Signal transduction through epidermal growth factor receptor is altered in HeLa monolayer cells during mitosis

Cell Cycle Synchronization of HeLa Cells to Assay EGFR Pathway Activation

Progression through the cell cycle causes changes in the cell's signaling pathways that can alter EGFR signal transduction. Here, we describe drug-derived protocols to synchronize HeLa cells in various phases of the cell cycle, including G1 phase, S phase, G2 phase, and mitosis, specifically in the mitotic stages of prometaphase, metaphase, and anaphase/telophase. The synchronization procedures are designed to allow synchronized cells to be treated for EGF and collected for the purpose of Western blotting for EGFR signal transduction components.S phase synchronization is performed by thymidine block, G2 phase with roscovitine, prometaphase with nocodazole, metaphase with MG132, and anaphase/telophase with blebbistatin. G1 phase synchronization is performed by culturing synchronized mitotic cells obtained by mitotic shake-off. We also provide methods to validate the synchronization methods. For validation by Western blotting, we provide the temporal expression of various cell cycle markers that are used to check the quality of the synchronization. For validation of mitotic synchronization by microscopy, we provide a guide that describes the physical properties of each mitotic stage, using their cellular morphology and DNA appearance. For validation by flow cytometry, we describe the use of imaging flow cytometry to distinguish between the phases of the cell cycle, including between each stage of mitosis.

Investigation of MEK activity in COS7 cells entering mitosis

Although the mitogen-activated protein kinase (MAPK) pathway has been extensively investigated, numerous events remain unclear. In the present study, we examined mitogen-activated protein kinase kinase (MEK) expression from interphase to mitosis. Following nocodazole treatment, COS7 cells gradually became round as early as 4 h after treatment. Cyclin B1 expression gradually increased from 4 to 24 h in the presence of nocodazole. When cells were treated with nocodazole for 4 h, the level of epidermal growth factor (EGF)-mediated MEK phosphorylation did not significantly change between nocodazole-untreated and -treated (4 h) cells (P>0.05). However, EGF-mediated MEK phosphorylation was significantly inhibited upon treatment with nocodazole for 8 and 24 h compared to nocodazole-untreated cells (P<0.05). MEK phosphorylation levels were comparable between 1, 5, 10 and 50 ng/ml EGF treatments. Phorbol 12-myristic 13-acetate (PMA) did not activate MEK in mitotic cells. Following treatment of COS7 cells at the interphase with AG1478 or U0126, MEK phosphorylation was blocked. In addition, the investigation of the expression of proteins downstream of MEK demonstrated that EGF does not significantly affect the phosphorylation level of extracellular-signal-regulated kinase (ERK), ribosomal protein S6 kinase (RSK) and Elk in mitotic cells (P>0.05). The results showed that MEK expression is gradually inhibited from cell interphase to mitosis, and that MEK downstream signaling is affected by this inhibition, which probably reflects the requirements of cell physiology during mitosis.

Visualization of polarized membrane type 1 matrix metalloproteinase activity in live cells by fluorescence resonance energy transfer imaging

Membrane type 1 matrix metalloproteinase (MT1-MMP) plays a critical role in cancer cell biology by proteolytically remodeling the extracellular matrix. Utilizing fluorescence resonance energy transfer (FRET) imaging, we have developed a novel biosensor, with its sensing element anchoring at the extracellular surface of cell membrane, to visualize MT1-MMP activity dynamically in live cells with subcellular resolution. Epidermal growth factor (EGF) induced significant FRET changes in cancer cells expressing MT1-MMP, but not in MT1-MMP-deficient cells. EGF-induced FRET changes in MT1-MMP-deficient cells could be restored after reconstituting with wild-type MT1-MMP, but not MMP-2, MMP-9, or inactive MT1-MMP mutants. Deletion of the transmembrane domain in the biosensor or treatment with tissue inhibitor of metalloproteinase-2, a cell-impermeable MT1-MMP inhibitor, abolished the EGF-induced FRET response, indicating that MT1-MMP acts at the cell surface to generate FRET changes. In response to EGF, active MT1-MMP was directed to the leading edge of migrating cells along micropatterned fibronectin stripes, in tandem with the local accumulation of the EGF receptor, via a process dependent upon an intact cytoskeletal network. Hence, the MT1-MMP biosensor provides a powerful tool for characterizing the molecular processes underlying the spatiotemporal regulation of this critical class of enzymes.

