Functional Network Analysis Reveals the Relevance of SKIIP in the Regulation of Alternative Splicing by p38 SAPK

Alternative splicing is a prevalent mechanism of gene regulation that is modulated in response to a wide range of extracellular stimuli. Stress-activated protein kinases (SAPKs) play a key role in controlling several steps of mRNA biogenesis. Here, we show that osmostress has an impact on the regulation of alternative splicing (AS), which is partly mediated through the action of p38 SAPK. Splicing network analysis revealed a functional connection between p38 and the spliceosome component SKIIP, whose depletion abolished a significant fraction of p38-mediated AS changes. Importantly, p38 interacted with and directly phosphorylated SKIIP, thereby altering its activity. SKIIP phosphorylation regulated AS of GADD45α, the upstream activator of the p38 pathway, uncovering a negative feedback loop involving AS regulation. Our data reveal mechanisms and targets of SAPK function in stress adaptation through the regulation of AS.

An RB insensitive to CDK regulation

The N-term phosphorylation of Retinoblastoma (RB) by the p38 stress-activated protein kinase (SAPK) makes RB insensitive to cyclin-dependent kinase (CDK)-Cyclin inhibition, which enhances the transcriptional repression of E2F-driven promoters and delays tumor cell growth. This novel mechanism of RB regulation opens up a window for developing new cancer drug treatments for tumors harboring high CDK-Cyclin activity and a wild-type RB gene.

The N-Terminal Phosphorylation of RB by p38 Bypasses Its Inactivation by CDKs and Prevents Proliferation in Cancer Cells

Control of the G1/S phase transition by the Retinoblastoma (RB) tumor suppressor is critical for the proliferation of normal cells in tissues, and its inactivation is one of the most fundamental events leading to cancer. Cyclin-dependent kinase (CDK) phosphorylation inactivates RB to promote cell cycle-regulated gene expression. Here we show that, upon stress, the p38 stress-activated protein kinase (SAPK) maximizes cell survival by downregulating E2F gene expression through the targeting of RB. RB undergoes selective phosphorylation by p38 in its N terminus; these phosphorylations render RB insensitive to the inactivation by CDKs. p38 phosphorylation of RB increases its affinity toward the E2F transcription factor, represses gene expression, and delays cell-cycle progression. Remarkably, introduction of a RB phosphomimetic mutant in cancer cells reduces colony formation and decreases their proliferative and tumorigenic potential in mice.

The p57 CDKi integrates stress signals into cell-cycle progression to promote cell survival upon stress

The stress-activated protein kinase p38 phosphorylates p57/Kip2, resulting in enhanced CDK2 inhibition and a cell-cycle delay that helps cells to survive under stress.

The p57^Kip2 cyclin-dependent kinase inhibitor (CDKi) has been implicated in embryogenesis, stem-cell senescence and pathologies, but little is known of its role in cell cycle control. Here, we show that p57^Kip2 is targeted by the p38 stress-activated protein kinase (SAPK). Phosphorylation of p57^Kip2 at T143 by p38 enhances its association with and inhibition of Cdk2, which results in cell-cycle delay upon stress. Genetic inactivation of the SAPK or the CDKi abolishes cell-cycle delay upon osmostress and results in decreased cell viability. Oxidative stress and ionomycin also induce p38-mediated phosphorylation of p57 and cells lacking p38 or p57 display reduced viability to these stresses. Therefore, cell survival to various stresses depends on p57 phosphorylation by p38 that inhibits CDK activity. Together, these findings provide a novel molecular mechanism by which cells can delay cell cycle progression to maximize cell survival upon stress.

The p38 SAPK is recruited to chromatin via its interaction with transcription factors

