Ras and Rho small GTPases possessing a C-terminal polybasic region (PBR) are vital signaling proteins whose misregulation can lead to cancer. Signaling by these proteins depends on their ability to bind guanine nucleotides and their prenylation with a geranylgeranyl or farnesyl isoprenoid moiety and subsequent trafficking to cellular membranes. There is little previous evidence that cellular signals can restrain nonprenylated GTPases from entering the prenylation pathway, leading to the general belief that PBR-possessing GTPases are prenylated as soon as they are synthesized. Here, we present evidence that challenges this belief. We demonstrate that insertion of the dominant negative mutation to inhibit GDP/GTP exchange diminishes prenylation of Rap1A and RhoA, enhances prenylation of Rac1, and does not detectably alter prenylation of K-Ras. Our results indicate that the entrance and passage of these small GTPases through the prenylation pathway is regulated by two splice variants of SmgGDS, a protein that has been reported to promote GDP/GTP exchange by PBR-possessing GTPases and to be up-regulated in several forms of cancer. We show that the previously characterized 558-residue SmgGDS splice variant (SmgGDS-558) selectively associates with prenylated small GTPases and facilitates trafficking of Rap1A to the plasma membrane, whereas the less well characterized 607-residue SmgGDS splice variant (SmgGDS-607) associates with nonprenylated GTPases and regulates the entry of Rap1A, RhoA, and Rac1 into the prenylation pathway. These results indicate that guanine nucleotide exchange and interactions with SmgGDS splice variants can regulate the entrance and passage of PBR-possessing small GTPases through the prenylation pathway.

SmgGDS is a guanine nucleotide exchange factor with the unique ability to activate multiple small GTPases, implicating it in cancer development and progression. Here, we investigated the role of SmgGDS in prostate cancer by studying the expression of SmgGDS in benign and malignant prostatic tissues. We also probed SmgGDS function in three prostate carcinoma cell lines using small interfering RNA (siRNA). Immunohistochemical analysis revealed that SmgGDS levels were elevated in prostatic intraepithelial neoplasia (PIN), prostate carcinoma, and metastatic prostate carcinoma. In addition, expression of SmgGDS positively correlated with that of cyclooxygenase-2 (COX-2), a protein believed to promote the development of prostate carcinoma. Reduction of SmgGDS expression in prostate carcinoma cells inhibited proliferation and migration, irrespective of androgen receptor status. These effects were accompanied by a reduction in COX-2 expression and in activity of NF-kappaB, a known regulator of COX-2. Taken together, these findings suggest that SmgGDS promotes the development and progression of prostate cancer, possibly associated with NF-kappaB-dependent up-regulation of COX-2.

A polyphenol-rich reagent, referred to as CSC, was isolated from cigarette smoke condensate and shown to prime purified human neutrophils. A mouse monoclonal anti-idiotypic antibody directed against the polyphenol-reactive determinants on a rabbit polyclonal anti-tobacco glycoprotein antibody was generated and shown to also prime neutrophils. After priming by CSC or tobacco anti-idiotypic antibody, there was a 2.5-fold to threefold increase in CD11b/18 expression and doubling of the number of formylmethionyl-leucyl-phenylalanine receptors on the cells. The primed cells showed a twofold increase, compared with unprimed cells, in production of superoxide and release of neutrophil elastase after stimulation with formyl-methionyl-leucyl-phenylalanine. Neutrophils in peripheral blood of cigarette smokers have been shown to be primed and more responsive to activating agents. The priming effects attributed to whole cigarette smoke have been demonstrated in these studies using purified neutrophils and CSC or tobacco anti-idiotypic antibody. These studies are a first step in testing the hypothesis that the inflammatory process contributing to progression of chronic obstructive pulmonary disease in exsmokers may be driven , in part , by tobacco antiidiotypic antibodies. This hypothesis is novel and carries with it the implication of a heretofore unrecognized autoimmune component in the disease process manifested through production of anti-idiotypic antibodies with tobacco-like activity. (Am J Pathol 2000, 157:1735-1743)