Acute leukemias express a functional receptor for the human growth hormone

Src transduces signaling via growth hormone (GH)-activated GH receptor (GHR) tyrosine-phosphorylating GHR and STAT5 in human leukemia cells

Most human leukemia cells are shown to express growth hormone receptor (GHR) and some of them proliferate in response to GH. We demonstrate that Src contributes to GHR-mediated signal transduction via STAT5 activation in F-36P human leukemia cells stimulated with GH. The tyrosine phosphorylation levels of GHR and STAT5 induced by GH decreased in the presence of PP2 Src kinase inhibitor. When GHR and wild-type Src were co-expressed in COS7 cells, GHR was markedly tyrosine phosphorylated as well as when Jak2 was co-expressed with GHR, but not when kinase-inactive Src co-expressed. The treatment of F-36P cells with the antisense src oligonucleotides, which selectively decreased the Src expression, reduced the rhGH-induced tyrosine phosphorylation of the STAT5 activation sites.

Growth hormone protects the intestines but not the tumour from 5-fluorouracil toxicity in the short term in the rat

OBJECTIVE

Growth hormone has been proposed as an effective preventative treatment against radiotherapy- and chemotherapy-induced toxicity in the gut. The aim of this study was to determine whether exogenously administered growth hormone modified the effect of 5-fluorouracil on the gut and an implanted colon adenocarcinoma in the rat

METHODS

An adenocarcinoma was implanted into rats that had been treated with 5-fluorouracil and growth hormone 3 days previously. Tumour growth, plus tumour and intestinal pathology, proliferation, apoptosis and p53 expression were determined.

RESULTS

Growth hormone protected the intestines against 5-fluorouracil by increasing proliferation and mucosal length, and decreasing apoptosis and p53 expression. Growth hormone did not modify the effects of 5-fluorouracil on the tumour.

CONCLUSIONS

Growth hormone protects the intestines from the deleterious effects of 5-fluorouracil while preserving its antitumoural action on the adenocarcinoma in the short term. Within the crypt, p53 expression is likely to be modulated by growth hormone after 5-fluorouracil treatment.

Acute leukaemia in acromegaly patients

Acromegaly patients are known to have an increased risk of malignancies, especially colonic adenocarcinoma. This may be as a result of the growth-stimulating effect of growth hormone (GH). The clustering of leukaemia in children treated with GH has also caused concern. There have been a few reports of leukaemia in acromegaly patients. We report two patients with acute lymphoblastic leukaemia and one patient with acute myeloid leukaemia among 106 acromegaly patients treated over a 15-year period. Two of the cases received radiotherapy as part of their treatment. Adjusted for age and follow-up years, the incidence of leukaemia in this cohort is significantly higher than the general population. The incidence is also higher than would be expected as a result of radiotherapy alone, suggesting that GH may play a synergistic role.

No evidence of tumor growth stimulation in human tumors in vitro following treatment with recombinant human growth hormone

In a recent study we demonstrated that recombinant human growth hormone (r-hGH; Saizen) delayed tumor-induced cachexia in human tumor xenografts in vivo. Such a therapeutic effect could have a great impact in the supportive care of advanced cancer patients. Before large clinical studies are initiated possible growth stimulation should be excluded. This question was investigated in vitro in 20 human tumor models, which had been established in serial passage in nude mice. The effect of continuous exposure of r-hGH was investigated at dose levels ranging from 0.3 ng/ml up to 0.1 microg/ml in colorectal (n=2), gastric (n=1), non-small cell lung (n=4), small cell lung (n=1), mammary (n=3), ovarian (n=2), prostate (n=2) and renal cancers (n=2), and melanoma (n=3) using a modified Hamburger and Salmon clonogenic assay. The results show that there was neither tumor growth inhibition nor any evidence for tumor growth stimulation in any of the tumors studies. Therefore this preclinical study in 20 human tumor models indicated no direct risk for tumor growth enhancement.

Role of growth hormone receptor in HL-60 cell survival

Although it is presently well established that locally produced growth hormone (GH) plays a major role in the regulation of survival mechanisms in hemopoietic cells, the responsible mechanisms are poorly understood, and the involvement of the GH receptor (GHR) has not even been demonstrated to date. In this work we investigated the presence of GHR in the human promyelocytic leukemia cell line HL-60, as well as the ability of GH treatment to stimulate both GHR and survival signaling pathways downstream GHR. Our results demonstrate that (1) both GHR mRNA and GHR immunoreactivity are present in HL-60 cells; (2) GH treatment results in an increase in the phosphorylation of the GHR-associated Jak2 and Stat3 proteins, indicating the ability of the hormone to induce receptor activation; and (3) activation of GHR increases the activity of Akt, a serine/threonine kinase that plays a prominent role in the regulation of cell survival. Taken together, these results demonstrate that GHR activation promotes survival of HL-60 cells, thus suggesting that GH plays a major role in the regulation of cell survival in the hemopoietic system, via an autocrine/paracrine mechanism.

