The role of the cytoskeleton in hormone action.

The structure and functions of microfilaments and microtubules is briefly reviewed. Based on evidence from a variety of cells with various experimental approaches, it is proposed that the cytoskeleton is important in the functions of cells as follows: the cytoskeleton appears to be involved in attachment (to other cells and to substrata used in cell culture) and in movement; the cytoskeleton appears to be involved in the complex relationship between the surface and the interior of the cell. This is a two-way relationship in which the cell uses the cytoskeleton to influence the distribution of proteins within the plasma membrane and perhaps as a go-between to transmit information from the surface of the cell to the interior. Within the cell, the cytoskeleton appears to coordinate intracellular activities (e.g., between nucleus and cytoplasm) and to provide a compartmentation which restricts the movements of intracellular structures (from molecules to organelles), thereby facilitating productive encounters between such structures with an efficiency that is presumed to exceed that of random motion. The cytoskeleton may arrange the various components of the cell in space; it may direct intracellular traffic and promote intracellular movement. This loose picture of cytoskeletal activity may be more emphatic than the available data warrant. It should be regarded as informed extrapolation based upon fragments of information. When these ideas are applied to endocrine function, we can see that the secretion of hormones (exemplified by thyroid hormones and insulin) requires cytoskeletal activity to provide direction and perhaps to promote movement. In steroidogenic cells the movement of cholesterol from cytoplasm to mitochondria requires microfilaments. The antidiuretic hormone requires microfilaments and microtubules to promote transport of water through cells in response to osmotic gradients. Microfilaments adjust the compliance of the cell to provide a path for water and microtubules influence the permeability of the cell membrane to water.