GHRH proliferative action on somatotrophs is cell-type specific and dependent on Pit-1/GHF-1 expression

To investigate the mechanisms by which the hypothalamic peptide GHRH influences cell division, we analyzed its effects on the proliferation of two different cell lines: CHO-4, an ovary-derived cell line, and GH3, a pituitary-derived cell line. We found that GHRH induces the proliferation of pituitary-derived cells but inhibits the proliferation of ovary-derived cells. We further characterized this dual effect of GHRH to find that the cytoplasmic signals induced by this hormone are similar in both cell lines. Moreover, in CHO-4 cells GHRH stimulates two well-known positive cell cycle regulators, c-myc and cyclin D1, but is unable to induce the degradation of the negative cell cycle regulator p27(Kip1). Significantly, when the Pit-1/GHF-1 gene is exogenously expressed in CHO-4 cells, the negative effect of GHRH on the proliferation of these cells is attenuated. Furthermore, when the levels of Pit-1 are downregulated by siRNA in GH3-GHRHR cells, the positive effects of GHRH on the proliferation of these cells are diminished. These findings add to our understanding of the molecules involved in the regulation of cell proliferation by GHRH, as we demonstrate for the first time that Pit-1 is not only required to drive the expression of the GHRH receptor, as previously described, but is also needed for the downstream effects that occur after its activation to modulate cell proliferation. These data suggest that the regulation of cell proliferation in response to a specific growth factor depends in certain cell populations on the presence of a tissue-specific transcription factor.

Bcl-2 retards cell cycle entry through p27(Kip1), pRB relative p130, and altered E2F regulation

Independent of its antiapoptotic function, Bcl-2 can, through an undetermined mechanism, retard entry into the cell cycle. Cell cycle progression requires the phosphorylation by cyclin-dependent kinases (Cdks) of retinoblastoma protein (pRB) family members to free E2F transcription factors. We have explored whether retarded cycle entry is mediated by the Cdk inhibitor p27 or the pRB family. In quiescent fibroblasts, enforced Bcl-2 expression elevated levels of both p27 and the pRB relative p130. Bcl-2 still slowed G(1) progression in cells deficient in pRB but not in those lacking p27 or p130. Hence, pRB is not required, but both p27 and p130 are essential mediators. The ability of p130 to form repressive complexes with E2F4 is implicated, because the retardation by Bcl-2 was accentuated by coexpressed E2F4. A plausible relevant target of p130/E2F4 is the E2F1 gene, because Bcl-2 expression delayed E2F1 accumulation during G(1) progression and overexpression of E2F1 overrode the Bcl-2 inhibition. Hence, Bcl-2 appears to retard cell cycle entry by increasing p27 and p130 levels and maintaining repressive complexes of p130 with E2F4, perhaps to delay E2F1 expression.

Growth hormone-releasing hormone stimulates mitogen-activated protein kinase

GH-releasing hormone (GHRH) can induce proliferation of somatotroph cells. The pathway involving adenylyl cyclase/cAMP/protein kinase A pathway in its target cells seems to be important for this action, or at least it is deregulated in some somatotroph pituitary adenomas. We studied in this work whether GHRH can also stimulate mitogen-activated protein (MAP) kinase. GHRH can activate MAP kinase both in pituitary cells and in a cell line overexpressing the GHRH receptor. Although both protein kinase A and protein kinase C could activate MAP kinase in the CHO cell line studied, neither protein kinase A nor protein kinase C appears to be required for GHRH activation of MAP kinase in this system. However, sequestration of the betagamma-subunits of the G protein coupled to the receptor inhibits MAP kinase activation mediated by GHRH. This pathway also involves p21ras and a phosphatidylinositol 3-kinase, probably phosphatidylinositol 3-kinase-gamma. Despite the involvement of p21ras, the protein kinase Raf-1 is not hyperphosphorylated in response to GHRH, contrary to what usually occurs when the Ras-Raf-MAP kinase pathway is activated. In summary, this work describes for the first time the activation of MAP kinase by GHRH and outlines a path for this activation that is different from the cAMP-dependent mechanism that has been traditionally described as mediating the mitogenic actions of GHRH.

