Insulin-like growth factors, their binding proteins, and transforming growth factor-beta 1 in oxidant-arrested lung alveolar epithelial cells.

The epithelium of the pulmonary alveolus is a major target for oxidant injury, and its proper repair following injury is dependent on the proliferative response of its stem cells, the type 2 cells. We have recently shown that hyperoxia arrests proliferation of an immortalized type 2 cell line (SV40T-T2) and that expression of several growth-related genes, normally induced near the G1/S and boundary was altered with a block of translation of their mRNA. In the present study we examined the possible role of the insulin-like growth factor (IGF) system and of transforming growth factor-beta 1 (TGF-beta 1) in the arrest of proliferation induced by hyperoxia. We show that IGF-binding protein 2 (IGFBP-2) accumulates to higher levels in culture medium of SV40T-T2 cells whose proliferation has been arrested by hyperoxia. This proliferation arrest is associated with increased expression of IGFBP-2 mRNA and with induction of type 2 IGF receptor and IGF-II mRNAs. When O2-arrested cells were allowed to resume proliferation in normoxia, the level of expression of these genes rapidly decreased to control levels. We also, found that TGF-beta 1 was induced by O2 exposure, that TGF-beta 1 inhibited SV40T-T2 proliferation, and that TGF-beta 1 itself was a potent stimulator of IGFBP-2 expression. These studies suggest a regulatory link between components of the IGF system and TGF-beta 1 in hyperoxic control of cell proliferation of alveolar epithelial cells.

Presence and characterization of the somatostatin precursor in normal human pituitaries and in growth hormone secreting adenomas.

Normal and tumoral human pituitaries release in vitro SRIH and contain messenger RNAs encoding preproSRIH. In the present study, we document the presence and characterization of the SRIH precursor in both human normal pituitaries and in GH-secreting adenomas. Molecular sieve filtration of normal pituitary and adenoma acid extracts revealed the presence of three immunoreactive SRIH peaks, distinct from SRIH 1-28 and SRIH 1-14. Western blot analysis performed under both nonreducing and reducing conditions confirmed the presence of the three different immunoreactive forms with respective molecular masses estimated as 21, 17, and 12 kilodaltons. When analyzed in reverse phase high performance liquid chromatography, the three immunoreactive SRIH material contained in the three peaks coeluted with the proSRIH extracted from rat hypothalamus and from a neuroblastoma cell line (N2A) transfected with the human preproSRIH complementary DNA. None of these three forms could bind concanavalin A. Digestion of the three forms with a specific endopeptidase resulted in the generation of SRIH 1-14, indicating that they can be considered as SRIH precursor or related forms. Estimation of proSRIH amount after molecular sieve filtration indicated that normal human pituitaries (n = 4) contained proSRIH in variable amounts (from 26.5-303 pg/mg proteins), the 21-, 17-, and 12-kilodalton forms being always present, in addition to SRIH 1-28. In contrast, among the 21 adenomas tested, only 11 contained proSRIH material (from 9-4480 pg/mg proteins). Moreover, neither SRIH 1-28 nor SRIH 1-14 could be detected in these adenomas. These data demonstrate the presence of high molecular mass SRIH in normal pituitaries which represent unprocessed proSRIH material. This provides an additional argument for a local synthesis of SRIH. Additionally, the fact that only 50% of GH-secreting pituitary adenomas here studied contain exclusively proSRIH may reflect an alteration in messenger RNA translation or in proSRIH processing, or both, resulting in the absence of mature SRIH in these adenomas.

Autocrine and paracrine effects of peptides on human pituitary cells.

In contrast to normal human pituitaries, GH-secreting adenomas cannot process in vivo ProSRIH whereas they do it in vitro. The existence of an endogenous factor able to inhibit ProSRIH processing in vivo was postulated and such a role was analyzed for GHRH. Results showed that when GH adenomas are incubated in vitro with GHRH 10(-8) M, their ProSRIH contents are decreased, percent inhibition being negatively correlated to the amount of endogenously released GHRH. When incubation is performed in the presence of GHRH antibody in order to block the effect of endogenous GHRH, Pro-SRIH content is increased. The same effects are observed on SRIH release: inhibition by GHRH, stimulation by GHRH antibody. Normal rabbit serum had no effect. It may therefore be concluded that the absence of ProSRIH maturation observed in adenomas in vivo may be the consequence of the GHRH release that is known to be higher from GH adenomas than from normal pituitaries.