Growth hormone gene expression in normal lymph nodes and lymphomas of the dog

Growth hormone (GH) is involved in the development, maturation, and function of the immune system. Recent studies have demonstrated that GH can be synthesized and secreted by lymphoid tissues, where it may act as an autocrine/paracrine growth factor. To determine whether GH may be involved in the development of hematological malignancies, GH gene expression in canine lymphomas was investigated. GH mRNA was detected in non-tumorous lymph nodes and in the majority of the lymphomas, by RT-PCR analysis. In situ and Northern blot hybridizations were negative. Analysis of the transcriptional start sites of the GH gene using 5'-RACE (rapid amplification of cDNA ends) showed that the canine lymphoid transcripts contained a 33-85 bp enlarged 5'-untranslated region compared to the pituitary and mammary GH transcripts. Part of the lymphoid GH transcripts contained intron 1, which would result in early termination of the translation due to an in-frame stopcodon. GH measurements in lymphoid tissues revealed a low content of immunoreactive GH. The results presented demonstrate that canine lymphoid tissue is an extrapituitary site of GH gene expression. However, GH production appeared to be low, indicating that lymphoid GH is probably not a major factor in the development or progression of canine lymphoma.