Differential isoform distribution between stored and secreted prolactin

Area of Science:

• Endocrinology
• Molecular Biology
• Cell Biology

Background:

• Prolactin (PRL) is synthesized and secreted by mammotroph cells in the anterior pituitary.
• PRL exists in multiple molecular forms, including three isoforms with the same molecular weight but different net molecular charges.

Purpose of the Study:

• To investigate the relative proportions of newly synthesized PRL isoforms in intracellular stores versus secreted forms.
• To determine if the secretion of PRL isoforms is proportional to their intracellular abundance.

Main Methods:

• Primary rat anterior pituitary cells were cultured and labeled with [35S]methionine.
• Intracellular and secreted proteins were analyzed using one- and two-dimensional polyacrylamide gel electrophoresis.
• Autoradiography and densitometric scanning quantified the relative proportions of PRL isoforms.
• Cysteamine was used to differentiate between newly synthesized and stored hormone.

Main Results:

• Intracellular PRL isoforms were found in proportions of ~14% (isoform 1), ~72% (isoform 2), and ~15% (isoform 3).
• Secreted PRL showed significantly different proportions: ~60% (isoform 1), ~20% (isoform 2), and ~11% (isoform 3).
• Cysteamine experiments confirmed that newly synthesized isoforms were secreted non-proportionally.

Conclusions:

• The secretion of PRL isoforms is a complex process, not a simple proportional release of all forms.
• Non-proportional release cannot be explained by the release of a single isoform per functional mammotroph subpopulation.