PRB protein

RB1 Codes for the pRB protein, a master brake of Retinoblastoma, osteosarcoma, and bladder, small cell... 

Figure 1.3 Western blotting of pRB protein extracted from two fresh cell lines, T24 and J82. The pRB proteins in fresh T24 cell line showed a stronger band than that obtained from J82 cell line. The Western blotting results correlated well with IHC staining intensity (Table 1.3 and Fig. 1.1). Reproduced with permission from Shi et al., Biotech. Histochem. 2007 82 301-309.

The histone deacetylases are found in large protein complexes, often together with repressive transcription factors. By this token, interactions of the repressive heterodi-meric transcription factor Mad-Max and a complex with the histone deacetylase HDAC I and the mSinSA protein have been demonstrated. A complex of HDAC I and the nuclear receptor-corepressor (see chapter 4) binds to unliganded nuclear receptors and is believed to exercise a repressive effect. A further example is the tumor suppressor protein pRb (see chapters 13,14), which can occur as a transcription repressor in the hypo-phosphorylated form and transcriptionally activating in the hyperphosphorylated form. The repressive form of the pRb protein recruits the histone deacetylase HDAC 1 to the DNA and thereby initiates an active repression of the gene (see 13.3.2). 

The inhibitor pl6 4 is attributed properties as a tumor suppressor since the gene for pl6 4a jg mutated in many tumor cell lines. There is evidence that the pRb protein (see 13.3.2) performs a regulating function on transcription of pl6 . 

Cyclin E also performs its function in Gi/S phase. It demonstrates a periodic concentration change with a maximal value at the start of S phase. Afterwards, its concentration falls off sharply within S phase. The gene for cyclin E is also induced by transcription factor E2F which explains the increase in cyclin E at the Gi/S transition. Cyclin E binds and activates CDK2. The activated CDK2 complex is also involved in phosphorylation of the pRb protein. As a consequence, a signal is transmitted, with cooperation of cyclin D, in the direction of the transcription of genes that are essential for the continuation of the cell cycle. 

The pRb protein is a nuclear phosphoprotein of ca. 100 kDa. The domain structure of pRb is shown in Fig. 13.13 (review Riley et al., 1994 Weinberg, 1995). The pRb proteins possesses numerous Ser/Thr phosphorylation sites, binding sites for the transcription factor E2F, for the viral oncoproteins TAg, ElA and E7 (see Chapter 14) and a non-specific DNA binding site. Furthermore, an N-terminal sequence section has been identified in pRb protein that is required for oligomerization. 

The pRb protein has the characteristics of a tumor suppressor protein. Loss of its function is associated with deregulation of cell division and favors tumor formation (see Chapter 14). 

A scheme of the control function of pRb and E2F is shown in Fig. 13.14. pRb controls the function of E2F by entering into a complex with the latter. In the underphos-phorylated form, the pRb-E2F complex actively represses transcription of DNA. The hypophosphorylated pRb protein represents the active form of pRb since this form mediates repression of the E2F controlled genes. If pRb exists in the hyperphosphory-lated form, transcription repression is removed and E2F can stimulate transcription of the target genes. 

Tliese findings, and the regulating activity on E2F function performed in the promo-ter-boimd state, indicate that pRb protein is able to interact with components of the... 

The pRb protein is also involved in differentiation processes. It is therefore assumed that pRb controls other effector molecules in addition to E2F. 

The regulatory network, of which the pRb protein is a part, can be changed by various mechanisms in the sense of stimulation of proliferation (Fig. 14.6). First, an error in the function of proteins involved in phosphorylation of pRb may initiate a dominating activation of the pRb function. Second, the pRb protein itself may be activated in an improgrammed maimer by mutation or by binding of viral proteins. 

An imcontroUed increase in the concentration and activity of the cyclins D1 and CDK4 can lead to unprogrammed phosphorylation of the pRb protein. Overexpression of cycUn D1 or CDK4 has been found in various tumors of the bladder, breast, pancreas and the lungs (Weintraub, 1995). 

It is also important that the MDM2 protein has a regulating influence on the pRb protein (see 14.4.4) so that the MDM2 protein regulates two important cell cycle control proteins. 

Another essential function of cyclin D-CDK4/6 complexes is to activate metabolism and to promote cell growth. Cells must exceed a critical size in order to be able to pass through the restriction point R. The best characterized substrate of cyclin D-CDK4/6 complexes is the pRb protein, which is transformed from an unphosphorylated state to a hypophosphorylated state by cyclin D-CDK4/6 complexes. 

The activity of cyclin E-CDK2 complexes is mainly directed toward two substrates, the pRb protein and the inhibitor p27KIP1. Phosphorylation of pRb inactivates pRb and releases the transcription factor E2F from a repressed state, allowing transcription of key proteins for Gj/S transition. The gene for cyclin E is also induced by transcription... 

The E2F proteins are DNA-binding proteins, which, in addition to the DNA-binding domain, also have a binding site for pRb protein and - except E2F4 and E2F5 - a transactivating domain (review Trimarchi and Lees, 2001). In some cell types, the E2F... 

The oncoprotein of the SV40 virus, Tag (see Section 14.6), binds to the p53 protein and can inactivate the p53 function in a similar way to that assumed for inactivation of the pRb protein. 

The tumor-promoting activity of OA class compounds is dne to their inhibitory activity of protein phosphatases, which increases phosphorylation/activation of varions proteins involved in the signal transduction pathways regulating the cell cycle and cell transformation. ° Different effects of OA leading to transformation of cells were reported, such as induction of mitosis, ° ° cycle progression, " as well as cell proliferation genes and proto-oncogenes expression. " It has been demonstrated that OA induces the release and expression of tumor necrosis factor-a (TNF-a) and other cytokines, phosphorylation of the tnmor snppressors p53 and retinoblastoma pRb proteins, and activation and induction of the nnclear transcription factors, such as activator protein-1 (AP-1) andNF-KB. ... 

---