Commitment to differentiation and expression of early differentiation markers in murine keratinocytes in vitro are regulated independently of extracellular calcium concentrations.

In the epidermis, one of the earliest characterized events in keratinocyte differentiation is the coordinate induction of a pair of keratins specifically expressed in suprabasal cells, keratin 1 (K1) and keratin 10 (K10). Both in vivo and in vitro, extracellular calcium is necessary for several biochemical and structural changes during keratinocyte differentiation. However, it has been unclear if calcium serves as a differentiation signal in keratinocytes. In these studies, expression of suprabasal keratin mRNA and protein is used to test whether the initial differentiation of primary mouse keratinocytes in vitro is dependent on changes in the concentration of extracellular calcium. K1 mRNA was expressed at low levels in cultures of keratinocytes growing on plastic in 0.05 mM calcium but in attached cells was not further induced by increases in the concentration of extracellular calcium. Suspension of the keratinocytes into semi-solid medium induced a rapid and substantial increase in both expression of K1 mRNA and in the percentage of cells expressing suprabasal keratin proteins. The induction was unaffected by the concentration of calcium in the semi-solid medium and could not be enhanced by exposing attached cells to higher calcium before suspension. The induction of K1 mRNA could be inhibited by exposure of the keratinocytes to either EGF or fibronectin. These results suggest that commitment of mouse keratinocytes to terminal differentiation is independent of extracellular calcium and may be regulated primarily by extracellular factors other than calcium.

Platelet-derived growth factor-induced alterations in vinculin and actin distribution in BALB/c-3T3 cells.

Exposure of BALB/c-3T3 cells (clone A31) to platelet-derived growth factor (PDGF) results in a rapid time- and dose-dependent alteration in the distribution of vinculin and actin. PDGF treatment (6-50 ng/ml) causes vinculin to disappear from adhesion plaques (within 2.5 min after PDGF exposure) and is followed by an accumulation of vinculin in punctate spots in the perinuclear region of the cell. This alteration in vinculin distribution is followed by a disruption of actin-containing stress fibers (within 5 to 10 min after PDGF exposure). Vinculin reappears in adhesion plaques by 60 min after PDGF addition while stress fiber staining is nondetectable at this time. PDGF treatment had no effect on talin, vimentin, or microtubule distribution in BALB/c-3T3 cells; in addition, exposure of cells to 5% platelet-poor plasma (PPP), 0.1% PPP, 30 ng/ml epidermal growth factor (EGF), 30 ng/ml somatomedin C, or 10 microM insulin also had no effect on vinculin or actin distribution. Other competence-inducing factors (fibroblast growth factor, calcium phosphate, and choleragen) and tumor growth factor produced similar alterations in vinculin and actin distribution as did PDGF, though not to the same extent. PDGF treatment of cells for 60 min followed by exposure to EGF (0.1-30 ng/ml for as long as 8 h after PDGF removal), or 5% PPP resulted in the nontransient disappearance of vinculin staining within 10 min after EGF or PPP additions; PDGF followed by 0.1% PPP or 10 microM insulin had no effect. Treatment of cells with low doses of PDGF (3.25 ng/ml), which did not affect vinculin or actin organization in cells, followed by EGF (10 ng/ml), resulted in the disappearance of vinculin staining in adhesion plaques, thus demonstrating the synergistic nature of PDGF and EGF. These data suggest that PDGF-induced competence and stimulation of cell growth in quiescent fibroblasts are associated with specific rapid alterations in the cellular organization of vinculin and actin.

Persistence of the mitogenic response to platelet-derived growth factor (competence) does not reflect a long-term interaction between the growth factor and the target cell.

Quiescent BALB/c-3T3 cells exposed briefly to platelet-derived growth factor (PDGF) become "competent" to replicate their DNA even if PDGF is removed from cell culture medium prior to the onset of DNA synthesis. We have suggested that persistence of the PDGF-induced competent state reflects a rapidly induced and relatively stable biochemical change within the target cells. Others suggest that the phenomenon reflects a long-term association between PDGF and its target cells or perhaps between PDGF and the cell culture dish. This controversy has been addressed (a) by examining the effect of anti-PDGF antibodies on PDGF-induced competence and (b) by studying the chemical fate of 125I-labeled PDGF. Anti-PDGF antibodies inactive both soluble and surface-bound PDGF. However, if quiescent 3T3 cells are exposed to PDGF for as little as 30 min, subsequent addition of these antibodies to the culture medium does not prevent the mitogenic response. Under conditions where the PDGF-induced competent state decays stochastically with a t1/2 of 18-20 h, cell-associated 125I-PDGF decays with a t1/2 of approximately 50 min. These data do not support the concept that persistence of the PDGF-induced competent state reflects a long-term association between PDGF and the target cells or between PDGF and the culture dish.

