Numerous pathological conditions may induce the phosphorylation of the focal adhesion partners paxillin and FAK, with the consequent activation of intracellular signals leading to the alteration of cell cytoskeleton, cell division, and cell motility.
Stress and oxidation
Conditions involving oxidation, mechanical traction, or hypotonic challenge (HTS) have been shown to induce FAK and paxillin phosphorylation and the translocation/activation of RhoA. Athough FAK and paxillin tyrosine phosphorylation is not required for the activation of RhoA/RhoK signaling, the simultaneous activation of RhoA/Rho kinases and FAK/paxillin, driven either by mechanical stress or by receptor activation-induced ATP release, has been found to promote actin cytoskeleton reorganization leading to functional repercussion [
98].
Oxidative stress is one of the most common apoptosis-inducing factors in several organs, from the intestinal and cardiovascular systems to neuronal cells, including the sensory organs [
99]. In pathological events involving ischemia–reperfusion, reactive oxygen species (ROS), such as superoxide anion (O2·2), hydrogen peroxide (H
2O
2), and hydroxyl radical (HO·), are released by the ischemic tissue. ROS, particularly the hydroxyl radical (HO·), are capable of stimulating the tyrosine phosphorylation of paxillin, FAK, and p130cas in human endothelial cells which, in turn, triggers the activation of signaling cascades that regulate cell proliferation and differentiation. Importantly, recent studies show that PAX-FAK-p130cas phosphorylation induced by intracellular hydroxyl radical (HO·) stimulates polymorphonuclear lymphocyte (PMN) adhesion to human endothelial cells and their subsequent migration, suggesting an important role of FAK/PAX activation by ROS in PMN recruitment during inflammation [
100].
Cancer
Paxillin involvement in cell migration was initially suggested by the high levels of expression of the phosphorylated protein determined in several cancer tissues and metastatic cancer cells, in parallel to increased epithelial-mesenchymal transition [
104].
A main characteristic of transformed epithelial cancer cells is the ability to survive and proliferate in the absence of contact with immobilized ECM [
105]. Paxillin is known to acquire gain of function mutations that are associated with alterations in the malignant progression of many tumors including breast, lung, prostate, melanoma, and colorectal cancer [
106]. Among these, glioblastoma multiforme (GBM), the most common and malignant type of glioma, remains one of the most lethal cancers in the central nervous system (CNS). Recent studies have demonstrated that the overexpression of paxillin at the RNA and protein level is associated with GBM tumor malignancy and hence, predictive of poor survival [
107]. These data identify paxillin as a novel prognostic biomarker with potential anti-invasion therapeutic implications in GBM. On this line, a total of 21 unique paxillin mutations were identified in lung cancer tissue specimens and cell lines. A127T, the most common of these mutations, increases tumor growth and invasion in vivo [
108]. Importantly, paxillin δ is able to suppress full-length (α)-paxillin signaling as well as interactions with actin-binding and integrin-linked proteins due to the lack of LD1 domain, thus suggesting a potential role in suppressing the migratory phenotype of cancer cells [
10].
Recent data show that maximal tyrosine phosphorylation of paxillin by Src and FAK is required for the induction of anchorage-independent signal transduction and proliferation, characteristic of metastatic cells [
105,
109]. Along this line, studies by Sero et al. [
110] in embryonic fibroblasts (MEF) show that paxillin integrates physical and chemical motility signals by spatially constraining the location of motile processes, thereby regulating directional migration. These findings indicate that complex multimeric and competing interactions on paxillin are required to augment anchorage-independent cell proliferation in cancer development.
A growing number of signaling molecules have been shown to promote or regulate cell migration of cancer cells through the phosphorylation of paxillin at Tyr118 and Ser178 by FAK, which alters the organization of FAs, with the consequent promotion of cell motility [
2,
111]. In fact, the increased phosphorylation of these residues is considered as indicative of metastasis.
