Interdependent SMAD and JNK signaling in transforming growth factor-beta-mediated transcription

SMAD and JNK cascades are essential components of the transforming growth factor-beta (TGF-beta) signaling machinery and are implicated in common transcriptional responses. However, the relationship of these pathways to one another downstream of the TGF-beta receptor complex is unknown. We show that JNK is rapidly activated by TGF-beta in a SMAD-independent manner and phosphorylates Smad3 outside its -SSXS motif. Smad3 phosphorylation by JNK facilitates both its activation by the TGF-beta receptor complex and its nuclear accumulation. JNK regulates SMAD- and TGF-beta-mediated transcriptional responses, yet JNK activators only partially stimulate transcriptional responses characteristic of TGF-beta without coincident SMAD pathway activation. These results suggest an interdependent relationship between the JNK and SMAD pathways in TGF-beta-mediated transcription.

Synergistic induction of cyclooxygenase-2 by transforming growth factor-beta1 and epidermal growth factor inhibits apoptosis in epithelial cells

Increased expression of cyclooxygenase-2 (COX-2) expression has been observed in several human tumor types and in selected animal and cell culture models of carcinogenesis, including lung cancer. Increased expression of COX-2 and production of prostaglandins appear to provide a survival advantage to transformed cells through the inhibition of apoptosis, increased attachment to extracellular matrix, increased invasiveness, and the stimulation of angiogenesis. In the present studies, we found that transforming growth factor beta1 (TGF-beta1) and epidermal growth factor (EGF) synergistically induced the expression of COX-2 and prostaglandin E2 (PGE2) production in mink lung epithelial (Mv1Lu) cells. EGF, but not PDGF or IGF-1, was able to inhibit TGF-beta1-induced apoptosis in Mv1Lu cells and this effect was blocked by NS-398, a selective inhibitor of COX-2 activity, suggesting a possible role for COX-2 in the anti-apoptotic effect of EGF receptor ligands. The combination of TGF-beta1 and EGF also significantly induced COX-2 expression in rat intestinal epithelial (RIE-1) cells and completely prevented sodium butyrate (NaBu)-induced apoptosis. The synergistic induction of COX-2 by TGF-beta1 and EGF was not observed in R1B-L17 cells, a line derived from Mv1Lu cells that lacks the TGF-beta type-I receptor. AG1478, a selective inhibitor of EGF receptor tyrosine kinase activity, completely suppressed the induction of COX-2 expression by either EGF or TGF-beta1+EGF. Also, PD98059, a specific inhibitor of MEK/ERK pathway, and SB203580, a specific inhibitor of p38 MAPK activity, significantly inhibited the induction of COX-2 in response to combined EGF and TGF-beta1. These results suggest an important collaborative interaction of TGF-beta1 and EGF signaling in the induction of COX-2 and prostaglandin production in Mv1Lu cells.

Overexpression of a kinase-deficient transforming growth factor-beta type II receptor in mouse mammary stroma results in increased epithelial branching

Members of the transforming growth factor-beta (TGF-beta) superfamily signal through heteromeric type I and type II serine/threonine kinase receptors. Transgenic mice that overexpress a dominant-negative mutation of the TGF-beta type II receptor (DNIIR) under the control of a metallothionein-derived promoter (MT-DNIIR) were used to determine the role of endogenous TGF-betas in the developing mammary gland. The expression of the dominant-negative receptor was induced with zinc and was primarily localized to the stroma underlying the ductal epithelium in the mammary glands of virgin transgenic mice from two separate mouse lines. In MT-DNIIR virgin females treated with zinc, there was an increase in lateral branching of the ductal epithelium. We tested the hypothesis that expression of the dominant-negative receptor may alter expression of genes that are expressed in the stroma and regulated by TGF-betas, potentially resulting in the increased lateral branching seen in the MT-DNIIR mammary glands. The expression of hepatocyte growth factor mRNA was increased in mammary glands from transgenic animals relative to the wild-type controls, suggesting that this factor may play a role in TGF-beta-mediated regulation of lateral branching. Loss of responsiveness to TGF-betas in the mammary stroma resulted in increased branching in mammary epithelium, suggesting that TGF-betas play an important role in the stromal-epithelial interactions required for branching morphogenesis.