In mammals, the stress-activated protein kinase (SAPK) p38 coordinates a rapid and complex transcriptional program to adapt to sudden changes in the extracellular environment. Although a number of genes have been reported to be under the control of p38, the basic mechanisms of transcriptional regulation by this SAPK remain uncharacterized. Here we show that in response to osmotic shock, anisomycin- or TNFα-activated p38 SAPK is recruited to stress-induced genes. The MAPKK MKK6 is also found at stress-responsive promoters. The recruitment of RNA polymerase II complex to the target promoters requires p38 activity. Moreover, when tethered to DNA as a LexA fusion protein, p38 activates transcription in a stress-regulated manner. Thus, p38 activity allows for recruitment of RNA polymerase and transcription initiation. p38 directly phosphorylates and interacts with the transcription factor Elk1. p38 activity is necessary for the recruitment of Elk1 to the c-Fos promoter, and knocking down Elk1 by siRNAs compromises both p38 recruitment to the c-Fos promoter and c-Fos transcriptional up-regulation upon osmostress. In addition, p38 recruitment to the osmoinducible gene Cox2 and the TNFα target gene IL8 is mediated by the transcription factors AP1 and NFκB, respectively. Therefore, anchoring of active SAPK to target genes is mediated by transcription factors. The presence of active p38 at open reading frames also suggests the involvement of the SAPK in elongation. Taken together, SAPK recruitment to target genes appears to be a broad mechanism to regulate transcription that has been preserved from yeast to mammals.

Whole genome analysis of p38 SAPK-mediated gene expression upon stress

Background

Cells have the ability to respond and adapt to environmental changes through activation of stress-activated protein kinases (SAPKs). Although p38 SAPK signalling is known to participate in the regulation of gene expression little is known on the molecular mechanisms used by this SAPK to regulate stress-responsive genes and the overall set of genes regulated by p38 in response to different stimuli.

Results

Here, we report a whole genome expression analyses on mouse embryonic fibroblasts (MEFs) treated with three different p38 SAPK activating-stimuli, namely osmostress, the cytokine TNFα and the protein synthesis inhibitor anisomycin. We have found that the activation kinetics of p38α SAPK in response to these insults is different and also leads to a complex gene pattern response specific for a given stress with a restricted set of overlapping genes. In addition, we have analysed the contribution of p38α the major p38 family member present in MEFs, to the overall stress-induced transcriptional response by using both a chemical inhibitor (SB203580) and p38α deficient (p38α^-/-) MEFs. We show here that p38 SAPK dependency ranged between 60% and 88% depending on the treatments and that there is a very good overlap between the inhibitor treatment and the ko cells. Furthermore, we have found that the dependency of SAPK varies depending on the time the cells are subjected to osmostress.

Conclusions

Our genome-wide transcriptional analyses shows a selective response to specific stimuli and a restricted common response of up to 20% of the stress up-regulated early genes that involves an important set of transcription factors, which might be critical for either cell adaptation or preparation for continuous extra-cellular changes. Interestingly, up to 85% of the up-regulated genes are under the transcriptional control of p38 SAPK. Thus, activation of p38 SAPK is critical to elicit the early gene expression program required for cell adaptation to stress.

Whole genome analysis of p38 SAPK-mediated gene expression upon stress

BACKGROUND

Cells have the ability to respond and adapt to environmental changes through activation of stress-activated protein kinases (SAPKs). Although p38 SAPK signalling is known to participate in the regulation of gene expression little is known on the molecular mechanisms used by this SAPK to regulate stress-responsive genes and the overall set of genes regulated by p38 in response to different stimuli.

RESULTS

Here, we report a whole genome expression analyses on mouse embryonic fibroblasts (MEFs) treated with three different p38 SAPK activating-stimuli, namely osmostress, the cytokine TNFalpha and the protein synthesis inhibitor anisomycin. We have found that the activation kinetics of p38alpha SAPK in response to these insults is different and also leads to a complex gene pattern response specific for a given stress with a restricted set of overlapping genes. In addition, we have analysed the contribution of p38alpha the major p38 family member present in MEFs, to the overall stress-induced transcriptional response by using both a chemical inhibitor (SB203580) and p38alpha deficient (p38alpha-/-) MEFs. We show here that p38 SAPK dependency ranged between 60% and 88% depending on the treatments and that there is a very good overlap between the inhibitor treatment and the ko cells. Furthermore, we have found that the dependency of SAPK varies depending on the time the cells are subjected to osmostress.

CONCLUSIONS

Our genome-wide transcriptional analyses shows a selective response to specific stimuli and a restricted common response of up to 20% of the stress up-regulated early genes that involves an important set of transcription factors, which might be critical for either cell adaptation or preparation for continuous extra-cellular changes. Interestingly, up to 85% of the up-regulated genes are under the transcriptional control of p38 SAPK. Thus, activation of p38 SAPK is critical to elicit the early gene expression program required for cell adaptation to stress.