Complex regulation of prothymosin alpha in mammary tumors arising arising in transgenic mice

Expression of prothymosin alpha (PTA) has been related to cell proliferation, both normal and pathological. PTA has also been proposed to be a target of the c-myc protooncogene. To study PTA mRNA levels during pathological cell growth, and especially the effect of the activation of specific oncogenes on PTA expression, we have studied its expression in tumors that arise in transgenic mice. We found high PTA levels in mammary tumors arising in c-myc, c-neu, and v-ras transgenic mice. Levels of this protein were variable between different tumors, and there is a differential regulation of PTA respect to other putative c-myc target genes, such as Ornithine Decarboxylase (ODC). Furthermore, expression of PTA is not absolutely dependent of c-myc expression, as shown by MYC depletion experiments performed with antisense oligonucleotides. We conclude that regulation of PTA in these tumors is complex and depends on more than a single activated oncogene.

TGF-beta1 actions on FRTL-5 cells provide a model for the physiological regulation of thyroid growth

Little is known about the TGF-beta1 mechanism that promotes thyroid cell growth arrest. We assessed TGF-beta1 effects on Fisher rat thyroid cell line (FRTL-5). This allowed us to study TGF-beta1 action on thyroid cells in various physiological situations such as actively proliferating cells, resting cells stimulated to proliferate by the action of various mitogens, and resting cells. TGF-beta1 arrested proliferating FRTL-5 cells, increasing c-myc mRNA levels and reducing p27-free cyclin D1 protein levels, without affecting either the cellular content of p27 or the cyclin D1-p27 complexes. Moreover, TGF-beta1 treatment reduced the activity of cyclin E-CDK2 complexes and, consequently, pRB was found to be hypophosphorylated. TGF-beta1 prevented resting cells to enter in the cell cycle when stimulated with growing medium (newborn calf serum plus a mixture of five hormones) but not when TSH (thyroid stimulating hormone) plus IGF-1 (Insulin-like growth factor I) were used as mitogens. Both stimuli increased the levels of cyclins D1, D3 and E but TGF-beta1 had a greater effect in decreasing these cyclin levels in growing-medium stimulated cells than in TSH + IGF-1. This suggests that for FRTL-5 cells, the content of these cyclins must exceed a threshold to progress through the cell cycle. TGF-beta1 induced apoptosis in quiescent cells, accompanied by a reduction in p27 protein levels and an increase in c-myc expression. Interestingly, TGF-beta1-induced variations in prothymosin alpha and c-myc mRNA levels were not correlated. TGF-beta1 always promoted an increase of p15 mRNA levels. In summary, our results point to the fact that TGF-beta1 could play a physiological role in the control of thyroid growth through the modification of cell cycle regulatory proteins.

The J domain of simian virus 40 large T antigen is required to functionally inactivate RB family proteins

Transformation by simian virus 40 large T antigen (TAg) is dependent on the inactivation of cellular tumor suppressors. Transformation minimally requires the following three domains: (i) a C-terminal domain that mediates binding to p53; (ii) the LXCXE domain (residues 103 to 107), necessary for binding to the retinoblastoma tumor suppressor protein, pRB, and the related p107 and p130; and (iii) an N-terminal domain that is homologous to the J domain of DnaJ molecular chaperone proteins. We have previously demonstrated that the N-terminal J domain of TAg affects the RB-related proteins by perturbing the phosphorylation status of p107 and p130 and promoting the degradation of p130 and that this domain is required for transformation of cells that express either p107 or p130. In this work, we demonstrate that the J domain of TAg is required to inactivate the ability of each member of the pRB family to induce a G1 arrest in Saos-2 cells. Furthermore, the J domain is required to override the repression of E2F activity mediated by p130 and pRB and to disrupt p130-E2F DNA binding complexes. These results imply that while the LXCXE domain serves as a binding site for the RB-related proteins, the J domain plays an important role in inactivating their function.

