Question

Cell Biology Question;

How can cell signalling control gene expression? (please explain)

Answer 1

Cell signaling is a process by which a signalling molecule such as a hormone or a protein binds to a target receptor on a cell and changes the metabolic rate or gene expression in them.

Genes which are present in every cell of an organism are expressed by the process of transcription and translation. The expressed products are in the form of polypeptides which are then folded to form proteins and then destined for their particular functions.

Through cell signaling it is not possible to alter the genetic makeup of an organism but it is possible to alter the rate of gene expression by changing the rates of mRNA formation through transcription or protein formation through translation.

Hence when a hormone or protein (for example growth hormone) binds to its target cell a cellular response is initiated.

Let's understand this by taking GH response as an example.

When GH binds to its receptor GHR receptor a conformational change in receptor occurs. This change further activates JAK-STAT kinase pathway in which JAK kinase phosphorylates it's target Molecules and activate them and than the phosphorylated molecules further activates others and like this a change of response molecules are initiated.

One of the targets of this chain is a molecule in cell cytoplasm- MNK1. This MNK1 becomes active upon phosphorylation and binds to hairpin loops of mRNA which prevents translation, after the binding of MNK1 hairpin loop gets resolved and translation of mRNA continues hence the proteins are formed. Many key genes and mRNAs of cell division have these hairpin loop mechanism to control the rate of translation, hence MNK1 signalling enhances the rate of cell division.

Second target of the secondary messengers produced by GH signaling are molecules present in nucleus. c-Myc is a molecule which gets activated upon phosphorylation and binds to DNA to enhance the process of transcription. Hence c-Myc is an enhancer of mRNA formation and hence gene expression.

Like this signalling cascades intiatiated by signaling molecules regulates the level of gene expression in target cells.