Inactivation of the transforming growth factor beta type II receptor in human small cell lung cancer cell lines

Transforming growth factor beta (TGF-beta) exerts a growth inhibitory effect on many cell types through binding to two types of receptors, the type I and II receptors. Resistance to TGF-beta due to lack of type II receptor (RII) has been described in some cancer types including small cell lung cancer (SCLC). The purpose of this study was to examine the cause of absent RII expression in SCLC cell lines. Northern blot analysis showed that RII RNA expression was very weak in 16 of 21 cell lines. To investigate if the absence of RII transcript was due to mutations, we screened the poly-A tract for mutations, but no mutations were detected. Additional screening for mutations of the RII gene revealed a GG to TT base substitution in one cell line, which did not express RII. This mutation generates a stop codon resulting in predicted synthesis of a truncated RII of 219 amino acids. The nature of the mutation, which has not previously been observed in RII, has been linked to exposure to benzo[a]-pyrene, a component of cigarette smoke. Since RII has been mapped to chromosome 3p22 and nearby loci are often hypermethylated in SCLC, it was examined whether the lack of RII expression was due to hypermethylation. Southern blot analysis of the RII promoter did not show altered methylation patterns. The restriction endonuclease pattern of the RII gene was altered in two SCLC cell lines when digested with Smal. However, treatment with 5-aza-2'-deoxycytidine did not induce expression of RII mRNA. Our results indicate that in SCLC lack of RII mRNA is not commonly due to mutations and inactivation of RII transcription was not due to hypermethylation of the RII promoter or gene. Thus, these data show that in most cases of the SCLC cell lines, the RII gene and promoter is intact in spite of absent RII expression. However, the nature of the mutation found could suggest that it was caused by cigarette smoking.

Inhibition of DNA synthesis by a farnesyltransferase inhibitor involves inhibition of the p70(s6k) pathway

Previously, the protein farnesyltransferase inhibitor (FTI), L-744, 832, has been shown to inhibit the proliferation of a number of tumor cell lines in vitro in a manner that correlated with the inhibition of the mitogen-activated protein kinase cascade. Here we show that FTI inhibits p70(s6k) phosphorylation in mammary tumors in vivo in transgenic mice. Furthermore, in a mouse keratinocyte cell line, FTI inhibits p70(s6k) phosphorylation and activity and inhibits PHAS-1 phosphorylation in vitro in both rapidly growing cells and in growth factor-stimulated quiescent cells. Dominant-negative Ras expression inhibits p70(s6k) stimulation by epidermal growth factor, demonstrating a requirement for Ras activity during p70(s6k) activation. FTI does not inhibit protein kinase B phosphorylation on Ser473, indicating that FTI does not act by inhibiting phosphatidylinositol 3-kinase. FTI also inhibits DNA synthesis in keratinocytes, and inhibition of DNA synthesis correlates closely with p70(s6k) inhibition. Rapamycin, an inhibitor of p70(s6k) and PHAS-1 phosphorylation, causes a 30-45% reduction in DNA synthesis in keratinocytes, while FTI induces an 80-90% reduction in DNA synthesis. These observations suggest that alteration of p70(s6k) and PHAS-1 function by FTI are responsible for a substantial portion of the growth-inhibitory properties of FTI. Together, these data demonstrate that p70(s6k) and PHAS-1 are novel downstream targets of FTI and suggest that the anti-tumor properties of FTI are probably due to the inhibition of multiple mitogenic pathways.

