Protean PTEN: form and function

Germline mutations distributed across the PTEN tumor-suppressor gene have been found to result in a wide spectrum of phenotypic features. Originally shown to be a major susceptibility gene for both Cowden syndrome (CS), which is characterized by multiple hamartomas and an increased risk of breast, thyroid, and endometrial cancers, and Bannayan-Riley-Ruvalcaba syndrome, which is characterized by lipomatosis, macrocephaly, and speckled penis, the PTEN hamartoma tumor syndrome spectrum has broadened to include Proteus syndrome and Proteus-like syndromes. Exon 5, which encodes the core motif, is a hotspot for mutations likely due to the biology of the protein. PTEN is a major lipid 3-phosphatase, which signals down the PI3 kinase/AKT pro-apoptotic pathway. Furthermore, PTEN is a protein phosphatase, with the ability to dephosphorylate both serine and threonine residues. The protein-phosphatase activity has also been shown to regulate various cell-survival pathways, such as the mitogen-activated kinase (MAPK) pathway. Although it is well established that PTEN's lipid-phosphatase activity, via the PI3K/AKT pathway, mediates growth suppression, there is accumulating evidence that the protein-phosphatase/MAPK pathway is equally important in the mediation of growth arrest and other crucial cellular functions.     

p53 Down Regulates PDGF-Induced Formation of Circular Dorsal Ruffles in Rat Aortic Smooth Muscle Cells

The tumor suppressor, p53, negatively regulates cell migration and invasion in addition to its role in apoptosis, cell cycle regulation and senescence. Here, we study the roles of p53 in PDGF-induced circular dorsal ruffle (CDR) formation in rat aortic smooth muscle (RASM) cells. In primary and immortalized RASM cells, up-regulation of p53 expression or increase in activity with doxorubicin inhibits CDR formation. In contrast, shRNA-knockdown of p53 or inhibition of its activity with pifithrin α promotes CDR formation. p53 acts by up-regulating PTEN expression, which antagonizes Rac and Cdc42 activation. Both lipid and protein phosphatase activities of PTEN are required for maximal suppression of CDR, but the lipid activity clearly plays the dominant role. N-WASP, the downstream effector of Cdc42, is the major positive contributor to CDR formation in RASM, and is an indirect target of p53. The Rac effector, WAVE2, appears to also play a minor role, while WAVE1 has no significant effect in CDR formation. In sum, we propose that p53 suppresses PDGF-induced CDR formation in RASM cells by upregulating PTEN leading mainly to the inhibition of the Cdc42-N-WASP pathway.     

Tks5 recruits AFAP-110, p190RhoGAP, and cortactin for podosome formation

Podosome formation in vascular smooth muscle cells is characterized by the recruitment of AFAP-110, p190RhoGAP, and cortactin, which have specific roles in Src activation, local down-regulation of RhoA activity, and actin polymerization, respectively. However, the molecular mechanism that underlies their specific recruitment to podosomes remains unknown. The scaffold protein Tks5 is localized to podosomes in Src-transformed fibroblasts and in smooth muscle cells, and may serve as a specific recruiting adapter for various components during podosome formation. We show here that induced mislocalization of Tks5 to the surface of mitochondria leads to a major subcellular redistribution of AFAP-110, p190RhoGAP, and cortactin, and to inhibition of podosome formation. Analysis of a series of similarly mistargeted deletion mutants of Tks5 indicates that the fifth SH3 domain is essential for this recruitment. A Tks5 mutant lacking the PX domain also inhibits podosome formation and induces the redistribution of AFAP-110, p190RhoGAP, and cortactin to the perinuclear area. By expressing a catalytically inactive point mutant and by siRNA-mediated expression knock-down we also provide evidence that p190RhoGAP is required for podosome formation. Together our findings demonstrate that Tks5 plays a central role in the recruitment of AFAP-110, p190RhoGAP, and cortactin to drive podosome formation.     

Regulatory signals for endothelial podosome formation

Podosomes are punctate actin-rich adhesion structures which spontaneously form in cells of the myelomonocytic lineage. Their formation is dependent on Src and RhoGTPases. Recently, podosomes have also been described in vascular cells. These podosomes differ from the former by the fact that they are inducible. In endothelial cells, such a signal can be provided by either constitutively active Cdc42, the PKC activator PMA or TGFbeta, depending on the model. Consequently, other regulatory pathways have been reported to contribute to podosome formation. To get more insight into the mechanisms by which podosomes form in endothelial cells, we have explored the respective contribution of signal transducers such as Cdc42-related GTPases, Smads and PKCs in three endothelial cell models. Results presented demonstrate that, in addition to Cdc42, TC10 and TCL GTPases can also promote podosome formation in endothelial cells. We also show that PKCalpha can be either necessary or entirely dispensable, depending on the cell model. In contrast, PKCdelta is essential for podosome formation in endothelial cells but not smooth muscle cells. Finally, although podosomes vary very little in their molecular composition, the signalling pathways involved in their assembly appear very diverse.     

