The 'LXCXE' hydropathic superfamily of ligands for retinoblastoma protein: a proposal

Structural evaluation of new human polyomaviruses provides clues to pathobiology

In the past three years, remarkable discoveries have added three new human polyomaviruses (KI virus (KIV), WU virus (WUV) and Merkel cell virus (MCV)) to a class that previously had only two disease-causing members (BK virus (BKV) and JC virus (JCV)) identified. Two monkey polyomaviruses, simian virus (SV)40 and B-cell lymphotropic polyomavirus (LPV) are also present in humans. KIV and WUV lack the agnoprotein coding sequence and regulatory micro (mi)RNA clusters of BKV, JCV and SV40. MCV lacks the agnoprotein sequence but generates miRNAs. KIV, WUV and MCV are all widespread in humans. Although they have distinctive tissue tropisms, all these viruses are probably acquired in childhood. Of these viruses, only MCV has thus far been strongly linked to cancer. Marshalled evidence from diverse sources implicates MCV as an etiological agent of Merkel cell carcinoma. This review compares the structural features of the new and previously known polyomaviruses, with the aim of identifying approaches to molecular pathology.

Abrogation of cyclin D1 expression predisposes lung cancer cells to serum deprivation-induced apoptosis

Cyclin D1 antisense (D1AS)-transfected lung epithelial cell lines were serum deprived and then analyzed for three hallmarks of apoptosis: appearance of single-strand DNA breaks, alteration of apoptosis-related protein expression, and induction of chromatin condensation. Single-strand DNA breaks appeared at significant levels 24 h after serum deprivation, whereas induction of chromatin condensation was observed after 72 h. The antioxidants dimethyl sulfoxide, ascorbate, and glutathione, as well as insulin-like growth factor-I, inhibited induction of DNA damage in this assay. Additionally, proliferating cell nuclear antigen expression is completely suppressed in the D1AS cells, indicating a mechanism to explain the reduced capacity for DNA repair. Increased expression of cyclin D1, which is a common lesion in lung cancer, may thus prevent induction of apoptosis in an oxidizing and growth factor-poor environment. Reducing cyclin D1 expression in lung cancer cells by expression of D1AS RNA disrupted these protective pathways.

Functional interaction between a novel protein phosphatase 2A regulatory subunit, PR59, and the retinoblastoma-related p107 protein

The proteins of the retinoblastoma family are potent inhibitors of cell cycle progression. It is well documented that their growth-inhibitory activity can be abolished by phosphorylation on serine and threonine residues by cyclin dependent kinases. In contrast, very little is known about the dephosphorylation of retinoblastoma-family proteins. We report here the isolation, by virtue of its ability to associate with p107, of a novel Protein Phosphatase 2A (PP2A) regulatory subunit, named PR59. PR59 shares sequence homology with a known regulatory subunit of PP2A, PR72, but differs from PR72 in its expression pattern and its functional properties. We show that PR59 co-immunoprecipitates with the PP2A catalytic subunit, indicating that PR59 is a genuine component of PP2A holo-enzymes. In vivo, PR59 associates specifically with p107, but not with pRb. Elevated expression of PR59 results in dephosphorylation of p107, but not of pRb, and inhibits cell proliferation by causing cells to accumulate in G1. These data support a model in which the distinct PP2A regulatory subunits act to target the PP2A catalytic subunit to specific substrates and suggest a role for PP2A in regulation of p107.

Differential regulation of androgen and glucocorticoid receptors by retinoblastoma protein

The androgen receptor (AR) plays a major role in the development and maintenance of male primary and secondary sexual characteristics. The growth promoting effects of androgens are clearly seen in prostate cancer where treatment by androgen ablation usually leads to tumor regression, followed sometime later, by growth of tumor cells that are resistant to endocrine therapy. We have found that the level of pRB in cells controls AR activity. Overexpression of pRB leads to increased transcriptional activity of the AR. This is similar to the previously reported potentiation of glucocorticoid receptor activity by pRB. In contrast, loss of pRB activity inhibits AR but not glucocorticoid receptor activity. This inhibition correlates with the unique ability of the AR to form a protein-protein complex with pRB in vitro. The site of interaction with pRB lies within the N-terminal domain of the AR and co-localizes with the region of the AR that specifies a requirement for pRB. Thus, the AR has a novel requirement for pRB raising the possibility that the growth promoting activity of AR is due to its direct interaction with pRB. Furthermore, loss of pRB activity during progression of prostate cancer may directly result in a decreased response to androgens.

Neuronal Cdc2-like kinase (Nclk) binds and phosphorylates the retinoblastoma protein

The tumor suppressor retinoblastoma protein (RB) plays a central role in cellular growth regulation, differentiation, and apoptosis. Phosphorylation of RB results in a consequent loss of its ability to inhibit cell cycle progression. However, how RB phosphorylation might be regulated in apoptotic or postmitotic cells, such as neurons, remains unclear. Here we report that neuronal Cdc2-like kinase (Nclk), composed of Cdk5 and a neuronal Cdk5 activator (p25(nck5a)), can bind and phosphorylate RB. Since RB has been shown recently to associate with D-type G1 cyclins and viral oncoproteins through a common peptide sequence motif of LXCXE, Nclk binding may be mediated by a related sequence motif (LXCXXE) found in p25(nck5a). We demonstrate (i) in vitro binding of bacterially expressed p25(nck5a) to a GST-RB fusion protein, (ii) coprecipitation of GST-RB and reconstituted Cdk5.p25(nck5a), and (iii) phosphorylation of GST-RB by bacterially expressed Cdk5.p25(nck5a) kinase and by Cdk5.p25(nck5a) kinase purified from bovine brain. Finally, we show that immunoprecipitation of RB from embryonic mouse brain homogenate results in the coprecipitation of Cdk5 and that Cdk5 kinase activity is maximal during late embryonic development, a period when programmed cell death of developing neurons is greatest. Taken together, these results suggest that Nclk can bind to and phosphorylate RB in vitro and in vivo. We infer that Nclk may play an important role in regulating the activity of RB in the brain, including perhaps in apoptosing neurons.