Interferon inhibits the establishment of competence in Go/S-phase transition.

Addition of mouse interferon-alpha/beta (IFN) to confluent, quiescent BALB/c 3T3 (clone A31) mouse fibroblasts resulted in a block or delay in serum-induced activation of the cell cycle. It was necessary to add IFN within 6 hours after serum stimulation to inhibit nuclear labeling with [3H]thymidine. This is consistent with the time required for platelet-derived growth factor (PDGF) to induce cells to become competent to respond to additional growth factors present in platelet-poor plasma. Simultaneous addition of IFN with PDGF inhibited the PDGF-induced synthesis of a 29-kilodalton and a 35-kilodalton protein that normally occurs within 1 hour after PDGF addition. IFN also suppressed the general increase in protein synthesis that occurs by the fifth hour after PDGF addition. These results show that IFN antagonizes the action of PDGF, thereby interfering with the activation of Go cells for G1 traverse and S-phase entry.

Repression of p27kip1 synthesis by platelet-derived growth factor in BALB/c 3T3 cells.

We have investigated the regulation of p27kip1, a cyclin-dependent kinase inhibitor, in BALB/c 3T3 cells during growth factor-stimulated transition from quiescence (G0) to a proliferative (G1) state. The level of p27kip1 protein falls dramatically after mitogenic stimulation and is accompanied by a decrease in cyclin E associated p27kip1, as well as a transient increase in cyclin D1-associated p27kip1 that later declines concomitantly with the loss of total p27kip1. Analysis of metabolically labelled cells revealed that cyclin D2, cyclin D3, and cdk4 were also partnered with p27kip1 in quiescent BALB/c 3T3 cells and that this association decreased after platelet-derived growth factor (PDGF) treatment. Furthermore, the decline in p27kip1 and reduced association with cyclin D3, initiated by the addition of PDGF but not plasma-derived factors, suggested that these changes are involved in competence, the first step in the exit from G0. Synthesis of p27kip1 as determined by incorporation of [35S]methionine was repressed upon mitogenic stimulation, and PDGF was sufficient to elicit this repression within 2 to 3 h. Pulse-chase experiments demonstrated the reduced rate of synthesis was not the result of an increased rate of degradation. Full repression of p27kip1 synthesis required the continued presence of PDGF and failed to occur in the presence of the RNA polymerase inhibitor 5,6-dichlorobenzimidazole riboside. These characteristics demonstrate that repression was a late effect of PDGF and was consistent with our finding that conditional expression of activated H-ras did not affect synthesis of p27kip1. Northern (RNA) analysis of p27kip1 mRNA revealed that the repression was not accompanied by a corresponding decrease in p27kip1 mRNA, suggesting that the PDGF-regulated decrease in p27kip1 expression occurred through a translational mechanism.

Analysis of cyclin D3-cdk4 complexes in fibroblasts expressing and lacking p27(kip1) and p21(cip1).

Our studies examined the effects of p27(kip1) and p21(cip1) on the assembly and activity of cyclin D3-cdk4 complexes and determined the composition of the cyclin D3 pool in cells containing and lacking these cyclin-dependent kinase inhibitors. We found that catalytically active cyclin D3-cdk4 complexes were present in fibroblasts derived from p27(kip1)-p21(cip1)-null mice and that immunodepletion of extracts of wild-type cells with antibody to p27(kip1) and/or p21(cip1) removed cyclin D3 protein but not cyclin D3-associated activity. Similar results were observed in experiments assaying cyclin D1-cdk4 activity. Data obtained using mixed cell extracts demonstrated that p27(kip1) interacted with cyclin D3-cdk4 complexes in vitro and that this interaction was paralleled by a loss of cyclin D3-cdk4 activity. In p27(kip1)-p21(cip1)-deficient cells, the cyclin D3 pool consisted primarily of cyclin D3 monomers, whereas in wild-type cells, the majority of cyclin D3 molecules were complexed to cdk4 and either p27(kip1) or p21(cip1) or were monomeric. We conclude that neither p27(kip1) nor p21(cip1) is required for the formation of cyclin D3-cdk4 complexes and that cyclin D3-cdk4 complexes containing p27(kip1) or p21(cip1) are inactive. We suggest that only a minor portion of the total cyclin D3 pool accounts for all of the cyclin D3-cdk4 activity in the cell regardless of whether the cell contains p27(kip1) and p21(cip1).

