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Human thymocytes express a prolactin-like messenger ribonucleic acid and synthesize bioactive prolactin-like proteins
D W Montgomery1, G K Shen, E D Ulrich
1Department of Pharmacology, University of Arizona College of Medicine, Tucson 85724.
Insights
Normal human lymphocytes, including thymocytes and peripheral blood lymphocytes (PBL), synthesize bioactive prolactin (PRL). This discovery reveals PRL
Area of Science:
- Immunology
- Endocrinology
- Cell Biology
Background:
- Pituitary prolactin (PRL) plays a key immunoregulatory role.
- PRLs are produced by transformed lymphocytes and implicated in regulating lymphocyte proliferation.
- PRL synthesis by normal human lymphocytes has not been previously reported.
Purpose of the Study:
- To investigate the synthesis and characteristics of PRL produced by normal human lymphocytes.
- To determine if human thymocytes and peripheral blood lymphocytes (PBL) produce PRL.
- To analyze the size, bioactivity, and regulation of lymphocyte-derived PRL.
Main Methods:
- Primary culture of human thymocytes and peripheral blood lymphocytes (PBL).
- Analysis of PRL size variants using gel electrophoresis and Western blotting.
- Assessment of PRL bioactivity using the Nb2 node lymphoma bioassay.
- Detection of PRL mRNA via Northern blot analysis.
Main Results:
- Human thymocytes and PBL synthesize PRL in primary culture.
- Thymocytes primarily produce a 24-kDa PRL, while PBLs produce a 27-kDa variant, with heterogeneity observed (21-29 kDa).
- A low molecular weight (11 kDa) PRL form is also synthesized and released; both 24- and 11-kDa forms exhibit bioactivity.
- PRL expression is mitogen-regulated in thymocytes but constitutive in PBL.
- A single, larger PRL-like mRNA transcript was detected in thymocytes.
Conclusions:
- Normal human lymphocytes synthesize bioactive PRLs comparable to pituitary PRL.
- Size variations in lymphocyte-derived PRL are likely due to post-translational modifications like proteolysis and glycosylation.
- This finding expands the known sources and functions of PRL within the immune system.
Abstract:
Recent evidence has demonstrated an important immunoregulatory role for pituitary PRL. Moreover, PRLs have been identified as products of transformed human lymphocyte cell lines and normal murine lymphocytes, and implicated as regulators of their proliferative responses. However, PRL synthesis by normal human lymphocytes has not yet been reported. Here we demonstrate that human thymocytes and peripheral blood lymphocytes (PBL) synthesize PRL in primary culture. The principal form produced by thymocytes is 24 kilodaltons (kDa), essentially the same size as pituitary PRL, while PBL produced a 27-kDa variant. Size heterogeneity was evident, with products detected ranging from 21-29 kDa in various tissue samples, a phenomenon also found to occur in human pituitary and decidual PRL. Thymocytes and PBLs also synthesized a low mol wt form (11 kDa) that was released into culture supernatants concurrently with the larger PRL. The 24- and 11-kDa forms expressed PRL-like bioactivity in the Nb2 node lymphoma bioassay, further supporting their PRL-like nature. Expression of these PRLs was regulated by mitogen stimulation in thymocytes, but was constitutively produced in PBL. Northern blot analysis of thymocyte RNA using a human PRL cDNA probe detected a single PRL-like mRNA, which was significantly larger than human pituitary PRL mRNA. This was constitutively present in unstimulated thymocytes. Taken together, these data demonstrate that normal human lymphocytes synthesize bioactive PRLs similar in size to those produced by the pituitary. The presence of a single PRL mRNA suggests that the size variation observed in these proteins is probably due to posttranslational modification, such as proteolysis and glycosylation.