Responses of the cell to its environment

In its simplest form, an environmental signal may result in a specific cellular result-for example, the opening of an ion channel or the alteration of a cell's synthetic repertoire by altered transcription of a particular gene, or altered protein stability. Complex cellular responses depend on the integration of signals from several sources, as illustrated in the following key examples determining the overall fate of a cell.

A series of mechanisms operate to ensure that progression to the next stage of the cell cycle 2@2 should only occur if the cell is in the appropriate environment, and if the preceding stage of the cell cycle has been completed successfully. The understanding of this sequential series of controls has stemmed from two observations made in lower organisms:

• Yeast mutants which did not complete the cell cycle stopped at discrete periods, particularly before S phase (G1-S phase arrest) and before mitosis (G2-M phase arrest).
• In frogs, the concentrations of certain proteins were shown to vary through the cell cycle (cyclins).

The cyclins subsequently were shown to be the regulatory subunits for a series of enzymes (cyclin-dependent kinases Cdks). When activated, Cdks phosphorylate downstream proteins to enable the cell cycle to progress. Regulation of Cdk activity after cellular assessment of environmental signals and intracellular events such as the status of DNA or cellular mass permits transient arrest of the cycle at a series of 'checkpoints', as observed in yeast. The same mechanisms have since been shown to operate through all eukaryotic organisms including humans. They facilitate physiological control of cell replication in the appropriate tissue environment or in response to external stimuli. Particularly important is their damage limitation function which allows pauses in the cell cycle to facilitate DNA repair before DNA replication transmits any uncorrected errors.

DNA synthesis in S phase requires the transcription of S-phase factors by members of the E2F transcription factor family. The active transcription factor is inhibited by the normal product of the retinoblastoma gene (PRB). The G1 cyclin-Cdk complexes (cyclin D-Cdk4 and cyclin D-Cdk6) phosphorylate pRB on serine and threonine residues, allowing the phosphorylated pRB to dissociate from E2F-DP1, enabling transcription to proceed (in a manner similar to the control of NFKB). The mRNAs for a variety of S-phase-specific proteins are then synthesised. Cyclin D complexes are activated by growth factors, but this can be overridden by a number of other signals. including inhibitory growth factors and signals indicating DNA damage (e.g. from p53), or that cell-cell contact has been achieved. If these negative signals are present, the cell suspends progression until, for example, DNA damage is repaired.

Escape from these regulatory mechanisms allows cells to pass through the G1-S phase checkpoint and synthesise DNA irrespective of their environment, a cardinal feature of malignancy. This can occur, for example, in the absence of PRB (due to deletion or other mutations in retinoblastomas and other tumours), or by inappropriate pRB phosphorylation-for example, by the human papillomavirus E7 protein in cervical carcinoma.

The complex series of cellular changes during mitosis are mediated by the phosphorylation of certain macromolecules at the onset of mitosis-for example, laminin (to dissolve the nuclear membrane), histones (involved in chromosomal condensation) and microtubules (leading to the formation of the mitotic spindle). The phosphorylation of these molecules is regulated by another cyclin-Cdk complex (cyclin B-Cdc2p34, itself regulated by signals derived from DNA damage and DNA synthesis. This complex is also thought to arrest membrane trafficking, allowing approximately equal redistribution of organelles to daughter cells at the end of mitosis.

The process of differentiation requires a cell to exit the cell cycle, a path which occurs when some cells are deprived of growth factor stimulation. Integration of signals in different cells—some maintained as self-renewing stem cells, others undergoing terminal differentiation—is the essence of development, in which the lineage of a particular cell, as well as the environmental signals which it receives, plays a crucial role in shaping its ultimate fate.

For example, in undifferentiated cells two sets of transcription factors, the MyoD gene family (including myogenin and MyoD itself) and the myocyte enhancer factor (MEF) 2 family, are inhibited by a large number of proteins including helix-loop-helix proteins, regulators of cell cycle progression, and regulators promoting non-muscle cell fates. Myocyte differentiation requires the activation of the MyoD and MEF2 proteins to establish the cell as a muscle cell precursor, ultimately inducing the transcription of muscle-specific genes such as myosin and actin.

Apoptosis refers to the morphological characteristics of cells undergoing programmed cell death, an alternative result of a withdrawal from the cell cycle. Cell types which depend upon continued growth factor stimulation for survival will undergo apoptosis if they are deprived of growth factors, a process again mediated in part by regulation of cyclin-Cdk complexes. Usually this occurs at the critical GI-S phase transition, but some cells such as thymocytes can undergo apoptosis anywhere in the cell cycle. Additional causes of apoptosis include signals of continued DNA damage, or inflammatory-related stimuli, to be discussed.

Programmed cell death occurs in three molecular phases: an initiation event (which is cell-type and stimulus specific), an effector stage when molecules of the Bcl-2 family act as opposing death agonists (e.g. Bax) and antagonists (e.g. Bcl-2) to interact with other apoptosis factors, ultimately activating a cascade of caspases leading to the degradation phase. Apoptosis is of crucial importance in regulating cell numbers without subjecting local tissues to damaging cellular enzymes and contents, since apoptotic cells are engulfed by local macrophages and do not discharge their contents to the exterior.