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Deficiency of the extracellular matrix protein latent transforming growth factor-β (TGF-β)-binding protein-4 (LTBP4) results in lack of intact elastic fibers, which leads to disturbed pulmonary development and lack of normal alveolarization in humans and mice. Formation of alveoli and alveolar septation in pulmonary development requires the concerted interaction of extracellular matrix proteins, growth factors such as TGF-β, fibroblasts, and myofibroblasts to promote elastogenesis as well as vascular formation in the alveolar septae. To investigate the role of LTBP4 in this context, lungs of LTBP4-deficient ( Ltbp4−/−) mice were analyzed in close detail. We elucidate the role of LTBP4 in pulmonary alveolarization and show that three different, interacting mechanisms might contribute to alveolar septation defects in Ltbp4−/− lungs: 1) absence of an intact elastic fiber network, 2) reduced angiogenesis, and 3) upregulation of TGF-β activity resulting in profibrotic processes in the lung.

Objective Metformin, the first line drug for treatment of type 2 diabetes, suppresses hepatic gluconeogenesis and reduces body weight in patients, the latter by an unknown mechanism. Methods Mice on a high fat diet were continuously fed metformin in a therapeutically relevant dose, mimicking a retarded formulation. Results Feeding metformin in pharmacologically relevant doses to mice on a high fat diet normalized HbA1c levels and ameliorated glucose tolerance, as expected, but also considerably slowed down weight gain. This was due to increased energy expenditure, since food intake was unchanged and locomotor activity was even decreased. Metformin caused lactate accumulation in the intestinal wall and in portal venous blood but not in peripheral blood or the liver. Increased conversion of glucose-1-13C to glucose-1,6-13C under metformin strongly supports a futile cycle of lactic acid production in the intestinal wall, and usage of the produced lactate for gluconeogenesis in liver. Conclusions The reported glucose–lactate–glucose cycle is a highly energy consuming process, explaining the beneficial effects of metformin given continuously on the development of a type 2 diabetic-like state in our mice., Highlights • Orally administered metformin slowed down weight gain on a high fat diet. • Metformin treatment led to increased energy expenditure, but decreased locomotion. • Metformin treatment caused a futile, energy consuming glucose–lactate–glucose cycle.

LTBP‑4L and LTBP‑4S are two isoforms of the extracellular matrix protein latent transforming growth factor β-binding protein 4 (LTBP‑4). The mutational inactivation of both isoforms causes autosomal recessive cutis laxa type 1C (ARCL1C) in humans and an ARCL1C‑like phenotype in Ltbp4‑/− mice, both characterized by high postnatal mortality and severely affected elastogenesis. However, genetic data in mice suggest isoform-specific functions for Ltbp-4 because Ltbp4S‑/− mice, solely expressing Ltbp‑4L, survive to adulthood. This clearly suggests a requirement of Ltbp‑4L for postnatal survival. A major difference between Ltbp4S‑/− and Ltbp4‑/− mice is the matrix incorporation of fibulin‑4, a key factor for elastogenesis, which is normal in Ltbp4S‑/− mice, whereas it is defective in Ltbp4‑/− mice, suggesting that the presence of Ltbp‑4L might be required for this process. To investigate the existence of a functional interaction between Ltbp‑4L and fibulin‑4, we studied the consequences of fibulin-4 deficiency in mice only expressing Ltbp‑4L. Resulting Ltbp4S‑/−;Fibulin‑4R/R mice showed a dramatically reduced life span compared to Ltbp4S‑/− and Fibulin‑4R/R mice, which survive to adulthood. This dramatic reduction in survival of Ltbp4S‑/−;Fibulin‑4R/R mice correlates with severely impaired elastogenesis resulting in defective alveolar septation and distal airspace enlargement in lung and increased aortic wall thickness with severely fragmented elastic lamellae. Additionally, Ltbp4S‑/−;Fibulin‑4R/R mice suffer from aortic aneurysm formation combined with aortic tortuosity in contrast to Ltbp4S‑/− and Fibulin‑4R/R mice. Together, in accordance with our previous biochemical findings of a physical interaction between Ltbp-4L and fibulin-4, these novel in vivo data clearly establish a functional link between Ltbp-4L and fibulin-4 as a crucial molecular requirement for survival and elastogenesis in mice.