Multiple receptor-activated signaling pathways have been involved in cell transformation and migration underlying metastasis in breast cancers. Particularly, paxillin has been shown to control the signaling of estrogen (17-β estradiol) to FAK/N-WASP/Arp2/3 complex in breast cancer cells [
112]. On this line, the vitamin A derivative retinoic acid (RA), frequently used in cancer therapy, has been shown to induce Src/FAK/PI3K complex signaling to cell attachment, migration, and invasion mediated by the rapid activation of the actin-binding protein moesin. Within this pathway, cell migration of breast cancer cells is prevented by RA through the regulation of the expression of Moesin and the downstream inhibition of Src, FAK, and paxillin activity, providing novel mechanistic clues for the development of new drugs for cancer treatment [
113]. Also on this matter, paxillin has been shown to be transcriptionally upregulated and phosphorylated by human epidermal growth factor receptor-2 (HER2) signaling in vitro. Since HER2 overexpression has been involved in metastatic cancer progression, particularly in breast cancer, paxillin has been identified as a marker of aggressive breast cancer and a promoter of neoplastic growth and invasion, supporting its role as a biomarker or therapeutic target [
114,
115].
Rho/ROCK signaling plays a crucial role in the regulation of FAs and cell motility [
116]. The serine/threonine protein phosphatase inhibitor calyculin A has been shown to promote focal adhesion assembly upon the inhibition of the ROCK downstream target serine/threonine myosin light chain (MLC) phosphatase, and the resulting tyrosine phosphorylation of FAK, paxillin, and p130cas, thus supporting calyculins as potential anticancer agents [
117]. The FAK/paxillin pathway regulates small Rho GTPases including RhoA, Rac1, and Cdc42, which are critical determinants of cancer cell migration. Importantly, the specific interaction of Rho GTPases with tumor supressor deleted in liver cancer-1 (DLC1) has been shown to decrease FAK-dependent localization of paxillin at immature focal adhesions, thus controlling the lifetime of nascent focal adhesions [
118]. Since this process is FAK-independent in migrated cells, these data indicate that the function of paxillin/FAK/RhoA signaling in the control of cell movement is developmentally regulated. On this matter, the serine/threonine kinase maternal embryonic leucine zipper kinase (MELK) has recently been shown to promote cell migration and invasion via the activation of RhoA and the downstream phosphorylation of FAK and paxillin. Hence, MELK has emerged as a potential cancer biomarker [
119]. The human phosphatase and tensin homolog (PTEN) tumor suppressor is mutated at high frequency in a large number of cancers. Recent work in colon cancer cells has shown that PTEN function as a tumor suppressor derives from the downregulation of paxillin expression by the inhibition of PI3K/AKT/NF-kB signaling. This allows the activation of NF-kB and the promotion of paxillin expression, thus stimulating cell invasion, migration, and cancer progression [
14]. In agreement with this evidence, proteolysis of paxillin by calpain between the LD1 and LD2 motifs in HeLa cells generates a carboxy-terminal 55 KDa fragment, similar to delta paxillin, which functions as an antagonist of endogenous paxillin and may function to limit cancer cell invasion by preventing the assembly/disassembly of FAs and cell migration [
120,
121].
Recent studies in human pancreatic ductal adenocarcinoma cells (PANC-1) have shown that the elevation of cAMP concentration by distinct stimuli results in the inhibition of migration of several PANC cell types in strict correlation with the inhibition of FA turnover, a highly significant loss of paxillin from FAs, and the consequent cessation of ruffling. These data clearly indicate that the inhibitory effect of cAMP on migration, ruffling, FA dynamics, and paxillin localization is mediated by the downstream activation of PKA, while the inhibition of this enzyme potentiates migration [
122].
Hepatocyte growth factor (HGF)-induced c-Met signaling plays critical roles in the progression of hepatocellular carcinoma (HCC). On this line, PKCε-mediated c-Met endosomal processing was shown to stimulate c-Met-JNK-induced paxillin (Ser178) phosphorylation, which is required for cell migration, invasion, and intrahepatic metastasis [
123].