Granulocyte-macrophage colony-stimulating factor stimulation results in phosphorylation of cAMP response element-binding protein through activation of pp90RSK

CREB in the pathophysiology of cancer: implications for targeting transcription factors for cancer therapy

Transcription factors are key regulators of the pattern of gene expression in a cell and directly control central processes such as proliferation, survival, self-renewal, and invasion. Given this critical role, the function of transcription factors is normally regulated closely, often through transient phosphorylation. Although transcription factors are not often directly modified by mutations in cancer cells, they frequently become activated constitutively through mutations affecting "upstream" pathways. By continually driving the expression of key target genes, these oncogenic transcription factors play a central role in tumor pathogenesis. One such transcription factor is the cAMP-regulatory element-binding protein (CREB), which can be activated through phosphorylation by a number of kinases, including Akt, p90Rsk, protein kinase A, and calcium/calmodulin-dependent kinases and regulates genes whose deregulated expression promotes oncogenesis, including cyclins, Bcl-2 family members, and Egr-1. CREB is overexpressed and constitutively phosphorylated in a number of forms of human cancer, including acute myeloid leukemia (AML) and non-small cell lung cancer, and appears to play a direct role in disease pathogenesis and prognosis. Although transcription factors have not been a central focus of drug development, recent advances suggest that CREB and other such proteins may be worthwhile targets for cancer therapy.

Beta2-microglobulin promotes the growth of human renal cell carcinoma through the activation of the protein kinase A, cyclic AMP-responsive element-binding protein, and vascular endothelial growth factor axis

PURPOSE

Beta(2)-microglobulin (beta2M), a soluble protein secreted by cancer and host inflammatory cells, has various biological functions, including antigen presentation. Because aberrant expression of beta2M has been reported in human renal cell carcinoma, we investigated the effects of beta2M overexpression on cancer cell growth and analyzed its molecular signaling pathway.

EXPERIMENTAL DESIGN

We established clonal cell lines that overexpressed beta2M in human renal cell carcinoma (SN12C) cells and then examined cell growth in vitro and in vivo and studied the beta2M-mediated downstream cell signaling pathway.

RESULTS

Our results showed that beta2M expression positively correlates with (a) in vitro growth on plastic dishes and as Matrigel colonies, (b) cell invasion and migration in Boyden chambers, and (c) vascular endothelial growth factor (VEGF) expression and secretion by cells. We found, in addition, that beta2M mediates its action through increased phosphorylation of cyclic AMP-responsive element-binding protein (CREB) via the protein kinase A-CREB axis, resulting in increased VEGF expression and secretion. In convergence with this signal axis, beta2M overexpression also activated both phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase pathways. Beta2M overexpression induced accelerated growth of SN12C in mouse subcutis and bone. Interrupting the beta2M signaling pathway using small interfering RNA led to apoptosis with increased activation of caspase-3 and caspase-9 and cleaved poly(ADP-ribose) polymerase.

CONCLUSIONS

Our results showed for the first time that the beta2M-protein kinase A-CREB-VEGF signaling axis plays a crucial role in support of renal cell carcinoma growth and progression and reveals a novel therapeutic target.

Testosterone activates mitogen-activated protein kinase and the cAMP response element binding protein transcription factor in Sertoli cells

The androgen testosterone is essential for the Sertoli cell to support the maturation of male germ cells and the production of spermatozoa (spermatogenesis). In the classical view of androgen action, binding of androgen to the intracellular androgen receptor (AR) produces a conformational change in AR such that the receptor-steroid complex has high affinity for specific DNA regulatory elements and is able to stimulate gene transcription. Here, we demonstrate that testosterone can act by means of an alternative, rapid, and sustainable mechanism in Sertoli cells that is independent of AR-DNA interactions. Specifically, the addition of physiological levels of testosterone to Sertoli cells stimulates the mitogen-activated protein kinase signaling pathway and causes phosphorylation of the cAMP response element binding protein transcription factor on serine 133, a modification known to be required for Sertoli cells to support spermatogenesis. Androgen-mediated activation of mitogen-activated protein kinase and cAMP response element binding protein occurs within 1 min, extends for at least 12 h and requires AR. Furthermore, androgen induces endogenous cAMP response element binding protein-mediated transcription in Sertoli cells. These newly identified mechanisms of androgen action in Sertoli cells suggest new targets for developing male contraceptive agents.

P21(Cip1) induced by Raf is associated with increased Cdk4 activity in hematopoietic cells

To investigate the functions of the different Raf genes in hematopoietic cell proliferation, the capacities of beta-estradiol-regulated Delta Raf:ER genes to induce cell cycle regulatory gene expression and cell cycle progression in FDC-P1 cells were examined. Raf activation increased the expression of Cdk2, Cdk4, cyclin A, cyclin D, cyclin E, p21(Cip1) and c-Myc and decreased the expression of p27(Kip1) which are associated with G(1) progression. However only the cell clones with moderate Raf activation, i.e. FD/Delta Raf-1:ER and FD/Delta A-Raf:ER, successfully underwent cell proliferation. The cell clones with the highest Delta Raf activity, FD/Delta B-Raf:ER, underwent apoptosis before cell proliferation. p21(Cip1) induced by Raf activation specifically bound with Cdk4/cyclin D complexes but not Cdk2/cyclin E complexes and this binding was associated with the increased Cdk4 activity. However, no binding of p27(Kip1) with either Cdk2/cyclin E or Cdk4/cyclin D was observed. Thus Raf mediated growth was associated with elevated p21(Cip1) expression, which may specifically bind with and activate Cdk4/cyclin D complexes and with decreased p27(Kip1) expression.