The role of phosphatidylinositol 3'-kinase and its downstream signals in erbB-2-mediated transformation

We previously demonstrated that erbB-2-overexpressing human mammary epithelial (HME) cells exhibit several transformed phenotypes including growth factor independence, anchorage-independent growth, motility, and invasiveness. Because phosphatidylinositol 3'-kinase (PI3K) is a major target of erbB-2 activation, we tested the contribution that PI3K and its downstream signaling pathways make to these phenotypes. Utilizing a constitutively active form of PI3K, p110CAAX, we show that PI3K can mediate most phenotypes observed in erbB-2-overexpressing cells. To identify pathways leading from PI3K to specific phenotypes, we expressed constitutively active AKT or PTEN in erbB-2-overexpressing cells or in HME cells. HME cells expressing constitutively active AKT were growth factor independent, anchorage independent and motile, but not invasive. PTEN expression blocked erbB-2-mediated invasion but none of the other phenotypes. Rottlerin blocked invasion induced by p110CAAX and erbB-2, suggesting that protein kinase C delta (PKC-delta) is the downstream effector of PI3K responsible for the invasive capacity of the cells. Consistent with these observations, phospho-AKT remained detectable in erbB-2 cells treated with LY294002 or expressing exogenous PTEN, but was abolished by treatment with the p38MAP kinase inhibitor SB202190. Thus, both PI3K-dependent and p38MAP kinase-dependent pathways lead to activation of AKT, and activation of PKC-delta, via PI3K, mediates invasion.     

Biophysical Induction of Vascular Smooth Muscle Cell Podosomes

Vascular smooth muscle cell (VSMC) migration and matrix degradation occurs with intimal hyperplasia associated with atherosclerosis, vascular injury, and restenosis. One proposed mechanism by which VSMCs degrade matrix is through the use of podosomes, transient actin-based structures that are thought to play a role in extracellular matrix degradation by creating localized sites of matrix metalloproteinase (MMP) secretion. To date, podosomes in VSMCs have largely been studied by stimulating cells with phorbol esters, such as phorbol 12,13-dibutyrate (PDBu), however little is known about the physiological cues that drive podosome formation. We present the first evidence that physiological, physical stimuli mimicking cues present within the microenvironment of diseased arteries can induce podosome formation in VSMCs. Both microtopographical cues and imposed pressure mimicking stage II hypertension induce podosome formation in A7R5 rat aortic smooth muscle cells. Moreover, wounding using a scratch assay induces podosomes at the leading edge of VSMCs. Notably the effect of each of these biophysical stimuli on podosome stimulation can be inhibited using a Src inhibitor. Together, these data indicate that physical cues can induce podosome formation in VSMCs.     

Cell migration and invasion in human disease: the Tks adaptor proteins

Cell invasion plays a central role in a wide variety of biological phenomena and is the cause of tumour growth and metastasis. Understanding the biochemical mechanisms that control cell invasion is one of the major goals of our laboratory. Podosomes and invadopodia are specialized cellular structures present in cells with physiological or pathological invasive behaviours. These transient structures are localized at the ventral cell surface, contain an array of different proteins and facilitate cell-substrate adhesion, as well as the local proteolytic activity necessary for extracellular matrix remodelling and subsequent cellular invasion. We have shown previously that the adaptor proteins and Src substrates Tks4 and Tks5 are required for podosome and invadopodia formation, for cancer cell invasion in vitro, and for tumour growth in vivo. We have also defined a role for the Tks-mediated generation of ROS (reactive oxygen species) in both podosome and invadopodia formation, and invasive behaviour. Tks4 and Tks5 are also required for proper embryonic development, probably because of their roles in cell migration. Finally, we recently implicated podosome formation as part of the synthetic phenotype of vascular smooth muscle cells. Inhibitors of podosome and invadopodia formation might have utility in the treatment of vascular diseases and cancer. We have therefore developed a high-content cell-based high-throughput screening assay that allows us to identify inhibitors and activators of podosome/invadopodia formation. We have used this assay to screen for small-molecule inhibitors and defined novel regulators of invadopodia formation. In the present paper, I review these recent findings.     