Inactivation of pRB-related proteins p130 and p107 mediated by the J domain of simian virus 40 large T antigen

Inactivation of the retinoblastoma tumor suppressor protein (pRB) contributes to tumorigenesis in a wide variety of cancers. In contrast, the role of the two pRB-related proteins, p130 and p107, in oncogenic transformation is unclear. The LXCXE domain of simian virus 40 large T antigen (TAg) specifically binds to pRB, p107, and p130. We have previously shown that the N terminus and the LXCXE domain of TAg cooperate to alter the phosphorylation state of p130 and p107. Here, we demonstrate that TAg promotes the degradation of p130 and that the N terminus of TAg is required for this activity. The N terminus of TAg has homology to the J domain of the DnaJ family of molecular chaperone proteins. Mutants with mutations in the J-domain homology region of TAg are defective for altering p130 and p107 phosphorylation and for p130 degradation. A heterologous J-domain from a human DnaJ protein can functionally substitute for the N terminus of TAg in the effect on p107 and p130 phosphorylation and p130 stability. We further demonstrate that the J-domain homology region of TAg confers a growth advantage to wild-type mouse embryo fibroblasts (MEFs) but is dispensable in the case of MEFs lacking both p130 and p107. This indicates that p107 and p130 have overlapping growth-suppressing activities whose inactivation is mediated by the J domain of TAg.

DnaJ/hsp40 chaperone domain of SV40 large T antigen promotes efficient viral DNA replication

The amino-terminal domain of SV40 large tumor antigen (TAg) is required for efficient viral DNA replication. However, the biochemical activity associated with this domain has remained obscure. We show here that the amino-terminal domain of TAg shares functional homology with the J-domain of DnaJ/hsp40 molecular chaperones. DnaJ proteins function as cofactors by regulating the activity of a member of the 70-kD heat shock protein family. Genetic analyses demonstrated that amino-terminal sequences of TAg comprise a novel J-domain that mediates a specific interaction with the constitutively expressed hsc70 and show that the J-domain is also required for efficient viral DNA replication in vivo. Furthermore, we demonstrated that the J-domain of two human DnaJ homologs, HSJ1 or DNAJ2, could substitute functionally for the amino-terminus of TAg in promoting viral DNA replication. Together, our findings suggest that TAg uses its J-domain to support SV40 DNA replication in a manner that is strikingly similar to the use of Escherichia coli DnaJ by bacteriophage lambda in DNA replication. However, TAg has evolved a more efficient strategy of DNA replication through an intrinsic J-domain to associate directly with a partner chaperone protein. Our observations provide evidence of a role for chaperone proteins in the process of eukaryotic DNA replication.

Ras signalling linked to the cell-cycle machinery by the retinoblastoma protein

The Ras proto-oncogene is a central component of mitogenic signal-transduction pathways, and is essential for cells both to leave a quiescent state (G0) and to pass through the G1/S transition of the cell cycle. The mechanism by which Ras signalling regulates cell-cycle progression is unclear, however. Here we report that the retinoblastoma tumour-suppressor protein (Rb), a regulator of G1 exit, functionally links Ras to passage through the G1 phase. Inactivation of Ras in cycling cells caused a decline in cyclin D1 protein levels, accumulation of the hypophosphorylated, growth-suppressive form of Rb, and G1 arrest. When Rb was disrupted either genetically or biochemically, cells failed to arrest in G1 following Ras inactivation. In contrast, inactivation of Ras in quiescent cells prevented growth-factor induction of both immediate-early gene transcription and exit from G0 in an Rb-independent manner. These data suggest that Rb is an essential G1-specific mediator that links Ras-dependent mitogenic signalling to cell-cycle regulation.