P27Kip1 and p21Cip1 are not required for the formation of active D cyclin-cdk4 complexes.

Our studies address questions pertaining to the regulation of D cyclin-cdk4 activity, and the following results were obtained. Conditions that increased the abundance of the D cyclins also increased the abundance of enzymatically active D cyclin-cdk4 complexes in mouse embryo fibroblasts (MEFs) lacking both p27(Kip1) and p21(Cip1) (p27/p21(-/-)). Such conditions included ectopic expression of cyclin D1 and inhibition of D cyclin degradation by the proteasome inhibitor MG132. However, as determined by treatment of wild-type MEFs with MG132, maximal accumulation of D cyclin-cdk4 complexes required p27(Kip1) and p21(Cip1) and coincided with the formation of inactive D cyclin-cdk4-p27(Kip1) or -p21(Cip1) complexes. p27(Kip1) or p21(Cip1) also increased the abundance of D cyclin-cdk4 complexes and reduced amounts of cdk4 activity when ectopically expressed in p27/p21(-/-) MEFs. Lastly, increases in the stability of the D cyclins accounted for their greater abundance in wild-type MEFs than in p27/p21(-/-) MEFs. We conclude that (i) D cyclin-cdk4 complexes are formed and become active in the absence of p27(Kip1) and p21(Cip1) and (ii) p27(Kip1) and p21(Cip1) maximize the accumulation but inhibit the activity of D cyclin-cdk4 complexes. We suggest that D cyclin-cdk4 complexes are more stable when bound to p27(Kip1) or p21(Cip1) and that formation of ternary complexes also stabilizes the D cyclins.

Platelet-derived growth factor induces rapid and sustained tyrosine phosphorylation of phospholipase C-gamma in quiescent BALB/c 3T3 cells.

Platelet-derived growth factor (PDGF) stimulates the proliferation of quiescent fibroblasts through a series of events initiated by activation of tyrosine kinase activity of the PDGF receptor at the cell surface. Physiologically significant substrates for this or other growth factor receptor or oncogene tyrosine kinases have been difficult to identify. Phospholipase C (PLC), a key enzyme of the phosphoinositide pathway, is believed to be an important site for hormonal regulation of the hydrolysis of phosphatidylinositol 4,5-bisphosphate, which produces the intracellular second-messenger molecules inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. Treatment of BALB/c 3T3 cells with PDGF led to a rapid (within 1 min) and significant (greater than 50-fold) increase in PLC activity, as detected in eluates of proteins from a phosphotyrosine immunoaffinity matrix. This PDGF-stimulated increase in phosphotyrosine-immunopurified PLC activity occurred for up to 12 h after addition of growth factor to quiescent cells. Interestingly, the PDGF stimulation occurred at 3 as well as 37 degrees C and in the absence or presence of extracellular Ca2+. Immunoprecipitation of cellular proteins with monoclonal antibodies specific for three distinct cytosolic PLC isozymes demonstrated the presence of a 145-kilodalton isozyme, PLC-gamma (formerly PLC-II), in BALB/c 3T3 cells. Furthermore, these immunoprecipitation studies showed that PLC-gamma is rapidly phosphorylated on tyrosine residues after PDGF stimulation. The results suggest that mitogenic signaling by PDGF is coincident with tyrosine phosphorylation of PLC-gamma.

Growth factor regulation of cyclin D1 mRNA expression through protein synthesis-dependent and -independent mechanisms.

