CHAPTER 15

Now we understand how Transcription Factors interact with DNA to regulate transcription. The next logical question that arises is what stimulates the production of Transcription Factors. A considerable amount of relevant research has been performed concerning this with regard to the regulation of the cell cycle.

First we should define a few terms. Kinases are proteins that phosphorylate other proteins thereby activating the phosphorylated recipient protein. Phosphatases are enzymes that remove phosphates from other proteins thereby inactivating them. Cyclins are proteins whose concentrations vary dependent on the stage of the cell cycle.

Cyclins form complexes with Cyclin Dependent Kinases (CDKs) which in turn allow the kinase to phosphorylate their target proteins. The phosphorylation events are transient and reversible. When cyclin concentrations decrease phosphatases dephosphorylate their target proteins. Figure 5-3. A key family of target proteins of cyclin-CDK complexes is the Rb protein which normally binds to the Transcription Factor E2F thereby inactivating it. This inactivation persists until the Rb is phosphorylated which changes its shape and hence its affinity for E2F. When the E2F is released it turns on genes for the next stage of the cell cycle. Figure 15-5.

Apoptosis is programmed cell death. It occurs sequentially via: DNA breakup, disruption of organelle structure, loss of cell shape, breaking cells up into small apoptotic bodies that are then phagocytized.

Caspases (cysteine containing aspartate specific proteases) are enzymes present as part of larger polypeptides that represent inactive forms (zymogens). Caspases cleave target proteins at specific aspartate sites. Caspases are classified as either initiators or executioners. Initiators are activated by signals from other classes of proteins and then activate executioners. One of the things that executioners do is to cleave sequestering proteins that normally bind endonucleases. This releases the endonucleases so that they can enter the nucleus and digest the DNA.

Within a cell an important protein p53 is induced in G1 due to DNA damage. This activates protein p21 that binds to the Cylin-CDK that would normally phosphorylate Rb. This prevents release of E2F Transcription Factor which is necessary for the cell to progress into the S-phase. Figure 15-8.

Other signals are generated extracellularly, often by other cells. If they originate from distant cells and arrive via the circulatory system the signals are hormones (often steroids that can cross cell membranes) and part of the endocrine system. If they arise from nearby cells they are part of the paracrine system and are referred to as growth factors.These are generally proteins that cannot move across the cell membrane. The general term for these signaling molecules is ligands.

Protein ligands require a transmembrane receptor site to bind to. A common receptor is the Tyrosine Kinase Receptor (RTK) Figure 15-10 and this is used for growth factors. Growth factors stimulate Mitosis. RTKs may cause a sequence of proteins to be activated leading to the activation of Transcription Factors. An important category of proteins is G-proteins, one of which important to the cell cycle (specifically cancer) is the Ras protein. Ras is activated when bound to GTP, and inactivated when bound to GDP. G-proteins have natural GTPase activity and soon degrade the GTP to GDP thereby inactivating themselves. As an aside G-proteins are now known to be the basis for most medicines (about 60 % of which target G-proteins). Activated Ras propagates a signal that promotes cell proliferation.

Cancer is a result of cell division occurring in an uncontrolled fashion thereby leading to tumor production. Normally a balance exists between genetic systems that encourage cell proliferation (oncogenes) and genetic systems that prevent excess cell proliferation (tumor-suppressor genes). Usually cancers require a sequential accumulation of mutations. Oncogenes may inappropriately promote cell cycle progression or inappropriately inhibit apoptosis. Tumor-suppresser gene mutations may remove cell cycle progression inhibition, prevent apoptosis or prevent DNA repair.

Oncogene mutations to the Ras protein which cause it to lose its GTPase activity cause Ras to be continuously activated, thereby signaling for cell proliferation. Another form of oncogene mutation may occur in the transmembrane signal receptor proteins such that they remain in a turned on configuration.

Examples of tumor-suppressor gene mutations include changes in the Rb gene that creates an Rb protein unable to bind E2F. This eliminates a significant cell cycle regulation system that would normally stop the cycle prior to the S phase. Some mutations of p53 can prevent it from stimulating p21 when DNA damage exists. Without p21 a cyclin-CDK complex is not inhibited from phosphorylating the Rb protein and releasing E2F. Therefore, tumor suppression is eliminated. Not only does p53 mutation remove suppression, but it removes constraints on DNA fidelity. This increases mutation rates thereby increases the probability sunsequent that cancer inducing mutations will develop. Another function of p53 is to initiate apoptosis when DNA damage goes unrepaired. About 50% of human tumors lack a function p53.