Signal transduction by transforming growth factor-beta: a cooperative paradigm with extensive negative regulation

Transforming growth factor-beta (TGF-beta) represents an evolutionarily conserved family of secreted factors that mobilize a complex signaling network to control cell fate by regulating proliferation, differentiation, motility, adhesion, and apoptosis. TGF-beta promotes the assembly of a cell surface receptor complex composed of type I (T beta RI) and type II (T beta RII) receptor serine/threonine kinases. In response to TGF-beta binding, T beta RII recruits and activates T beta RI through phosphorylation of the regulatory GS-domain. Activated T beta RI then initiates cytoplasmic signaling pathways to produce cellular responses. SMAD proteins together constitute a unique signaling pathway with key roles in signal transduction by TGF-beta and related factors. Pathway-restricted SMADs are phosphorylated and activated by type I receptors in response to stimulation by ligand. Once activated, pathway-restricted SMADs oligomerize with the common-mediator Smad4 and subsequently translocate to the nucleus. Genetic analysis in Drosophila melanogaster and Caenorhabditis elegans, as well as T beta RII and SMAD mutations in human tumors, emphasizes their importance in TGF-beta signaling. Mount ng evidence indicates that SMADs cooperate with ubiquitous cytoplasmic signaling cascades and nuclear factors to produce the full spectrum of TGF-beta responses. Operating independently, these ubiquitous elements may influence the nature of cellular responses to TGF-beta. Additionally, a variety of regulatory schemes contribute temporal and/or spatial restriction to TGF-beta responses. This report reviews our current understanding of TGF-beta signal transduction and considers the importance of a cooperative signaling paradigm to TGF-beta-mediated biological responses.

Treatment with farnesyl-protein transferase inhibitor induces regression of mammary tumors in transforming growth factor (TGF) alpha and TGF alpha/neu transgenic mice by inhibition of mitogenic activity and induction of apoptosis

Mouse mammary tumor virus-transforming growth factor alpha (MMTV-TGF alpha) and MMTV-TGF alpha/neu transgenic mice develop mammary tumors after a long latency and therefore provide useful model systems for breast cancer with its recognized activation of receptor tyrosine kinase signaling. We used these mice to study the antitumor effect of L-744,832 (FTI), a potent and selective inhibitor of farnesyl-protein transferase, and hence of Ras function. A total of 55 mice were assigned randomly to treatment with FTI or vehicle, and one-half of the mice were crossed over after initial treatment to the opposite group. L-744,832 induced reversible regression of mammary tumors that was paralleled by a decrease in serum levels of TGF alpha secreted by the tumor cells. There was no difference in response to treatment with FTI between MMTV-TGF alpha mice, in which tumorigenesis was accelerated by multiparity or the chemical carcinogen 7,12-dimethylbenzanthracene, and MMTV-TGF alpha/neu mice. The tumor histological type had no impact on FTI sensitivity. For mechanistic analyses, tumor excision biopsies were obtained from 12 mice before and after treatment with L-744,832. In these samples, tumor regression was paralleled biochemically by inhibition of mitogen-activated protein kinase activity and biologically by an increase in G1-phase and decrease in S-phase fractions, as well as induction of apoptosis. These results suggest that the potential clinical use of FTI could be expanded to include cancers harboring activated receptor tyrosine kinases as well as those containing activated Ras.

Identification of STRAP, a novel WD domain protein in transforming growth factor-beta signaling