Evaluation of mammalian cell-free systems of nuclear disassembly and assembly

Mammalian cell-free systems are very useful for the biochemical and structural study of nuclear disassembly and assembly. Through experimental manipulations, the role of specific proteins in these processes can be studied. Recently, we intended to examine the involvement of integral and peripheral inner nuclear membrane proteins in nuclear disassembly and assembly. However, we could not achieve proper disassembly when isolated interphase HeLa nuclei were exposed to mitotic soluble extracts obtained from the same cell line and containing cyclin B1. Homogenates of synchronized mitotic HeLa cells left to reassemble their nuclei generated incomplete nuclear envelopes on chromatin masses. Digitonin-permeabilized mitotic cells also assembled incomplete nuclei, generating a lot of cytoplasmic inclusions of inner nuclear membrane proteins as an intermediate. These results were therefore used as a basis for a critical evaluation of mammalian cell-free systems. We present here evidence that cell synchronization itself can interfere with the progress of nuclear assembly, possibly by causing aberrant nuclear disassembly and/or by inducing the formation of an abnormal number of mitotic spindles.

Extracellular signal-regulated kinase 1/2 activity is not required in mammalian cells during late G2 for timely entry into or exit from mitosis

Extracellular signal-regulated kinase (ERK)1/2 activity is reported to be required in mammalian cells for timely entry into and exit from mitosis (i.e., the G2-mitosis [G2/M] and metaphase-anaphase [M/A] transitions). However, it is unclear whether this involvement reflects a direct requirement for ERK1/2 activity during these transitions or for activating gene transcription programs at earlier stages of the cell cycle. To examine these possibilities, we followed live cells in which ERK1/2 activity was inhibited through late G2 and mitosis. We find that acute inhibition of ERK1/2 during late G2 and through mitosis does not affect the timing of the G2/M or M/A transitions in normal or transformed human cells, nor does it impede spindle assembly, inactivate the p38 stress-activated checkpoint during late G2 or the spindle assembly checkpoint during mitosis. Using CENP-F as a marker for progress through G2, we also show that sustained inhibition of ERK1/2 transiently delays the cell cycle in early/mid-G2 via a p53-dependent mechanism. Together, our data reveal that ERK1/2 activity is required in early G2 for a timely entry into mitosis but that it does not directly regulate cell cycle progression from late G2 through mitosis in normal or transformed mammalian cells.

Downregulation of protein phosphatase 2A activity in HeLa cells at the G2-mitosis transition and unscheduled reactivation induced by 12-O-tetradecanoyl phorbol-13-acetate (TPA)

In the cell cycle the transition from G2 phase to cell division (M) is strictly controlled by protein phosphorylation-dephosphorylation reactions effected by several protein kinases and phosphatases. Although much indirect and direct evidence point to a key role of protein phosphatase 2A (PP2A) at the G2/M transition, the control of the enzyme activity prior to and after the transition are not fully clarified. Using synchronized HeLa cells we determined the PP2A activity (i.e. the increment sensitive to inhibition by 2nM okadaic acid) in immunoprecipitates obtained with antibodies raised against a conserved peptide sequence (residues 169-182, Ab(169/182)) of the PP2A catalytic subunit (PP2A C). Two different substrates were offered: the phospho-peptide KR(p)TIRR and histone H1 phosphorylated by means of the cyclin-dependent protein kinase p34(cdc2). The results indicate that in HeLa cells the specific activity of PP2A towards both substrates goes through a minimum in late G2 phase and stays low until metaphase. Treatment of G2 cells with TPA (10(-7) M) caused a reactivation of the downregulated PP2A activity within 20 min, i.e. the same time frame within which TPA was shown earlier to block HeLa cells at the transition from G2 to mitosis [Kinzel et al., 1988. Cancer Res. 48, 1759-1762]. Activation of PP2A was also induced by TPA in mitotic cells. The low activity of PP2A in mitotic cells was accompanied by a strong reaction of mitotic PP2A C with anti-P-Tyr antibodies in Western blots, which was reversed by treatment of mitotic cells with TPA. The results suggest that the activity of cellular PP2A requires downregulation for the transition from G2 phase to mitosis. Unscheduled reactivation of PP2A induced by TPA in late G2 phase appears to inhibit the progress into mitosis.