Overexpression of the cyclin D1/PRAD1 oncogene has been observed in a number of tumorigenic cell lines, suggesting that regulation of D1 expression may represent an important step in the control of cellular proliferation. We have examined the mRNA expression of cyclin D1, as well as two related D-type cyclins, D2 and D3, in response to defined growth factors that control the growth of Balb/c-3T3 fibroblasts. Transcripts for all three D-type cyclins were expressed during the G1 phase of the Balb cell cycle, however only D1 and D3 exhibited periodic induction. Although redundantly expressed, message levels of cyclin D1 and D3 were differentially regulated in regard to kinetics of induction; a modest increase in D3 mRNA was detected near the G1/S boundary, 12 h after serum stimulation of quiescent cells, while abundance of D1 transcript increased 20 to 30-fold, peaking 6 h after addition of serum. Factors such as platelet-derived growth factor (PDGF) that induce competence formation in Balb cells, increased D1 message and protein levels to the same extent as serum but did not affect expression of cyclin D3 and did not stimulate entry into S phase. Progression factors contained within platelet-poor plasma stimulated D1 expression only weakly but acted synergistically with low concentrations of PDGF to increase D1 mRNA to maximum levels. Depletion of protein kinase C severely reduced the ability of PDGF and serum to induce D1 mRNA. PDGF- and serum-mediated elevation of steady-state D1 message levels was in part because of a transcriptional activation of the D1 gene that was independent of protein synthesis. However, protein synthesis was required 3-4 h after serum stimulation for the shut down of D1 transcription leading to the normal decline in message levels after peak induction. Our results indicate that overexpression of cyclin D1 message may result from a disruption of negative regulatory events that repress D1 transcription.

Cyclin D3-associated kinase activity is regulated by p27kip1 in BALB/c 3T3 cells.

We report that cyclin D3/cdk4 kinase activity is regulated by p27(kip1) in BALB/c 3T3 cells. The association of p27(kip1) was found to result in inhibition of cyclin D3 activity as measured by immune complex kinase assays utilizing cyclin D3-specific antibodies. The ternary p27(kip1)/cyclin D3/cdk4 complexes do exhibit kinase activity when measured in immune complex kinase assays utilizing p27(kip1)-specific antibodies. The association of p27(kip1) with cyclin D3 was highest in quiescent cells and declined upon mitogenic stimulation, concomitantly with declines in the total level of p27(kip1) protein. The decline in this association could be elicited by PDGF treatment alone; this was not sufficient, however, for activation of cyclin D3 activity, which also required the presence of factors in platelet-poor plasma in the culturing medium. Unlike cyclin D3 activity, which was detected only in growing cells, p27(kip1) kinase activity was present throughout the cell cycle. Since we found that the p27(kip1) activity was dependent on cyclin D3 and cdk4, we compared the substrate specificity of the active ternary complex containing p27(kip1) and the active cyclin D3 lacking p27(kip1) by tryptic phosphopeptide mapping of GST-Rb phosphorylated in vitro and also by comparing the relative phosphorylation activity toward a panel of peptide substrates. We found that ternary p27(kip1)/cyclin D3/cdk4 complexes exhibited a different specificity than the active binary cyclin D3/cdk4 complexes, suggesting that p27(kip1) has the capacity to both inhibit cyclin D/cdk4 activity as well as to modulate cyclin D3/cdk4 activity by altering its substrate preference.

Induction of NF-kappa B-like activity by platelet-derived growth factor in mouse fibroblasts.

Nuclear factor kappa B (NF-kappa B) modulates the expression of numerous genes via interaction with a specific DNA sequence termed the kappa B site. Its activity is modulated by a cytosolic inhibitor protein termed I kappa B, and its activation occurs in response to a variety of agents in a variety of cell types, most notably B and T lymphocytes. Data presented here show that an activity (designated complex I) that binds specifically to the kappa B site is induced in density-arrested Balb/c-3T3 mouse fibroblasts by platelet-derived growth factor (PDGF), a potent mitogen for these cells. Increased levels of complex I, as evaluated by electrophoretic mobility shift assays of nuclear extracts, were observed in cells treated for 1-4 h (but not 15 min) with the BB isoform of PDGF. 12-O-tetradecanoylphorbol 13-acetate (TPA) and the AA isoform of PDGF also stimulated this response and both isoforms, but not TPA, were effective in cells depleted of protein kinase C. Complex I most likely is authentic NF-kappa B, a p50-p65 heterodimer, or a closely related factor because it exhibited properties characteristic of those previously described for NF-kappa B including inducibility by deoxycholate and cycloheximide and sensitivity to I kappa B. A second kappa B binding activity (complex II), which apparently contained p50 homodimers, displayed limited induction by PDGF, whereas a third complex (complex III) migrated faster than but behaved similarly to complex I. These studies suggest that NF-kappa B or an NF-kappa B-like factor may participate in the expression of PDGF-inducible genes.

Density-dependent growth inhibition of fibroblasts ectopically expressing p27(kip1).

