High molecular weight isoforms of growth hormone in cells of the immune system

Abstract

A substantial body of research exists to support the idea that cells of the immune system produce growth hormone (GH). However, the structure and mechanism of action of lymphocyte-derived GH continues to remain largely unknown. Here we present the results of Western analysis of whole cell extracts showing that different molecular weight isoforms of GH of approximately 100 kDa, 65 kDa, and 48 kDa can be detected in primary mouse cells of the immune system and in the mouse EL4 cell line. The identity of the 65 kDa and 48 kDa isoforms of GH were confirmed by mass spectrometry. The various isoforms were detected in both enriched T and B spleen cell populations. The large molecular weight isoform appears to reside primarily in the cytoplasm whereas the lower molecular weight 65 kDa and 48 kDa isoforms were detected primarily in the nucleus. These results also suggest that GH isoforms are induced by oxidative stress. In EL4 cells overexpressing GH, the expression of luciferase controlled by a promoter containing the antioxidant response element is increased almost three-fold above control. The data suggest that the induction of isoforms of the GH molecule in cells of the immune system may be an important mechanism of adaptation and/or protection of lymphoid cells under conditions of oxidative stress.

Expression of lymphocyte-derived growth hormone (GH) and GH-releasing hormone receptors in aging rats

“…This observation was seen both in rats (unpublished observation) and mice [20]. We have also shown that greater levels of GH are induced in T cell-enriched populations compared to B cell-enriched populations after culture of cells under hypoxic conditions (unpublished observation).…”

“…6c). In the rat, isoforms of approximately 37, 44, and 48 kDa of lymphocyte GH can be detected where the predominant form is 48 kDa (unpublished observation) whereas in mice, the predominant GH isoform is approximately 62 kDa [20]. The increase in the level of a lower molecular weight GH isoform observed in old rats compared to younger animals could result from proteolysis while higher molecular weight isoforms may represent oligomers or the presence of different glycosylated forms.…”

“…Protein concentration was determined with the Bio-Rad protein assay reagent. The lysate was snap frozen and stored at −70°C until analyzed by Western blotting procedures as previously described [20]. Immunoreactive proteins were visualized using the ECL Western blotting analysis system.…”

“…The sites include the brain [14], mammary gland [15], placenta [16], skin [17], ovary [18], and cells of the immune system [19]. Our results in rodent spleen cells analyzing GH by mass spectrometry and Western analysis have shown that different molecular weight isoforms of GH can be detected in primary mouse spleen T and B cells [20]. In the mouse, we showed that GH isoforms could be induced by oxidative stress and that the larger molecular weight isoform appeared to reside primarily in the cytoplasm whereas the lower molecular weight isoform was primarily detected in the nucleus [20].…”

“…Our results in rodent spleen cells analyzing GH by mass spectrometry and Western analysis have shown that different molecular weight isoforms of GH can be detected in primary mouse spleen T and B cells [20]. In the mouse, we showed that GH isoforms could be induced by oxidative stress and that the larger molecular weight isoform appeared to reside primarily in the cytoplasm whereas the lower molecular weight isoform was primarily detected in the nucleus [20]. Most recently, in the rat we have shown that stressful cellular conditions likely to occur at sites of inflammation or tumor growth such as hypoxia and alterations in pH also induce the synthesis of lymphocyte GH [21].…”