Amino acids activate mTOR complex 1 via Ca2+/CaM signaling to hVps34

Excess levels of circulating amino acids (AAs) play a causal role in specific human pathologies, including obesity and type 2 diabetes. Moreover, obesity and diabetes are contributing factors in the development of cancer, with recent studies suggesting that this link is mediated in part by AA activation of mammalian target of rapamycin (mTOR) Complex 1. AAs appear to mediate this response through class III phosphatidylinositol 3-kinase (PI3K), or human vacuolar protein sorting 34 (hVps34), rather than through the canonical class I PI3K pathway used by growth factors and hormones. Here we show that AAs induce a rise in intracellular Ca(2+) ([Ca(2+)](i)), which triggers mTOR Complex 1 and hVps34 activation. We demonstrate that the rise in [Ca(2+)](i) increases the direct binding of Ca(2+)/calmodulin (CaM) to an evolutionarily conserved motif in hVps34 that is required for lipid kinase activity and increased mTOR Complex 1 signaling. These findings have important implications regarding the basic signaling mechanisms linking metabolic disorders with cancer progression.

Amino acids mediate mTOR/raptor signaling through activation of class 3 phosphatidylinositol 3OH-kinase

During the evolution of metazoans and the rise of systemic hormonal regulation, the insulin-controlled class 1 phosphatidylinositol 3OH-kinase (PI3K) pathway was merged with the primordial amino acid-driven mammalian target of rapamycin (mTOR) pathway to control the growth and development of the organism. Insulin regulates mTOR function through a recently described canonical signaling pathway, which is initiated by the activation of class 1 PI3K. However, how the amino acid input is integrated with that of the insulin signaling pathway is unclear. Here we used a number of molecular, biochemical, and pharmacological approaches to address this issue. Unexpectedly, we found that a major pathway by which amino acids control mTOR signaling is distinct from that of insulin and that, instead of signaling through components of the insulin/class 1 PI3K pathway, amino acids mediate mTOR activation by signaling through class 3 PI3K, hVps34.

Absence of S6K1 protects against age- and diet-induced obesity while enhancing insulin sensitivity

Elucidating the signalling mechanisms by which obesity leads to impaired insulin action is critical in the development of therapeutic strategies for the treatment of diabetes. Recently, mice deficient for S6 Kinase 1 (S6K1), an effector of the mammalian target of rapamycin (mTOR) that acts to integrate nutrient and insulin signals, were shown to be hypoinsulinaemic, glucose intolerant and have reduced beta-cell mass. However, S6K1-deficient mice maintain normal glucose levels during fasting, suggesting hypersensitivity to insulin, raising the question of their metabolic fate as a function of age and diet. Here, we report that S6K1-deficient mice are protected against obesity owing to enhanced beta-oxidation. However on a high fat diet, levels of glucose and free fatty acids still rise in S6K1-deficient mice, resulting in insulin receptor desensitization. Nevertheless, S6K1-deficient mice remain sensitive to insulin owing to the apparent loss of a negative feedback loop from S6K1 to insulin receptor substrate 1 (IRS1), which blunts S307 and S636/S639 phosphorylation; sites involved in insulin resistance. Moreover, wild-type mice on a high fat diet as well as K/K A(y) and ob/ob (also known as Lep/Lep) mice-two genetic models of obesity-have markedly elevated S6K1 activity and, unlike S6K1-deficient mice, increased phosphorylation of IRS1 S307 and S636/S639. Thus under conditions of nutrient satiation S6K1 negatively regulates insulin signalling.

Cell cycle regulation by the B-Myb transcription factor

The expression of genes required for progression through the cell cycle is highly modulated through a regulatory axis containing the E2F transcription factor and retinoblastoma tumour suppressor protein families. One of the genes regulated through this mechanism encodes the B-Myb transcription factor, which has been shown to be critically required for early embryonal development in the mouse. Transcriptional activity of B-Myb is substantially enhanced in S phase through modification by cyclin A/cdk2, and the evidence points squarely to the major role being played by B-Myb during this phase of the cell cycle. We discuss in this review recent findings suggesting that B-Myb is a multifunctional protein that has, in addition to its transcriptional properties, the ability to interact directly with other regulators of the cell cycle.