Transforming growth factor-beta1 (TGF-beta1) is the prototype of a large family of proteins that regulate a variety of biological processes. The pleiotropic responses to TGF-beta are mediated via ligand-induced heteromeric complex formation by type I (TbetaR-I) and type II (TbetaR-II) serine-threonine kinase receptors. Several studies have shown that TbetaR-II acts as a primary receptor, binding TGF-beta and phosphorylating TbetaR-I, whose kinase activity then propagates the signals. Therefore, intracellular proteins that interact with type I receptors are likely to play important roles in TGF-beta signaling. We have identified a novel WD domain-containing protein, designated STRAP (serine-threonine kinase receptor-associated protein), which interacts with TbetaR-I in a yeast two-hybrid system. STRAP associates with both functional TbetaR-I and TbetaR-II in vivo. Overexpression of STRAP leads to inhibition of TGF-beta-mediated transcriptional activation. It also shows synergistic inhibition of TGF-beta signaling in concert with Smad7, but not with Smad6, as measured by TGF-beta-dependent transcriptional reporters. The existence of the STRAP gene from yeast to mammals indicates an evolutionarily conserved function in eukaryotes. The data suggest a potential role for STRAP in TGF-beta signal transduction.

c-myc-enhanced S phase entry in keratinocytes is associated with positive and negative effects on cyclin-dependent kinases

The function of the c-myc proto-oncogene in cell cycle progression remains unclear. In order to examine the role c-myc may play in cell cycle progression, we have expressed the hormone-inducible MycER protein in the nontransformed, EGF-dependent mouse keratinocyte cell line BALB/MK. We have found that activation of MycER, but not a mutant MycER, Gal4ER, or FosER, leads to an EGF-dependent and hormone-dependent increased incorporation of labeled thymidine only during the S phase of the cell cycle in BALB/MK cells. A possible explanation for the increase in thymidine incorporation comes from flow cytometric analyses that reveal that activation of MycER leads to an increase in the total number of cells that enter S phase after EGF restimulation. Investigation of the intracellular effects of Myc activation shows that the expression of several putative Myc-sensitive proteins, cyclins A, E, and D1, and the E2F-1 protein are unaffected by Myc induction. Interestingly, we find that the histone H1 kinase activity associated with an E2F-1 complex containing Cyclin A and Cdk-2, but not that associated with Cyclin E, in late G1 and early S phases is increased in cells containing hormone-activated MycER, but not FosER. Although the mechanism for this Myc-dependent effect on E2F-1-associated kinase activity is still unknown, it does not appear to involve dissociation of the Cdk inhibitor p27Kip1 from the complexes as suggested by others. However, we have also found that hormone-treated cells actually show more p16INK4A inhibitor associated with another kinase, Cdk-4, as the cells are entering S phase. Altogether, the data suggest that the presence of excessive Myc protein in keratinocytes can stimulate otherwise noncycling cells to enter the cell cycle, and that this effect of Myc involves both positive effects on E2F-1-associated Cdk-2 and negative effects on Cdk-4 in late G1.

RhoB is stabilized by transforming growth factor beta and antagonizes transcriptional activation

Transforming growth factor beta (TGF-beta) is the prototype for an evolutionarily conserved superfamily of secreted factors implicated in diverse biological phenomena. The pleiotropic responses to TGF-beta are initiated by a heteromeric receptor complex that binds and phosphorylates downstream effectors. Among these, the Smads have been extensively studied. However, less attention has been directed toward alternative downstream effectors and their participation in TGF-beta signal transduction. We show that TGF-beta promotes accumulation of the labile monomeric GTPase RhoB by antagonizing its normal proteolytic destruction, presumably via the 26 S proteasome. RhoB accumulates in its isoprenylated form. Transient overexpression of wild type RhoB but not its dominant negative mutant RhoB-N19 antagonizes TGF-beta-mediated transcriptional activation. These results suggest a novel mechanism of regulation by TGF-beta and implicate RhoB as a negative regulator of TGF-beta signal transduction.