The cyclin/cyclin-dependent kinase (cdk) inhibitor p27(kip1) is thought to be responsible for the onset and maintenance of the quiescent state. It is possible, however, that cells respond differently to p27(kip1) in different conditions, and using a BALB/c-3T3 cell line (termed p27-47) that inducibly expresses high levels of this protein, we show that the effect of p27(kip1) on cell cycle traverse is determined by cell density. We found that ectopic expression of p27(kip1) blocked the proliferation of p27-47 cells at high density but had little effect on the growth of cells at low density whether exponentially cycling or stimulated from quiescence. Regardless of cell density, the activities of cdk4 and cdk2 were markedly repressed by p27(kip1) expression, as was the cdk4-dependent dissociation of E2F4/p130 complexes. Infection of cells with SV40, a DNA tumor virus known to abrogate formation of p130- and Rb-containing complexes, allowed dense cultures to proliferate in the presence of supraphysiological amounts of p27(kip1) but did not stimulate cell cycle traverse when cultures were cotreated with the potent cdk2 inhibitor roscovitine. Our data suggest that residual levels of cyclin/cdk activity persist in p27(kip1)-expressing p27-47 cells and are sufficient for the growth of low-density cells and of high-density cells infected with SV40, and that effective disruption of p130 and/or Rb complexes is obligatory for the proliferation of high-density cultures.

Accumulation of p53 and reductions in XIAP abundance promote the apoptosis of prostate cancer cells.

Toward the goal of developing effective treatments for prostate cancers, we examined the effects of cyclin-dependent kinase inhibitors on the survival of prostate cancer cells. We show that roscovitine, R-roscovitine, and CGP74514A (collectively referred to as CKIs) induce the apoptosis of LNCaP and LNCaP-Rf cells, both of which express wild-type p53. Apoptosis required caspase-9 and caspase-3 activity, and cytochrome c accumulated in the cytosol of CKI-treated cells. Amounts of p53 increased substantially in CKI-treated cells, whereas amounts of the endogenous caspase inhibitor XIAP decreased. CKIs did not appreciably induce the apoptosis of LNCaP cells treated with pifithrin-alpha, which prevents p53 accumulation, or of prostate cancer cells that lack p53 function (PC3 and DU145). Ectopic expression of p53 in PC3 cells for 44 hours did not reduce XIAP abundance or induce apoptosis. However, p53-expressing PC3 cells readily apoptosed when exposed to CKIs or when depleted of XIAP by RNA interference. These findings show that CKIs induce the mitochondria-mediated apoptosis of prostate cancer cells by a dual mechanism: p53 accumulation and XIAP depletion. They suggest that these events in combination may prove useful in the treatment of advanced prostate cancers.

Induction of expression of c-fos and c-myc protooncogenes by basic calcium phosphate crystal: effect of beta-interferon.

Basic calcium phosphate (BCP) crystals control the traverse of cells from the G0-G1 to S-phase of the cell cycle and initiate proliferation by rendering fibroblasts competent to respond to insulin-like growth factors in plasma. The present study examines whether BCP crystals induce transcription of the protooncogenes c-fos and c-myc and the effect of beta-interferon (IFN-beta) on protooncogene transcription as well as BCP crystal-induced DNA synthesis. Stimulation of density-arrested BALB/c-3T3 cells with either BCP crystal or platelet-derived growth factor (PDGF) results in maximal accumulation of c-fos mRNA at 30 min after stimulation. Induction of c-myc transcription by BCP crystal or PDGF occurs within 1 h and is maximal at around 3 h after stimulation. Simultaneous addition of IFN-beta with either BCP crystals or PDGF had little effect on c-fos induction but delayed both c-myc message accumulation and entry into S phase. The delay in c-myc message induction after IFN-beta treatment cannot account for the observed delay in the onset of DNA synthesis, since IFN-beta can be added at up to 6 h after stimulation with either PDGF or BCP crystals, and a similar delay in the onset of DNA synthesis is still observed.

Primary keratinocytes have an adhesion dependent S phase checkpoint that is absent in immortalized cell lines.

