junctional communication [24]. Subsequent cloning of connexins enabled molecular characterization of how Src modifies Cx43 to affect gap junctional communication [25]. For example, Crow et al. demonstrated that Src induces tyrosine phosphorylation on Cx43 and that this event
MAPK phosphorylation of Cx43
Src utilizes MAPK to promote tumor cell growth. The Src kinase initiates signaling cascades that are required for malignant cell growth [7]. For example, Src activates the Ras GTPase, thus enabling MAPKKK to activate MAPKK which, in turn, activates MAPK. MAPK is a serine kinase that promotes transformed growth characteristics required for cancer progression [31], [32].
Src may induce MAPK to phosphorylate Cx43 and block gap junctional communication. As shown in Fig. 2 and Table 1, MAPK can
PKC phosphorylation of Cx43
PKC is a well established Src effector molecule that promotes tumor cell growth. Src stimulates phospholipase C activity, which increases diacylglycerol and calcium mobilization. These lead to increased activity of the calcium dependent PKC. In addition, Src may directly activate PKC by phosphorylation [34], [35].
Src may induce PKC to phosphorylate Cx43 and alter channel permeability. PKC is a serine/threonine protein kinase that phosphorylates Cx43 on S368 and S372 (see Fig. 2 and Table 1) [36]
Cdc2 phosphorylation of Cx43
Cdc2 is a cyclin dependent serine kinase that is active during mitosis. The Cdc2 kinase moves to the plasma membrane at the beginning of mitosis where it phosphorylates Cx43 at S255 and S262 (see Fig. 2 and Table 1). This Cdc2 kinase-dependent phosphorylation of Cx43 correlates with gap junction internalization at the onset mitosis [15], [20], [21], [44]. Thus, Cdc2 kinase activity may phosphorylate Cx43 to inhibit gap junctional communication between mitotically active Src transformed cells.
PKA phosphorylation of Cx43
PKA is a cAMP responsive protein kinase that can ultimately lead to phosphorylation of Cx43 at S364, S365, S369, and S373 [22], [45] (see Fig. 2 and Table 1) [22], [46]. In contrast to PKC and MAPK, PKA activity can increase gap junctional communication, stabilize gap junctions, and facilitate the assembly of new gap junctions [15], [17], [44], [47], [48].
Along with promoting gap junctional communication by Cx43, PKA can counteract Src and MAPK activity [49], [50], [51] (see Fig. 1). However
CK1 phosphorylation of Cx43
CK1 is a serine/threonine kinase that can phosphorylate Cx43 at S325, S328, and S330 (see Fig. 2 and Table 1). CK1 activity appears to be required for, or at least promote, the ability of Cx43 to form functional gap junction channels in some cells [44], [52]. While interplay between Src and CK1 in transformed cells has not been established, it should be noted that Src can modulate the activity of other casein kinases including CK2 [53], [54].
Other considerations and factors
As discussed above Src orchestrates cross talk between protein kinases that phosphorylate Cx43. However, Src also causes significant effects on the transcriptome, proteome, and cytoskeleton that can also affect gap junctional communication. For example, the expression of approximately 8% of transcribed genes is altered in Src transformed cells, indicating that Src can regulate the expression of hundreds of genes [55], [56]. Some of these genes may encode kinases or factors that regulate kinase
Additional complexities and stoichiometry
The structure of the gap junction channel should be taken into account when considering the effects of Cx43 phosphorylation on intercellular communication. We have assembled available structural data to create gap junction models shown in Fig. 2 [66], [67], [68]. These models suggest that negative regulatory phosphorylation sites may be located towards the actual transmembrane channel and positive regulatory sites oriented farther out into the cytoplasm. However, the stoichiometry of
Conclusion
Although this review is focused on Src, it should be noted that transformation by a variety of other oncogenes, carcinogens, and tumor promoters can also lead to decreased gap junctional communication by Cx43 [4], [5]. Moreover, although the effects of cell transformation on gap junctions are apparent, the consequences of this decreased intercellular communication on tumor cell growth and progression are not well defined. More complete understanding of this relationship will require in depth
Acknowledgments
This work was supported in part by grants from the United States National Institutes of Health CA88805 and EY014479 to GSG.
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