The cell cycle-regulated B-Myb transcription factor overcomes cyclin-dependent kinase inhibitory activity of p57(KIP2) by interacting with its cyclin-binding domain

The cell cycle-regulated B-Myb transcription factor is required for early embryonic development and is implicated in regulating cell growth and differentiation. In addition to its transcriptional regulatory properties, recent data indicate that B-Myb can release active cyclin/Cdk2 activity from the retinoblastoma-related p107 protein by directly interacting with the p107 N terminus. As this p107 domain has homology to the cyclin-binding domains of the p21(Waf1/Cip1) family of cyclin-dependent kinase inhibitors (CKIs), we investigated in this study whether B-Myb could also interact with these CKIs. No in vivo interaction was found with either p21(Waf1/Cip1) or p27(KIP1), however, binding to p57(KIP2) was readily detectable in both in vivo and in vitro assays. The B-Myb-interacting region of p57(KIP2) mapped to the cyclin-binding domain. Consistent with this, B-Myb competed with cyclin A2 for binding to p57(KIP2), resulting in release of active cyclin/Cdk2 kinase. Moreover, B-Myb partially overcame the ability of p57(KIP2) to induce G1 arrest in Saos-2 cells. Despite similarities with previous p107 studies, the B-Myb domains required for interaction with p57(KIP2) were quite different from those implicated for p107. Thus, it is evident that B-Myb may promote cell proliferation by a non-transcriptional mechanism that involves release of active cyclin/Cdk2 from p57(KIP2) as well as p107.

Insulin activation of Rheb, a mediator of mTOR/S6K/4E-BP signaling, is inhibited by TSC1 and 2

Tumor suppressor genes evolved as negative effectors of mitogen and nutrient signaling pathways, such that mutations in these genes can lead to pathological states of growth. Tuberous sclerosis (TSC) is a potentially devastating disease associated with mutations in two tumor suppressor genes, TSC1 and 2, that function as a complex to suppress signaling in the mTOR/S6K/4E-BP pathway. However, the inhibitory target of TSC1/2 and the mechanism by which it acts are unknown. Here we provide evidence that TSC1/2 is a GAP for the small GTPase Rheb and that insulin-mediated Rheb activation is PI3K dependent. Moreover, Rheb overexpression induces S6K1 phosphorylation and inhibits PKB phosphorylation, as do loss-of-function mutations in TSC1/2, but contrary to earlier reports Rheb has no effect on MAPK phosphorylation. Finally, coexpression of a human TSC2 cDNA harboring a disease-associated point mutation in the GAP domain, failed to stimulate Rheb GTPase activity or block Rheb activation of S6K1.

B-Myb overcomes a p107-mediated cell proliferation block by interacting with an N-terminal domain of p107

B-Myb is a cell-cycle regulated transcription factor which is implicated in cell proliferation and has an essential role in early embryonic development. In this study we examined the functions of B-Myb required to overcome G1 arrest in Saos-2 cells induced by the retinoblastoma-related p107 protein. Our results demonstrated that this activity was independent of B-Myb transactivation function, but correlated with its capacity to form an in vivo complex with p107. A large proportion of B-Myb formed complexes with p107 in cotransfected cells, however, B-Myb bound weakly to the related p130 protein and not at all to pRb. In contrast to the E2F transcription factors, which bind the p107 C-terminal pocket domain, B-Myb recognizes an N-terminal p107 region which overlaps the larger cyclin-binding domain. B-Myb and cyclin A2 formed mutually exclusive complexes with p107, and B-Myb enhanced the activity of co-transfected cyclin E kinase activity, implying that B-Myb affects the cell cycle by preventing sequestration of active cyclin/cdk2 complexes. This study defines a novel function of B-Myb and further suggests that the p107 N-terminus provides an interaction domain for transcription factors involved in cell cycle control.

An intronic AP-1 sequence mediates the transcriptional activation of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by serum

The expression of F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is rapidly induced by growth factors. We report here that an AP-1 intragenic sequence located at position +612 of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase transcription initiation site is involved in the transcriptional activation of this gene by serum. We have demonstrated in vitro DNA-protein interaction on this AP-1 site of the F-promoter. Indeed, this element was recognized by c-Fos and JunD in vitro, and mutation or deletion of this element reduced the early response to serum stimulation by 60%. We conclude that the serum response of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene requires the co-ordinated function of ets, E2F and AP-1 binding sites.