Cyclin E, a redundant cyclin in breast cancer

Cyclin E is an important regulator of cell cycle progression that together with cyclin-dependent kinase (cdk) 2 is crucial for the G1/S transition during the mammalian cell cycle. Previously, we showed that severe overexpression of cyclin E protein in tumor cells and tissues results in the appearance of lower molecular weight isoforms of cyclin E, which together with cdk2 can form a kinase complex active throughout the cell cycle. In this study, we report that one of the substrates of this constitutively active cyclin E/cdk2 complex is retinoblastoma susceptibility gene product (pRb) in populations of breast cancer cells and tissues that also overexpress p16. In these tumor cells and tissues, we show that the expression of p16 and pRb is not mutually exclusive. Overexpression of p16 in these cells results in sequestering of cdk4 and cdk6, rendering cyclin D1/cdk complexes inactive. However, pRb appears to be phosphorylated throughout the cell cycle following an initial lag, revealing a time course similar to phosphorylation of glutathione S-transferase retinoblastoma by cyclin E immunoprecipitates prepared from these synchronized cells. Hence, cyclin E kinase complexes can function redundantly and replace the loss of cyclin D-dependent kinase complexes that functionally inactivate pRb. In addition, the constitutively overexpressed cyclin E is also the predominant cyclin found in p107/E2F complexes throughout the tumor, but not the normal, cell cycle. These observations suggest that overexpression of cyclin E in tumor cells, which also overexpress p16, can bypass the cyclin D/cdk4-cdk6/p16/pRb feedback loop, providing yet another mechanism by which tumors can gain a growth advantage.

Simian virus 40 large T antigen alters the phosphorylation state of the RB-related proteins p130 and p107

p130 and p107 are nuclear phosphoproteins related to the retinoblastoma gene product (pRb). pRb, p107, and p130 each undergo cell cycle-dependent phosphorylation, form complexes with the E2F family of transcription factors, and associate with oncoproteins of DNA tumor viruses, including simian virus 40 (SV40) large T antigen (TAg) and adenovirus ElA protein. The results of recent studies with mouse embryo fibroblasts (MEFs) lacking the retinoblastoma gene (Rb-1) have suggested that p130 and p107 may be important targets for SV40 large TAg-mediated transformation (J.B. Christensen and M.J. Imperiale, J. Virol. 65:3945-3948, 1995; J. Zalvide and J.A. DeCaprio, Mol. Cell. Biol. 15:5800-5810, 1995). In this report, we demonstrate that the expression of TAg affects the phosphorylation state of p130 and p107. In cells expressing wild-type TAg, only un(der)phosphorylated p130 and p107 were detected. To determine the domains within TAg that contribute to this effect on the phosphorylation of p130, we performed transient expression assays. While transiently expressed p130 was apparently phosphorylated normally, only un(der)phosphorylated p130 was detected when p130 was coexpressed with TAg. Using this assay, we found that the first 147 amino acids of TAg were sufficient to alter the phosphorylation state of p130. Within this region, the LXCXE domain of TAg, required for binding to the retinoblastoma family of proteins, was necessary but not sufficient to affect p130 phosphorylation. Residues within the first 82 amino acids of TAg were also required. TAg with mutations in the N terminus retained the ability to efficiently associate with p130 but did not affect its phosphorylation state. This demonstrates that the effect of SV40 TAg on p130 is not simply the result of binding and suggests that TAg has a novel effect on p130 and p107 that differs from its effect on pRb.