In order to understand the mechanism through which loss of anchorage inhibits growth, we have investigated the events that occur in murine keratinocytes upon substratum detachment utilizing both primary cells and established immortalized cell lines. Our data has revealed that while both primary and immortalized cells undergo growth arrest in suspension, the nature of this arrest is markedly different. Primary cells exhibit a growth arrest that is characterized by rapid cessation of DNA synthesis resulting in a static S phase population. In contrast, an immortalized non-tumorgenic cell line, Balb MK, exhibits growth arrest as measured by thymidine incorporation, but does not prevent cells that have entered S phase from continuing into G2/M, and accumulating as a 4N population. In contrast to both primary and MK cells, the tumorigenic SLC-1 cell line did not accumulate in a specific cell cycle interval and were able to undergo continuous growth in suspension. Examination of cyclin A protein and its associated activity revealed that cyclin A protein levels decreased in primary but not MK cells; suggesting the continued presence of cyclin A may allow continued DNA synthesis observed in MK cells. Furthermore, we demonstrate the accumulation of suspension cultured MK cells as a 4N population correlated with the loss of cyclin A/cdk2 kinase activity, which in turn occurred through the accumulation of p27kip1, whereas neither p27kip1 accumulation nor loss of cyclin A activity was observed in SLC-1 cells. Our results clearly reveal that the process of growth inhibition in suspension cultured cells may occur in several forms with distinct characteristics that are dependent on the status of cyclin/cdk complexes and CKI proteins. Tumor derived cells in suspension did not lose cyclin A dependent kinase activity and thus continued to grow and divide.

Role of Gadd45alpha in the density-dependent G1 arrest induced by p27(Kip1).

p27(Kip1), an inhibitor of cyclin-dependent kinases, is an important regulator of cell cycle progression. We have previously shown that p27(Kip1) inhibits the G0 to S transition when ectopically expressed in p27-47 mouse fibroblasts arrested at high but not low densities. In the study described here, we identify Gadd45alpha, a member of the growth arrest- and DNA damage-inducible family of proteins, as a potential mediator of the density-dependent effects of p27(Kip1) on cell proliferation. Gadd45alpha mRNA and protein were more abundant in p27-47 cells arrested at high densities than at low densities. Amounts of both decreased and remained low when cells arrested at high densities were exposed to mitogens in the absence, but not in the presence, of ectopically expressed p27(Kip1). Importantly, enforced expression of Gadd45alpha prevented density-arrested mouse fibroblasts from initiating DNA synthesis in response to mitogens. We suggest that amounts of Gadd45alpha above a certain threshold are growth inhibitory and that such amounts are achieved in cells arrested at high but not low densities. For cultures arrested at high densities, the resumption of cell cycle traverse requires a sustained reduction in Gadd45alpha abundance, a process that is induced by mitogens and inhibited by p27(Kip1).

Regulation of the cell cycle machinery by oncogenic ras.

The ras proto-oncogene has been implicated during the formation of tumors in vivo as well as the transformation of cell lines in culture. Conditional expression of an activated ras mutant in Balb/c-3T3 fibroblasts failed to stimulate S phase entry in the absence of plasma-derived progression factors, but did shorten the G1 interval from 12 to 6 h and abrogate the normal proliferative requirement for platelet-derived growth factor. Ras-dependent alteration of the 3T3 cell cycle was accompanied by a dramatic increase in the expression of the G1 regulatory protein, cyclin D1, while expression of cyclin E and cyclin A proteins were only weakly induced. Cyclin/cdk complexes assembled in response to ectopic ras expression in the absence of growth factor stimulation bound the cdk inhibitory factor, Kip1, and were inactive. However, plasma-stimulated regulatory pathways functioned co-operatively with the oncogenic ras molecule to decrease Kip1 levels, induce the kinase activities associated with cyclins D, E and A, and trigger the initiation of DNA replication. Our results suggest that a ras-activated signal transduction pathway may link environmental mitogenic stimuli to the cell cycle machinery via modulation of G1 cyclin expression.

Activation of Stat3 preassembled with platelet-derived growth factor beta receptors requires Src kinase activity.