Regulation of the cell cycle by B-Myb

B-Myb is a cell-cycle-regulated member of the Myb transcription factor and, like c-Myb, has been implicated in regulation of hematopoietic cell proliferation and differentiation. In this study we have examined the mechanisms by which B-Myb regulates the cell cycle. We found that the ability of B-Myb both to promote Saos-2 cells into the S phase of the cell cycle and to overcome G1 arrest mediated by overexpression of the retinoblastoma-related p107 protein was correlated with the capacity of B-Myb to form an in vivo complex with p107, but was independent of its transactivation function. Further experiments using a B-Myb dominant-negative protein suggested that transcriptional activation of genes regulated through Myb DNA-binding sequences was required for cell proliferation. Our experiments suggest, therefore, that B-Myb influences cell cycle progression at two distinct levels: by inhibiting p107 and by inducing transcription of specific target genes.

Insulin inhibits glucocorticoid-stimulated L-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression by activation of the c-Jun N-terminal kinase pathway

The hepatic isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PF2K/Fru-2,6-BPase) is transcriptionally stimulated by glucocorticoids, whereas insulin blocks this stimulatory effect. Although this inhibitory effect has been extensively reported, nothing is known about the signalling pathway responsible. We have used well-characterized inhibitors for proteins involved in different signalling cascades to assess the involvement of these pathways on the transcriptional regulation of glucocorticoid-stimulated PF2K/Fru-2,6-BPase by insulin. Our results demonstrate that the phosphoinositide 3-kinase, p70/p85 ribosomal S6 kinase, extracellular signal-regulated protein kinase (ERK)1/2 and p38 mitogen-activated protein (MAP) kinase pathways are not involved in the inhibitory effect of insulin on glucocorticoid-stimulated PF2K/Fru-2,6-BPase. To evaluate the implication of the MAP kinase/ERK kinase (MEK)-4-stress-activated protein kinase-c-Jun-N-terminal protein kinase ('JNK-SAPK') pathway we overexpressed the N-terminal JNK-binding domain of the JNK-interacting protein 1 ('JIP-1'), demonstrating that activation of JNK is necessary for the insulin inhibitory effect. Moreover, overexpression of MEK kinase 1 and JNK-haemagglutinin resulted in the inhibition of the glucocorticoid-stimulated PF2K/Fru-2,6-BPase. These results provide clear and specific evidence for the role of JNK in the insulin inhibition of glucocorticoid-stimulated PF2K/Fru-2,6-BPase gene expression. In addition, we performed experiments with a mutant of the glucocorticoid receptor in which the JNK phosphorylation target Ser-246 had been mutated to Ala. Our results demonstrate that the phosphorylation of the glucocorticoid receptor on Ser-246 is not responsible for the JNK repression of glucocorticoid-stimulated PF2K/Fru-2,6-BPase gene expression.

Activation of phosphatidylinositol 3-kinase is required for transcriptional activity of F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: assessment of the role of protein kinase B and p70 S6 kinase

Previous studies have demonstrated that the F isoform of<hsp sp=0.5>6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase(6PF2K/Fru-2,6-BPase) is transcriptionally regulated by growth factors. The aim of this study was to investigate the importance of the phosphatidylinositol 3-kinase (PI 3-kinase) pathway in the regulation of 6PF2K/Fru-2,6-BPase gene expression. We have completed studies using chemical inhibitors and expression vectors for the proteins involved in this signalling cascade. Treatment of cells with LY 294002, an inhibitor of PI 3-kinase, blocked the epidermal growth factor (EGF)-dependent stimulation of 6PF2K/Fru-2,6-BPase gene transcription. Transient transfection of a constitutively active PI 3-kinase was sufficient to activate transcription from the F-type 6PF2K/Fru-2,6-BPase promoter. In contrast, co-transfection with a dominant-negative form of PI 3-kinase completely abrogated the stimulation by EGF, and down-regulated the basal promoter activity. In an attempt to determine downstream proteins that lie between PI 3-kinase and 6PF2K/Fru-2,6-BPase gene expression, the overexpression of a constitutively active form of protein kinase B (PKB) was sufficient to activate 6PF2K/Fru-2,6-BPase gene expression, even in the presence of either a dominant-negative form of PI 3-kinase or LY 294002. The over-expression of p70/p85 ribosomal S6 kinase or the treatment with its inhibitor rapamycin did not affect 6PF2K/Fru-2,6-BPase transcription. We conclude that PI 3-kinase is necessary for the transcriptional activity of F-type 6PF2K/Fru-2,6-BPase, and that PKB is a downstream effector of PI 3-kinase directly involved in the regulation of 6PF2K/Fru-2,6-BPase gene expression.

