Biology of cells Unit 2: Cell communication Homeostasis Cell signalling Hormones PART 1

What is a hormone?

A hormone is a substance secreted directly into the blood by discrete specialised cells in response to a specific stimulus (neural or blood borne) and in amounts which vary with the strength of the stimulus and transported to a distant target tissue where it exerts specific effects

Endocrine glands

• Responsible for the production of hormones • They secrete the hormones to the bloodstream


• Other organs can also produce hormones

Endocrine substances

(defined as hormones) secreted directly into the blood stream - transported to distant target tissue to exert its actions

Paracrine hormone released by

y a gland cell diffuses through the extracellular space to its target cell within the same gland

Autocrine

hormone released by a cell - exerts its actions on the same cell

Neuroendocrine

A neuronal hormone released into a synaptic cleft adjacent to where the neurone contacts the target cell

Classification of types of hormones

• Protein and peptide hormones eg. Insulin, luteinising hormone (LH)

• steroid hormones eg. Cortisol, oestradiol

• hormones derived from tyrosine eg. thyroxine and norepinephrine

• eicosanoids eg. prostaglandins and leukotrienes

Protein and peptide hormones

These hormones are synthesized from amino acids Stored in secretory vesicles until needed Vary in size:

some are very small and consist of only a single chain of amino acids

• Thyrotropin releasing hormone (TRH) consists of only 3 amino acid (aa) residues.

• Gonadotrophin releasing hormone (GnRH) consists of 10 aa.

Glycoprotein peptide hormones

Glycoprotein hormones are large molecules.

Gonadotrophins (luteinising hormone, LH and follicle stimulating hormone, FSH) and thyroid stimulating hormone (TSH) from the anterior pituitary consist of two distinct peptide chains (called a and b subunits) which are linked together by non-covalent bonds.

The α subunit is common to LH, FSH, TSH (also human chorionic gonadotrophin, hCG); biological specificity is conferred by the β subunit

They are glycosylated

Protein/peptide hormone action

Hydrophilic Bind to cell surface receptors (which receptor types?)
Initiate downstream signalling pathways that mediate biological effects

Steroid hormones

The steroids are a large class of lipids which are synthesized mainly in the adrenal cortex, testis, ovary and placenta from a common precursor cholesterol.

They all have a common ring structure which consists of 3 six-membered rings and 1 five membered ring.

Steroid hormone action

• Hydrophobic
• transported in the blood stream bound to carrier proteins
• can pass through the cell membrane
• Bind to receptors either in the cytoplasm or the nucleus
• These receptors move to the nucleus and dimerise in order to regulate gene transcription

• Also, alternative signalling pathways including non genomic action

Hormones derived from tyrosine

Include adrenaline, noradrenaline, dopamine and the thyroid hormones thyroxine (T4) and triiodothyronine (T3)


Whilst these hormones are all derived from tyrosine, they have very different properties.

Adrenaline, noradrenaline and dopamine are

e hydrophilic They bind to cell surface receptors (GPCRs)

Thyroid hormones

Thyroid hormones thyroxine (T4) and triiodothyronine (T3) are hydrophobic

They enter cells (via transporter proteins) and bind to nuclear receptors

Eicosanoids

Two major classes of messengers
• Prostaglandins (PGs)
• Leukotrienes (LTs)

They bind to GPCRs on target cells

Derived from arachidonic acid (AA) AA is converted to LTs by the action of 5-lipoxygenase

AA is converted to PGs by the action of cyclooxygenase (COX)

Summary

Hormones are produced primarily by endocrine glands • They can act in an endocrine, paracrine, autocrine or neuroendocrine manner
• They can be classified into four groups
• Protein and peptide hormones
• Hormones derived from tyrosine
• Steroid hormones
• Eicosanoids
• They exert their action by binding on receptors on target cells • Hydrophobic hormones are transported in the blood bound to carrier proteins