Role of pRb-related proteins in simian virus 40 large-T-antigen-mediated transformation

Simian virus 40 large T-antigen (TAg) transformation is thought to be mediated, at least in part, by binding to and modulating the function of certain cellular proteins, including the retinoblastoma tumor suppressor gene product, pRb. TAg can disrupt the inhibitory complexes formed by pRb with the oncogenic transcription factor E2F, and this mechanism has been suggested to be important for TAg-mediated transformation. Residues 102 to 114 of TAg (including the LXCXE motif) are required for binding to pRb. Mutations within this LXCXE motif abolish the ability of TAg to bind to pRb as well as to transform certain cell types. TAg can also bind to at least two other cellular proteins, p107 and p130, that are related to pRb by sequence homology and share the ability to bind E2F. However, whether p107 and p130 are also targets in TAg-mediated transformation is less clear. To assess the relative contribution of the inactivation of pRb, p107, and p130 to transformation by TAg, fibroblasts were prepared from embryos derived from matings of mice heterozygous for an Rb knockout allele. The ability of TAg to transform fibroblasts homozygous for either wild-type or knockout Rb alleles was evaluated. It is demonstrated that the integrity of the LXCXE motif provides a growth advantage in Rb+/+ and Rb-/- cells. Furthermore, wild-type TAg, but not the LXCXE mutants, could bind to p107 and p130 and disrupt p107-E2F and p130-E2F binding complexes. These results suggest that p107 and p130 participate in TAg-mediated transformation and that they may behave as tumor suppressors.

Distribution of prothymosin alpha in rat and human adrenal cortex

BACKGROUND

Prothymosin alpha (ProT) is a polypeptide widely distributed in the organism and expressed by cell types with a high proliferative capacity. The aim of the current work was to investigate if ProT was localized in the progenitor compartment of the adrenal cortex which, following the cell migration theory, corresponds to the zona glomerulosa.

METHODS

We studied by immunohistochemical and in situ hybridization methods the distribution of ProT in rat and human adrenal cortex. Immunohistochemical techniques for the study of the proliferating cell nuclear antigen (PCNA) and incorporation of 5-bromo-2'-deoxyuridin during DNA synthesis were also done. Immunoelectron microscopic procedures were performed to determine the exact subcellular localization of ProT.

RESULTS

ProT was found in the zona glomerulosa cells, but not in the cells of the remaining cortical layers (zonae fasciculata and reticularis). Glomerulosa cells showed immunostaining for ProT only in the nuclei, excluding the nucleoli. Variability in immunostaining intensity was found between different glomerulosa cells. In situ hybridization of ProT mRNA confirmed that ProT synthesis in adrenal cortex occurs only in the zona glomerulosa. The results obtained for proliferating cell nuclear antigen and incorporation of 5-bromo-2'-deoxyuridin confirmed that adrenocortical proliferation occurs in the zona glomerulosa. Ultrastructural immunocytochemistry showed labelling for ProT over the euchromatin, but not on the heterochromatin aggregations nor the nucleoli.

CONCLUSIONS

The results presented here: 1) support the migration theory for the adrenocortical cell renewal, 2) demonstrate that ProT is present in the nuclei of proliferating cells (being associated with euchromatin), and 3) suggest that the study of ProT expression would be useful in distinguishing cycling from resting cells.

Tissue concentrations of prothymosin alpha: a novel proliferation index of primary breast cancer

In 71 patients with classic invasive ductal carcinomas, levels of prothymosin alpha (PT alpha), as assayed by a radioimmunoassay that detects thymosin alpha 1 (the NH2-terminal fragment of PT alpha), were significantly greater in tumour samples than in normal breast tissue. PT alpha levels were correlated with (a) the number of positive axillary lymph nodes (rs = 0.5384, P < 0.01), and (b) the percentage of tumour cells in the S or G2/M phase as assessed by flow cytometry (rs = 0.5027, P < 0.01). Since the beginning of this study in 1989, 21 patients have presented distant metastases, all of whom were previously shown to have tumour PT alpha levels greater than 124 ng of thymosin alpha 1/mg protein. The present report indicates that PT alpha might be used to identify breast cancer patients at high risk for distant metastases.