Expression of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA during liver regeneration

The expression of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA was studied during liver regeneration by three independent assays: Northern blot analysis, reverse transcription-polymerase chain reaction and ribonuclease protection. We demonstrate the presence of F-type mRNA in foetal and adult rat livers and a transient increase in its levels with a maximum at 12 h after partial liver resection. The time course of F-type mRNA induction differs from that reported for the L-type isoform, suggesting differences in the regulation of the expression of F- and L-type isoforms of the bifunctional enzyme during liver regeneration.

Effect of growth factors on the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in Rat-1 fibroblasts

The activation of glycolytic flux is a biochemical characteristic of growing cells. Several reports have demonstrated the role of fructose 2,6-bisphosphate in this process. In this paper we show that the levels of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase (6PF2K/Fru-2,6-P2ase) mRNA are modulated in response to serum and growth factors and this effect is due to regulation of its transcription rate. The modulation of the expression of this enzyme by growth factors differs according their mitogenic effect; both lysophosphatidic acid and epidermal growth factor, when added alone, increased the mRNA levels, but endothelin had no effect. Furthermore, cAMP, which acts as an antimitogenic signal in Rat-1 fibroblasts, produced a decrease in 6PF2K/Fru-2, 6-P2ase mRNA and inhibited the effects of lysophosphatidic acid and epidermal growth factor on 6PF2K/Fru-2,6-P2ase expression. PD 098059, a specific inhibitor of the activation of the mitogen-activated protein kinase, was able to prevent the effect of EGF on 6PF2K/Fru-2, 6-P2ase gene expression. These results imply that activation of mitogen-activated protein kinase is required for the stimulation of the transcription of 6PF2K/Fru-2,6-P2ase by EGF.

Nitric oxide inhibits DNA synthesis and induces activation of poly(ADP-ribose) polymerase in cultured rat hepatocytes

Incubation of cultured rat hepatocytes with sodium nitroprusside or SIN-1, two nitric oxide (NO) donors, inhibited the mitogenic action of hepatocyte growth factor in a dose-dependent manner. The addition of 100 microM reduced hemoglobin, which is known to absorb NO, or the presence of 20 microM 1,5-isoquinolinediol, a poly(ADP-ribose) polymerase (PARP) inhibitor, decreased the cytostatic effects of SIN-1. By labeling the hepatocytes with [2-3H]adenine we studied whether nitric oxide induces ADP-ribosylation of proteins in a whole-cell system. At 0.4 mM, sodium nitroprusside increased the [3H]adenine labeling of two proteins of 116 and 130-135 kDa. This effect was time-dependent and was detected after 2 h. Only the 116-kDa protein was recognized by three different antibodies against poly (ADP-ribose) polymerase in Western blot assays. These results demonstrate that NO has antimitogenic effects in cultured hepatocytes and that its action could be mediated by PARP activation.

Hepatocyte growth factor and transforming growth factor beta regulate 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression in rat hepatocyte primary cultures

Hepatocyte growth factor (HGF) and transforming growth factor beta (TGF-beta) are believed to be of major importance for hepatic regeneration after liver damage. We have studied the effect of these growth factors on fructose 2,6-bisphosphate (Fru-2,6-P2) levels and the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-BPase) in rat hepatocyte primary cultures. Our results demonstrate that HGF activates the expression of the 6PF2K/Fru-2,6-BPase gene by increasing the levels of its mRNA. As a consequence of this activation, the amount of 6PF2K/Fru-2,6-BPase protein and 6-phosphofructo-2-kinase activity increased, which was reflected by a rise in Fru-2,6-P2 levels. In contrast, TGF-beta decreased the levels of 6PF2K/Fru-2,6-BPase mRNA, which led to a decrease in the amount of 6PF2K/Fru-2,6-BPase protein and Fru-2,6-P2. The different actions of HGF and TGF-beta on 6PF2K/Fru-2,6-BPase gene expression are concomitant with their effect on cell proliferation. Here we show that, in the absence of hormones, primary cultures of hepatocytes express the F-type isoenzyme. In addition, HGF increases the expression of this isoenzyme, and dexamethasone activates the L-type isoform. HGF and TGF-beta were able to inhibit this activation.