Cells usually do not live in isolation. Cells lie in the form of groups in diverse tissues and cells. Therefore, cell to cell communication network is needed to coordinate the growth, differentiation and metabolism of the cells. Direct cell to cell contact is a way of communication in cells present in small groups. But cells communicating over large distances need extra cellular products which act as signals. These extra cellular products are synthesized and released by signaling cells and then they move towards the other cells and induce response on the target cells that have receptors for signal molecules. Receptors are usually present on cell surface but sometimes they are present inside the cell as well.
Cell signaling stimulates various actions like animals overall growth and behavior, regulation of wound healing or stimulation of digestive enzymes. There are three types of signalling, depending upon distance over which the signal acts
- Autocrine Signaling Cells respond to substances that they themselves release. Example, cultured cells often response to growth factors that they secrete, cytokine interleukin-1 in monocytes.
- Paracrine Signaling The target cell in this type of signaling is close to the signaling cell and the signaling compound only effects group of target cells adjacent to it. Example neurotransmitters and neurohormones.
- Endocrine Signaling Signaling substances act on a distant group of target cells. These types of substances in animals are usually carried by the blood from its site of release to its target.
Communication by cellular signals usually involves six steps
- Synthesis of signaling molecule.
- Release of the chemical i.e. signaling molecule by the signaling cell.
- Transport to the target cell.
- Detection of the signal by a specific receptor protein.
- A change in cellular metabolism, triggered by the receptor-signaling molecule complex.
- Removal of the signal, often terminating the cellular response.
Hormones and their receptors
Hormones are the chemical substances produced by the body that regulate the activity of certain cells or organs. Most hormones circulate in blood and come in contact with essentially all cells. A particular cell is target cell for a signaling substance (hormones) if it contains functional receptors for that hormone. A signaling substance is referred to as ‘ligand’. Some hormones bind to cell surface receptors and some bind to receptors in the cell. Hormones fall into three broad categories
- Small lipophilic molecules that diffuse through the plasma membrane and interact with cytosolic or nuclear receptors. e.g., steroid hormone, thyroxine.
- Hydrophilic molecules that bind to cell-surface receptors, e.g., eicosanoids.
- Lipophilic molecules that bind to plasma-membrane receptors. e.g., insulin, histamine.
Some signaling molecules alter the activity of one or more enzymes present in target cells and some alter pattern of gene expression. Hormones like steroid hormones are a group of hormones that interacts with intracellular receptors. After crossing through plasma membrane steroids interacts with nucleus or cytosolic receptors and form complexes that accumulate in the nucleus. These complexes can either increase or decrease transcription rate of adjacent genes and some can affect stability of specific mRNA‘s. Hormone Analogs These analogs are chemically synthesized molecules that help in understanding of the receptor action. These analogs fall into two categories i. Agonists These are the molecules which mimic hormone function by binding to the receptor causing normal response.
Antagonists These molecules bind to the receptor but do not activate hormone induced functions.
These are the proteins found on the surface of certain cells and within the cells. There are two types of receptors cell surface receptors and receptors present inside the cell. Some hormones bind to cell-surface receptors, others to the receptors inside the cell. Cell surface receptors, peptide and protein hormones, prostaglandins, amino acids, epinephrine, and related compounds, bind to cell-surface receptors, triggering an increase or decrease in the cytosolic concentration of cAMP, Ca2+, 1,2-diacylglycerol or some other second messengers. Cytosolic or nuclear receptors steroids, thyroxin and retinoic acid being very hydrophobic, are transported by carrier proteins in the blood. After dissociation from the carrier, the hormones enter the cell, bind to the specific receptors in the cytosol or nucleus and act on the nuclear DNA to alter transcription of specific genes.