Binding of these hormones to a cell surface receptor results in activation of a signaling pathway this triggers intracellular activity and carries out the specific effects associated with the hormone. Unlike steroid hormones, lipid insoluble hormones do not directly affect the target cell because they cannot enter the cell and act directly on DNA. Lipid insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors. In this way, the steroid hormone regulates specific cell processes as illustrated in Figure 18.5.Īmino acid derived hormones and polypeptide hormones are not lipid-derived (lipid-soluble) and therefore cannot diffuse through the plasma membrane of cells. This protein can be used either to change the structure of the cell or to produce enzymes that catalyze chemical reactions. This, in turn, determines the amount of corresponding protein that is synthesized by altering gene expression.
The hormones and receptor complex act as transcription regulators by increasing or decreasing the synthesis of mRNA molecules of specific genes. The cell signaling pathways induced by the steroid hormones regulate specific genes on the cell’s DNA. The steroid hormones pass through the plasma membrane of a target cell and adhere to intracellular receptors residing in the cytoplasm or in the nucleus. At the target cell, the hormones are released from the carrier protein and diffuse across the lipid bilayer of the plasma membrane of cells. Once outside the cell, they bind to transport proteins that keep them soluble in the bloodstream. Lipid-derived (soluble) hormones such as steroid hormones diffuse across the membranes of the endocrine cell. Depending on the location of the protein receptor on the target cell and the chemical structure of the hormone, hormones can mediate changes directly by binding to intracellular hormone receptors and modulating gene transcription, or indirectly by binding to cell surface receptors and stimulating signaling pathways. Receptor binding alters cellular activity and results in an increase or decrease in normal body processes. When the number of receptors decreases in response to rising hormone levels, called down-regulation, cellular activity is reduced. In up-regulation, the number of receptors increases in response to rising hormone levels, making the cell more sensitive to the hormone and allowing for more cellular activity. Additionally, the number of receptors that respond to a hormone can change over time, resulting in increased or decreased cell sensitivity. The number of receptors that respond to a hormone determines the cell’s sensitivity to that hormone, and the resulting cellular response. Cells can have many receptors for the same hormone but often also possess receptors for different types of hormones. For example, thyroid hormones act on many different tissue types, stimulating metabolic activity throughout the body. Receptors for a specific hormone may be found on many different cells or may be limited to a small number of specialized cells. In this way, even though hormones circulate throughout the body and come into contact with many different cell types, they only affect cells that possess the necessary receptors. Hormones mediate changes in target cells by binding to specific hormone receptors. Discuss the role of different types of hormone receptors.
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