The role of protein kinase C in GH secretion induced by GH-releasing factor and GH-releasing peptides in cultured ovine somatotrophs.
Wu. D D; Clarke. I J IJ; Chen. C C
Key Findings
- GHRP‑2 (but not GHRP‑6) moves PKC from the cytosol to the cell membrane, indicating PKC activation.
- Inhibiting PKC partially cuts GH release from GHRP‑2; inhibiting PKA completely blocks it.
- Down‑regulating PKC eliminates GH release from GHRP‑2, GRF, and even GHRP‑6, showing PKC’s essential role.
- PKC and PKA pathways interact (cross‑talk) to amplify GH secretion in ovine somatotrophs.
Practical Outcomes
- For biohackers using GHRP‑2 to boost growth hormone, the study suggests that the peptide’s effect depends on both PKC and PKA signaling. Strategies that support PKC activity (e.g., avoiding strong PKC inhibitors) might enhance GH release, while PKA activation remains crucial. However, because the work is in sheep cells, direct translation to human dosing is uncertain, so any protocol tweaks should be approached cautiously.
Summary
In sheep pituitary cells, the growth‑hormone‑releasing peptide GHRP‑2 triggers GH release by activating both the PKC and the classic cAMP‑PKA pathways, and these two signaling routes help each other. Blocking PKC or PKA reduces the GH‑boosting effect of GHRP‑2, showing both are needed for full activity.
Abstract
The involvement of protein kinase C (PKC) in the action of GH-releasing factor (GRF) and synthetic GH-releasing peptides (GHRP-2 and GHRP-6) was investigated in ovine somatotrophs in primary culture. In partially purified sheep somatotrophs, GRF and GHRP-2 caused translocation of PKC activity from the cytosol to the cell membranes and caused GH release in a dose- and time-dependent manner. GHRP-6 did not cause PKC translocation. The PKC inhibitors, calphostin C, staurosporine and chelerythrine, partially reduced GH release in response to GRF and GHRP-2 at doses which selectively inhibit PKC activity. These inhibitors totally abolished GH release caused by phorbol 12-myristate 13-acetate (PMA). Down-regulation of PKC by the treatment of cells with phorbol 12,13-dibutyrate for 16 h caused a significant (P < 0.001) reduction in total PKC activity and totally abolished PKC translocation in response to a challenge with GRF, GHRP-2 or PMA. In addition, down-regulation abolished GH release in response to GRF, GHRP-2 or GHRP-6. Treatment of cells with H89, a selective PKA inhibitor, totally blocked GH release caused by either GRF or GHRP-2 and partially reduced PMA-induced GH release. H89 had no effect on PKC translocation caused by GRF, GHRP-2 or PMA and did not affect GH release caused by GHRP-6. These data suggest that GHRP-2 and GRF activate PKC in addition to stimulating adenylyl cyclase activity. Although the cAMP-protein kinase A (PKA) pathway is the major signalling pathway employed by GRF and GHRP-2, the activation of PKC may potentiate signalling via the cAMP-PKA pathway in ovine GH secretion. Importantly, the effect of PMA in increasing the secretion of GH from ovine somatotrophs is effected, in part, by up-regulation of the cAMP-PKA pathway. We conclude that there is cross-talk between the PKC pathway and the cAMP-PKA pathway in ovine somatotrophs during the action of GRF or GHRP.
Study Information
pubmed
1997
10.1677/joe.0.1540219