Effects of tyrosine phosphorylation of cortactin on podosome formation in A7r5 vascular smooth muscle cells

Cortactin, a predominant substrate of Src family kinases, plays an important role in Arp2/3-dependent actin polymerization in lamellipodia and membrane ruffles and was recently shown to be enriched in podosomes induced by either c-Src or phorbol ester. However, the mechanisms by which cortactin regulates podosome formation have not been determined. In this study, we showed that cortactin is required for podosome formation, using siRNA knockdown of cortactin expression in smooth muscle A7r5 cells. Treatment with phorbol ester or expression of constitutively active c-Src induced genesis of cortactin-containing podosomes as well as increase in phosphorylation of cortactin at Y421 and Y466, the Src phosphorylation sites on cortactin. The Src kinase inhibitor SU-6656 significantly inhibited formation of podosomes induced by phorbol ester and phosphorylation of cortactin, whereas PKC inhibitor did not affect podosome formation in c-Src-transfected cells. Unexpectedly, expression of cortactin mutants containing Y421F, Y421D, Y466F, or Y466D mutated sites did not affect podosome formation or cortactin translocation to podosomes, although endogenous tyrosine-phosphorylated cortactin at Y421 and Y466 was present in podosomes. Our data indicate that 1) PKC acts upstream of Src in phosphorylation of cortactin and podosome formation in smooth muscle cells; 2) expression of cortactin is essential for genesis of podosomes; 3) phosphorylation at Y421 and Y466 is not required for translocation of cortactin to podosomes, although phosphorylation at these sites appears to be enriched in podosomes; and 4) tyrosine phosphorylation of cortactin may be involved in regulation of stability and turnover of podosomes, rather than targeting this protein to the site of podosome formation. actin cytoskeleton; Src; protein kinase C     

Inhibition of mycobacterial infection by the tumor suppressor PTEN

The tumor suppressor PTEN is a lipid phosphatase that is frequently mutated in various human cancers. PTEN suppresses tumor cell proliferation, survival, and growth mainly by inhibiting the PI3K-Akt signaling pathway through dephosphorylation of phosphatidylinositol 3,4,5-triphosphate. In addition to it role in tumor suppression, the PTEN-PI3K pathway controls many cellular functions, some of which may be important for cellular resistance to infection. Currently, the intersection between tumorigenic signaling pathways and cellular susceptibility to infection is not well defined. In this study we report that PTEN signaling regulates infection of both noncancerous and cancerous cells by multiple intracellular mycobacterial pathogens and that pharmacological modulation of PTEN signaling can affect mycobacterial infection. We found that PTEN deficiency renders multiple types of cells hyper-susceptible to infection by Mycoplasma and Mycobacterium bovis Bacillus Calmette-Guérin (BCG). The lipid phosphatase activity of PTEN is required for attenuating infection. Furthermore, we found mycobacterial infection activates host cell Akt phosphorylation, and pharmacological inhibition of Akt or PI3K activity reduced levels of intracellular infection. Intriguingly, inhibition of mTOR, one of the downstream components of the Akt signaling and a promising cancer therapeutic target, also lowered intracellular Bacillus Calmette-Guérin levels in mammary epithelial cancer MCF-7 cells. These findings demonstrate a critical role of PTEN-regulated pathways in pathogen infection. The relationship of PTEN-PI3K-Akt mTOR status and susceptibility to mycobacterial infection suggests that the interaction of mycobacterial pathogens with cancer cells may be influenced by genetic alterations in the tumor cells.     

Angiotensin II type 1 receptor-associated protein regulates carotid intimal hyperplasia through controlling apoptosis of vascular smooth muscle cells

Intimal hyperplasia is the main cause of restenosis after carotid artery injury, and the underlying mechanism involves the proliferation and migration of vascular smooth muscle cells (VSMCs). Angiotensin II Type 1 Receptor-Associated Protein (ATRAP) has been reported to withstand intimal hyperplasia by inhibiting VSMCs proliferation and migration; however, whether the beneficial effect of ATRAP associates with VSMCs apoptosis remains unclarified. We demonstrated that the adenoviral-mediated overexpression of ATRAP induced VSMC apoptosis, alleviating the balloon injury-induced neointima formation in rats. Under the condition of Angiotensin-II stimulation, ATRAP overexpression induced the apoptosis of rat VSMCs by depressing the PI3K-Akt signaling; whereas up-regulation of Akt by PTEN inhibitor abolished the apoptotic death. Thus, ATRAP regulates carotid intimal hyperplasia through controlling the PI3K-Akt signal-mediated VSMCs apoptosis.     

PTEN protects p53 from Mdm2 and sensitizes cancer cells to chemotherapy.