Dominant-negative interference of the transforming growth factor beta type II receptor in mammary gland epithelium results in alveolar hyperplasia and differentiation in virgin mice

Transforming growth factor (TGF)-beta1 and TGF-beta3 are normally expressed at high levels in the mammary gland during quiescence and at all stages of development, except lactation. Exogenously added TGF-beta1, -beta2, and -beta3 have been shown to regulate growth and differentiation of mammary epithelial cells in vitro and in vivo. TGF-betas signal through a heteromeric complex of type I and type II serine/threonine kinases. The type II receptor is necessary for ligand binding and growth suppression by TGF-betas. Deletions of the cytoplasmic domains of several kinase receptors known to function in multimeric complexes have been shown to act as dominant-negative mutations. To evaluate the role of endogenous TGF-betas in the growth and differentiation of the mammary gland in vivo, we have targeted expression of a truncated, kinase-defective TGF-beta type II receptor to mammary epithelial cells in transgenic mice using the mouse mammary tumor virus promoter/enhancer. Transgene expression was localized to the epithelial cells of terminal ducts and alveolar buds. At approximately 20 weeks of age, virgin female transgenic mice demonstrated varying degrees of mammary epithelial hyperplasia. Mammary glands from transgenic, virgin animals exhibited alveolar development and expression of the milk protein, beta-casein. The data suggest that impaired responsiveness in the epithelium to endogenous TGF-betas results in inappropriate alveolar development and differentiation in the mammary gland. We conclude that endogenous TGF-betas signal to the epithelium to maintain quiescence in the mammary glands of virgin animals.

Expression of a truncated, kinase-defective TGF-beta type II receptor in mouse skeletal tissue promotes terminal chondrocyte differentiation and osteoarthritis

Members of the TGF-beta superfamily are important regulators of skeletal development. TGF-betas signal through heteromeric type I and type II receptor serine/threonine kinases. When over-expressed, a cytoplasmically truncated type II receptor can compete with the endogenous receptors for complex formation, thereby acting as a dominant-negative mutant (DNIIR). To determine the role of TGF-betas in the development and maintenance of the skeleton, we have generated transgenic mice (MT-DNIIR-4 and -27) that express the DNIIR in skeletal tissue. DNIIR mRNA expression was localized to the periosteum/perichondrium, syno-vium, and articular cartilage. Lower levels of DNIIR mRNA were detected in growth plate cartilage. Transgenic mice frequently showed bifurcation of the xiphoid process and sternum. They also developed progressive skeletal degeneration, resulting by 4 to 8 mo of age in kyphoscoliosis and stiff and torqued joints. The histology of affected joints strongly resembled human osteo-arthritis. The articular surface was replaced by bone or hypertrophic cartilage as judged by the expression of type X collagen, a marker of hypertrophic cartilage normally absent from articular cartilage. The synovium was hyperplastic, and cartilaginous metaplasia was observed in the joint space. We then tested the hypothesis that TGF-beta is required for normal differentiation of cartilage in vivo. By 4 and 8 wk of age, the level of type X collagen was increased in growth plate cartilage of transgenic mice relative to wild-type controls. Less proteoglycan staining was detected in the growth plate and articular cartilage matrix of transgenic mice. Mice that express DNIIR in skeletal tissue also demonstrated increased Indian hedgehog (IHH) expression. IHH is a secreted protein that is expressed in chondrocytes that are committed to becoming hypertrophic. It is thought to be involved in a feedback loop that signals through the periosteum/ perichondrium to inhibit cartilage differentiation. The data suggest that TGF-beta may be critical for multifaceted maintenance of synovial joints. Loss of responsiveness to TGF-beta promotes chondrocyte terminal differentiation and results in development of degenerative joint disease resembling osteoarthritis in humans.

Kips off to Myc: implications for TGF beta signaling

Loss of sensitivity to the negative growth regulator transforming growth factor beta (TGF beta) is a feature of many different tumor types and is likely involved in tumor progression. In some cases this loss of sensitivity to TGF beta has been shown to be manifest in the absence of membrane-associated TGF beta receptor complexes, thus preventing initiation of antiproliferative signals from the cell surface. In others, loss of sensitivity to TGF beta-induced inhibitory signals has been attributed to loss of function of intracellular effectors of TGF beta-induced inhibitory signals due to mutation or allelic loss of effector genes and their products. The intracellular effectors of TGF beta inhibitory signals have been shown to be involved in the normal regulation of progression through the cell cycle, specifically during G1 phase. In this manner, elucidation of the mechanisms by which TGF beta inhibits cell growth not only helps us identify steps involved in tumor progression, but also allows us to better understand how cells regulate progression through the cell cycle.

