Caveolar Internalization of Growth Hormone

Caveolae are plasma membrane specializations formed by caveolin and characterized by their dependence on membrane cholesterol for structural integrity. We have investigated the role of caveolae in the internalization of GH in CHO cells stably transfected with GH receptor cDNA (CHO–GHR1–638). We show by immunogold electron microscopy that a portion of the GH receptor at the cell surface is localized to or near caveolin-containing structures and upon GH stimulation the receptor aggregates in caveolae. Similarly the hormone is observed to be aggregated in caveolae and a portion of the hormone is internalized into the cell in caveolin-containing vesicles. Disruption of caveolar integrity by sterol-binding agents (filipin, nystatin) partially inhibits internalization of125I-hGH whereas internalization of hormone is not affected by non-sterol-binding agents which also insert into the cell membrane (polymyxin B, xylazine). Transient transfection of caveolin cDNA into CHO cells concomitantly transfected with GH receptor cDNA increases both the internalization of hormone and the GH stimulation of STAT-mediated transcription. In conclusion, we demonstrate that caveolae constitute one pathway for the internalization of GH. Such an internalization pathway may also be utilized by other members of the cytokine receptor superfamily.