The PTEN tumor suppressor protein inhibits phosphatidylinositol 3-kinase (PI3K)/Akt signaling that promotes translocation of Mdm2 into the nucleus. When restricted to the cytoplasm, Mdm2 is degraded. The ability of PTEN to inhibit the nuclear entry of Mdm2 increases the cellular content and transactivation of the p53 tumor suppressor protein. Retroviral transduction of PTEN into U87MG (PTEN null) glioblastoma cells increases p53 activity and expression of p53 target genes and induces cell cycle arrest. U87MG/PTEN glioblastoma cells are more sensitive than U87MG/PTEN null cells to death induced by etoposide, a chemotherapeutic agent that induces DNA damage. Previously, tumor suppressor proteins have been supposed to act individually to suppress cancers. Our results establish a direct connection between the activities of two major tumor suppressors and show that they act together to respond to stresses and malignancies. PTEN protects p53 from survival signals, permitting p53 to function as a guardian of the genome. By virtue of its capacity to protect p53, PTEN can sensitize tumor cells to chemotherapy that relies on p53 activity. p53 induces PTEN gene expression, and here it is shown that PTEN protects p53, indicating that a positive feedback loop may amplify the cellular response to stress, damage, and cancer.     

Plectin deposition at podosome rings requires myosin contractility

Metalloproteinase-dependent tissue invasion requires the formation of podosomes and invadopodia for localized matrix degradation. Actin cytoskeleton remodeling via Arp2/3-mediated actin polymerization is essential for podosome formation, and dynamic microtubules have an important role in maintaining podosome turnover in macrophages and osteoclasts. Little is known, however, about the involvement of the intermediate filament cytoskeleton in formation, stabilization, and turnover of podosomes. Here we show that vimentin intermediate filaments colocalize with the early sites of podosome formation at the stress fiber - focal adhesion interface in cultured vascular smooth muscle cells, but do not directly contribute to podosome formation, or stabilization. In unstimulated A7r5 cells the cytolinker protein plectin poorly colocalized with vimentin and the microdomains, but following induction by phorbol ester accumulated in the rings that surround the podosomes. In plectin-deficient A7r5 cells actin stress fiber remodelling is reduced in response to PDBu, and small podosomes remain localized at stable actin stress fibres. Pharmacological inhibition of actomyosin contractility by blebbistatin leads to an aberrant localization of podosomes away from the cell periphery and induces failure of plectin to surround the outer perimeter of these invasive adhesions. Taken together, we conclude that plectin is involved in growth and maturation of podosomes by reducing focal adhesion and stress fiber turnover, and that actomyosin-dependent contractility is required for the peripheral localization and specific deposition of plectin at the podosome rings.     

Ubiquitination regulates PTEN nuclear import and tumor suppression

The PTEN tumor suppressor is frequently affected in cancer cells, and inherited PTEN mutation causes cancer-susceptibility conditions such as Cowden syndrome. PTEN acts as a plasma-membrane lipid-phosphatase antagonizing the phosphoinositide 3-kinase/AKT cell survival pathway. However, PTEN is also found in cell nuclei, but mechanism, function, and relevance of nuclear localization remain unclear. We show that nuclear PTEN is essential for tumor suppression and that PTEN nuclear import is mediated by its monoubiquitination. A lysine mutant of PTEN, K289E associated with Cowden syndrome, retains catalytic activity but fails to accumulate in nuclei of patient tissue due to an import defect. We identify this and another lysine residue as major monoubiquitination sites essential for PTEN import. While nuclear PTEN is stable, polyubiquitination leads to its degradation in the cytoplasm. Thus, we identify cancer-associated mutations of PTEN that target its posttranslational modification and demonstrate how a discrete molecular mechanism dictates tumor progression by differentiating between degradation and protection of PTEN.     

IRSp53 Mediates Podosome Formation via VASP in NIH-Src Cells

Podosomes are cellular “feet,” characterized by F-actin-rich membrane protrusions, which drive cell migration and invasion into the extracellular matrix. Small GTPases that regulate the actin cytoskeleton, such as Cdc42 and Rac are central regulators of podosome formation. The adaptor protein IRSp53 contains an I-BAR domain that deforms membranes into protrusions and binds to Rac, a CRIB motif that interacts with Cdc42, an SH3 domain that binds to many actin cytoskeletal regulators with proline-rich peptides including VASP, and the C-terminal variable region by splicing. However, the role of IRSp53 and VASP in podosome formation had been unclear. Here we found that the knockdown of IRSp53 by RNAi attenuates podosome formation and migration in Src-transformed NIH3T3 (NIH-Src) cells. Importantly, the differences in the IRSp53 C-terminal splicing isoforms did not affect podosome formation. Overexpression of IRSp53 deletion mutants suggested the importance of linking small GTPases to SH3 binding partners. Interestingly, VASP physically interacted with IRSp53 in NIH-Src cells and was essential for podosome formation. These data highlight the role of IRSp53 as a linker of small GTPases to VASP for podosome formation.