Regulation of differentiation by TGF-beta

Recent experiments in neural, skeletal, endothelial, and hematopoietic tissues have provided new insights into the way members of the transforming growth factor-beta (TGF-beta) superfamily regulate cellular differentiation. TGF-betas regulate the fate of multipotential stem cells instructively (in the neural crest) by regulating the expression or function of tissue-specific transcription factors, as well as selectively (in the mesenchyme) by regulating the expression of required growth factors and their receptors. During skeletal development, TGF-betas have unique functions and act sequentially to modulate chondrocyte and osteoblast differentiation. Responsiveness to TGF-betas changes as cells differentiate and evidence now suggests that changes in TGF-beta receptor profile may account for some of these differences. Drosophila and transgenic mouse models are now providing useful insights into mechanisms of TGF-beta action in vivo.

Characterization of the interaction of FKBP12 with the transforming growth factor-beta type I receptor in vivo

The type I transforming growth factor-beta receptor (TbetaR-I) is the efferent component of the receptor complex, which presumably phosphorylates intracellular targets. FKBP12, a binding protein for FK506 and rapamycin, is shown to associate with the cytoplasmic region of TbetaR-I in vitro. In this report, we investigated the interaction of FKBP12 with TbetaR-I in vivo. FKBP12 interacts with TbetaR-I in mammalian cells as well as in yeast. Ligand addition does not affect the interaction, and both constitutively active and kinase-negative mutants of TbetaR-I bind FKBP12. FKBP12 dissociates from TbetaR-I in the presence of a high concentration of FK506. The juxtamembrane region of TbetaR-I, containing the major phosphorylation sites by the type II receptor, is required for the interaction. One of the deletion mutants in this region, which was shown to mediate transcriptional response, does not bind FKBP12, suggesting that FKBP12 is not directly involved in TGF-beta signaling. Furthermore TbetaR-I does not phosphorylate FKBP12 in vitro. FKBP12 may not be a direct substrate of TbetaR-I but possibly modulates the TbetaR-I function through its interaction with the regulatory domain of the kinase.

Interaction of the transforming growth factor-beta type I receptor with farnesyl-protein transferase-alpha

Transforming growth factor-beta 1 (TGF-beta 1) is the prototype of a large family of molecules that regulate a variety of biological processes. The type I (T beta R-I) and type II (T beta R-II) receptors for TGF-beta 1 are transmembrane serine/threonine kinases, forming a heteromeric signaling complex. Recent studies have shown that T beta R-II is a constitutively active kinase and phosphorylates T beta R-I upon ligand binding, suggesting that T beta R-I is the effector subunit of the receptor complex, which transduces signals to intracellular targets. This model has been further confirmed by the identification of constitutively active T beta R-I that mediates TGF-beta 1-specific cellular responses in the absence of ligand and T beta R-II. To investigate signaling by TGF-beta 1, we have sought to isolate proteins that interact with the cytoplasmic region of T beta R-I. One of the proteins identified was the alpha subunit of farnesyl-protein transferase (FT alpha) that modifies a series of peptides including Ras. T beta R-I specifically interacts with FT alpha in the yeast two-hybrid system. Glutathione S-transferase-T beta R-I fusion proteins bind FT alpha translated in vitro. T beta R-I also phosphorylates FT alpha. We further show that the constitutively active T beta R-I interacted with FT alpha very strongly whereas an inactive form of T beta R-I did not. These results suggest that FT alpha may be one of the substrates of the activated T beta R-I kinase.