Members of the STAT family of transcriptional regulators modulate the expression of a variety of gene products that promote cell proliferation, survival and transformation. Although initially identified as mediators of cytokine signaling, the STAT proteins are also activated by, and thus may contribute to the actions of, polypeptide growth factors. To define the mechanism by which these factors activate STATs, we examined the process of Stat3 activation in Balb/c-3T3 fibroblasts treated with platelet-derived growth factor (PDGF). As STATs are activated by tyrosine phosphorylation, and as PDGF receptors are ligand-activated tyrosine kinases, we considered the possibility that Stat3 interacts with and is phosphorylated by PDGF receptors. We find that Stat3 associates with PDGF beta receptors in both the presence and, surprisingly, the absence of PDGF. Moreover, Stat3 was phosphorylated on tyrosine in PDGF beta receptor immunoprecipitates of PDGF-treated but not untreated cells. Although required, receptor activation was insufficient for Stat3 activation. When added to cells in combination with a pharmacologic agent (PD180970) that specifically inhibits the activity of Src family tyrosine kinases, PDGF did not activate Stat3 as monitored by electrophoretic mobility shift assay. PD180970 did not affect MAPK activation by PDGF or the JAK-dependent activation of Stat3 by interleukin-6. The necessity of Src activity for Stat3 activation by PDGF was further evidenced by data showing the presence of Src in complexes containing both Stat3 and PDGF beta receptors in PDGF-treated cells. These results suggest a novel mechanism of STAT activation in which inactive Stat3 pre-assembles with inactive PDGF receptors, and in response to ligand binding and in a manner dependent on Src kinase activity, is rapidly phosphorylated and activated. Additional data demonstrate that Src kinase activity is also required for PDGF stimulation of DNA synthesis in density-arrested cells.

Loss of p21WAF1/CIP1 accelerates Ras oncogenesis in a transgenic/knockout mammary cancer model.

Upregulation of the cyclin-dependent kinase inhibitor p21WAF1/CIP1 and subsequent cell growth arrest or senescence is one mechanism by which normal cells are believed to respond to stress induced by the constitutively activated GTPase Ras. We hypothesize that in the absence of p21, the onset of Ras-dependent oncogenesis is accelerated. To test this hypothesis, we crossed MMTV/v-Ha-ras transgenic mice into a p21-deficient background. By 63 days of age, all 8 ras/p21-/- mice developed either malignant (mammary and/or salivary adenocarcinomas) or benign (Harderian hyperplasia) tumors. In contrast, by the same age, only one out of nine of the ras/p21+/+ mice developed a tumor. Furthermore, by 94 days of age, half of the ras/p21-/- mice, but none of the ras/p21+/+ mice, developed mammary tumors. p21-deficiency also accelerated the development of salivary (T50=66 days for ras/p21-/- vs T50=136 days for ras/p21+/+) and Harderian (T50=52 days for ras/p21-/- vs T50>221 days for ras/p21+/+) tumors. Furthermore, two out of the eight ras/p21-/- mice had metastatic lesions, one in its lungs, the other in its abdomen. None of the nine ras/p21+/+ mice had metastatic lesions. By 4 months of age, the mammary tumor multiplicity was 10-fold greater in ras/p21-/- (average 3.40 tumors/mouse) than in ras/p21+/+ (average 0.33 tumor/mouse) mice. However, once the tumors appeared, their growth rate, apoptosis level, and mitotic index were not affected by the loss of p21, suggesting that loss of p21 is critical in early but not late events of Ras oncogenesis. Altogether, the results show that tumor onset in MMTV/v-Ha-ras mice is p21-dependent with loss of p21 associated with earlier tumor appearance and increased tumor multiplicity and aggressiveness.

Induction of p21WAF1/CIP1 and cyclin D1 expression by the Src oncoprotein in mouse fibroblasts: role of activated STAT3 signaling.

While the activated viral Src oncoprotein, v-Src, induces uncontrolled cell growth, the mechanisms underlying cell cycle deregulation by v-Src have not been fully defined. Previous studies demonstrated that v-Src induces constitutively active STAT3 signaling that is required for cell transformation and recent data have implicated STAT3 in the transcriptional control of critical cell cycle regulators. Here we show in mouse fibroblasts stably transformed by v-Src that mRNA and protein levels of p21 (WAF1/CIP1), cyclin D1, and cyclin E are elevated. Using reporter constructs in transient-transfection assays, the cyclin D1 and p21 promoters were both found to be transcriptionaly induced by v-Src in a STAT3-dependent manner. The kinase activities of cyclin D/CDK4, 6 and cyclin E/CDK2 complexes were only slightly elevated, consistent with the findings that coordinate increases in p21, cyclin D1 and cyclin E resulted in an increase in cyclin/CDK/p21 complexes. Similar results were obtained in NIH3T3 and BALB/c 3T3 cells stably transformed by v-Src, indicating that these regulatory events associated with STAT3 signaling represent common mechanisms independent of cell line or clonal variation. These findings suggest that STAT3 has an essential role in the regulation of critical cell cycle components in v-Src transformed mouse fibroblasts.