Activation of growth hormone receptor delivers an antiapoptotic signal: evidence for a role of Akt in this pathway

A signaling pathway was delineated by which GH promotes cell survival. Experiments were performed in human leukemic cells (HL-60) and Chinese hamster ovary (CHO) cells. In HL-60 cells, GH treatment reduced starvation-induced cell death. In contrast, when HL-60 cells were treated with an anti-GH antibody, cell survival was sharply reduced. In CHO cells stably expressing either the wild-type (wtGHR) or a truncated form (delta454GHR) of the GH receptor in which GH induces a sustained activation of the receptor-associated tyrosine kinase JAK2, we found that GH stimulation inhibited programmed cell death induced by withdrawal of survival factors. This effect was enhanced in cells expressing the truncated form. In contrast, GH did not affect cell survival in CHO cells transfected with either the empty vector or a mutated GHR unable to transduce the signal (4P/AGHR). We also showed that the inhibitory action of GH on apoptosis is probably mediated via stimulation of the serine-threonine kinase Akt, as 1) GH treatment induces a prompt phosphorylation of Akt; and 2) GH effects on cell survival are abolished by transfection of an Akt mutant that exhibits dominant negative function. Experiments with pharmacological inhibitors demonstrated that GH-induced Akt phosphorylation is dependent on phosphoinositide 3-kinase activation. In contrast, we found no changes in Bcl-2 levels secondary to GHR activation.

Leukemia in children treated with growth hormone

An association between treatment with growth hormone (GH) and the development of leukemia was described in 1988. This perceived association is best explained by the fact that there are more children with GH deficiency (GHD) with risk factors predisposing them to leukemia than in the general population. These factors include previous cancers and their treatment, as well as co-existing conditions such as Down, Bloom and Fanconi syndromes. Examination of large databases of GH-treated individuals shows that GH-treated patients without these risk factors do not have an increased incidence of leukemia.

The haematopoietic effects of growth hormone and insulin-like growth factor-I

The process of haemopoiesis, occurring primarily within the bone marrow, involves the proliferation and differentiation of pluripotent haemopoietic stem cells into committed, or pathway-restricted progenitors /1/. The latter further proliferate and undergo a process of maturation into circulating blood cells of myeloid and erythroid lineages /2/. Haemopoietic cell growth and differentiation is primarily regulated by the local production of various cytokines within the bone marrow micro-environment /3/, as well as by the circulating hormone, erythropoietin (EPO). The formation as well as functional activation of mature blood cells, are also modulated by a variety of hormones and growth peptides, including growth hormone (GH) and insulin-like growth factor-I (IGF-I) /4,5/. Early evidence for the role of GH in modulating haemopoiesis was provided in classical studies in rodents, which showed that removal of the pituitary gland affects blood cell formation and function /6/ and that impairment of the latter can be restored by GH administration /7/. GH exerts its effects on target cells by binding to its own receptor, which belongs to the class I cytokine receptor superfamily /8/. In humans, GH can also bind to and activate the prolactin receptor /9/. Based on the somatomedin hypothesis of Salmon and Daughaday /10/, it is now generally accepted that, in addition to the above, GH exerts many of its effects via autocrine or paracrine IGF-I, as well as via endocrine IGF-I produced in the liver. IGF-I, a small single-chain polypeptide, is one of two highly homologous peptides (IGF-I and IGF-II), that stimulate the proliferation and differentiation of a wide variety of cell types, including bone marrow cells /5,11/. Both IGF-I and IGF-II play an important role in prenatal growth and IGF-I is also essential for postnatal growth and development /12/. Two types of IGF receptors have been described. The type I IGF receptor, a tyrosine kinase receptor highly homologous to the insulin receptor, binds IGF-I and IGF-II with a high affinity. The type II/mannose 6-phosphate receptor which lacks intrinsic kinase activity, binds IGF-II with a high affinity and IGF-I with a low affinity /13,14/. Haemopoietic progenitors and mature blood cells have been shown to produce GH and IGF-I and to express receptors for these peptides. GH and IGF-I may act independently on these cells or, as more commonly observed, in a synergistic manner with primary haemopoietic cytokines. GH and IGF-I receptors are also present on freshly explanted leukemic cells and leukemic cell lines. Thus, their possible contribution to the development of leukemia should also be considered. This review summarizes our current understanding of the role of GH and IGF-I in normal and malignant haemopoiesis.