Cdc2-mediated Inhibition of Epidermal Growth Factor Activation of the Extracellular Signal-regulated Kinase Pathway during Mitosis

Inhibition of general transcription and translation occurs during mitosis to preserve the high energy requirements needed for the dynamic structural changes that are occurring at this time of the cell cycle. Although the mitotic kinase Cdc2 appears to directly phosphorylate and inhibit key proteins directly involved in transcription and translation, the role of Cdc2 in regulating up-stream growth factor receptor-mediated signal transduction pathways is limited. In the present study, we examined mechanisms involved in uncoupling receptor-mediated activation of the extracellular signal-regulated (ERK) signaling pathway in mitotic cells. Treatment with epidermal growth factor (EGF) failed to activate the ERK pathway in mitotic cells, although partial activation of ERK could be achieved in mitotic cells treated with phorbol 12-myristate 13-acetate (PMA). The discrepancy between EGF and PMA-mediated ERK activation suggested that multiple events in the ERK pathway were regulated during mitosis. We show that Cdc2 inhibits EGF-mediated ERK activation through direct interaction and phosphorylation of several ERK pathway proteins, including the guanine nucleotide exchange factor, Sos-1, and Raf-1 kinase. Inhibition of Cdc2 activity with roscovitine in mitotic cells restored ERK activation by EGF and PMA. Similarly, mitotic inhibition of ERK activity in cells expressing active mutants of H-Ras and Raf-1 kinase could also be reversed following Cdc2 inhibition. In contrast, ERK activation in cells expressing active MEK1 was not inhibited during mitosis or affected by roscovitine. These data suggest that Cdc2 inhibits growth factor receptor-mediated ERK activation during mitosis by primarily targeting signaling proteins that are upstream of MEK1.

Matrix metalloproteinase-1 associates with intracellular organelles and confers resistance to lamin A/C degradation during apoptosis

Since the first description of matrix metalloproteinase (MMP)-1 as an interstitial collagenase, great importance has been ascribed to this enzyme in extracellular matrix remodeling during tumoral, inflammatory, and angiogenic processes. As more evidence for the role of MMPs in targeting nonmatrix substrates emerges, casual observations that intracellular MMP-1 is found in vitro and in vivo prompt investigation of the role that MMP-1 may play on basic cell functions such as cell division and apoptosis. Here we show for the first time that MMP-1 not only has extracellular functions but that it is strongly associated with mitochondria and nuclei and accumulates within the cells during the mitotic phase of the cell cycle. On induction of apoptosis, MMP-1 co-localized with aggregated mitochondria and accumulated around fragmented nuclei. Inhibition of this enzyme by RNA interference or treatment with a broad MMP inhibitor caused faster degradation of lamin A, activation of caspases, and fragmentation of DNA when compared with untreated cells. These observations strongly suggest that intracellular association of MMP-1 to mitochondria and nuclei confers resistance to apoptosis and may explain the well-known association of this enzyme with tumor cell survival and spreading.

p42-MAP kinase is activated in EGF-stimulated interphase but not in metaphase-arrested HeLa cells

It is known that cellular signals produced in response to an inappropriate spindle formation cause the cell to be arrested at metaphase (M) in the cell cycle. We report here that the 42-kDa isoform of MAPK (ERK2) was tyrosyl-phosphorylated and activated in response to epidermal growth factor (EGF) in interphase but not in M-arrested HeLa cells. However, the basal level of activity of M-arrested cells was higher than that of interphase, although the overall tyrosyl phosphorylation content was small. Further, the EGF receptor and its associated proteins GTPase-activating protein and phospholipase C were phosphorylated in M-arrested cells to a lower extent than they were in interphase. This implies that in spite of its high level of basal activity, the scarcity of MAPK activation in mitosis in response to EGF stems from an early impairment of phosphorylation of the receptor and neighboring proteins. The biological significance of these results underlies the importance of keeping the cell sheltered from extracellular signals when it undergoes division.