The 16 kDa N-terminal fragment of prolactin (16K-prolactin) is a potent antiangiogenic factor. Here, we demonstrate that matrix metalloproteases (MMPs) produced and secreted by chondrocytes generate biologically functional 16K-prolactin from full-length prolactin. When incubated with human prolactin at neutral pH, chondrocyte extracts and conditioned medium, as well as chondrocytes in culture, cleaved the Ser155-Leu156 peptide bond in prolactin, yielding - upon reduction of intramolecular disulfide bonds - a 16 kDa N-terminal fragment. This 16K-prolactin inhibited basic fibroblast growth factor (FGF)-induced endothelial cell proliferation in vitro. The Ser155-Leu156 site is highly conserved, and both human and rat prolactin were cleaved at this site by chondrocytes from either species. Conversion of prolactin to 16K-prolactin by chondrocyte lysates was completely abolished by the MMP inhibitors EDTA, GM6001 or 1,10-phenanthroline. Purified MMP-1, MMP-2, MMP-3, MMP-8, MMP-9 and MMP-13 cleaved human prolactin at Gln157, one residue downstream from the chondrocyte protease cleavage site, with the following relative potency: MMP-8>MMP-13 >MMP-3>MMP-1=MMP-2>MMP-9. Finally, chondrocytes expressed prolactin mRNA (as revealed by RT-PCR) and they contained and released antiangiogenic N-terminal 16 kDa prolactin (detected by western blot and endothelial cell proliferation). These results suggest that several matrix metalloproteases in cartilage generate antiangiogenic 16K-prolactin from systemically derived or locally produced prolactin.

Activation of endothelial nitric oxide synthase (eNOS) and subsequent nitric oxide production (NO) are events that mediate the effect of important angiogenic, vasopermeability, and vasorelaxation factors, including vascular endothelial growth factor (VEGF), bradykinin (BK), and acetylcholine (ACh). The N-terminal 16-kDa fragment of prolactin (16K-PRL) acts on endothelial cells to inhibit angiogenesis both in vivo and in vitro. Here, we show that 16K-PRL inhibits VEGF-induced eNOS activation in endothelial cells. Inhibition of eNOS activation may mediate the antiangiogenic properties of 16K-PRL, because the NO donor (Z)-1-[2-(2-aminoethyl)- N-(2-ammonio-ethyl)amino]diazen-1-ium-1,2-diolate (DETANONOate) prevented 16K-PRL from blocking the VEGF-induced proliferation of endothelial cells. In addition, 16K-PRL inhibited eNOS activation by BK and blocked the BK-evoked transient increase in intracellular Ca(2+) in endothelial cells. This finding suggests that 16K-PRL interferes with the mobilization of intracellular Ca(2+), thereby inhibiting the Ca(2+)-dependent activation of eNOS. Blockage of eNOS activation can lead to inhibition of vasodilation. Consistently, 16K-PRL inhibited BK-induced relaxation of coronary vessels in isolated perfused guinea pig hearts. Moreover, 16K-PRL inhibited eNOS activation induced by ACh, and this action resulted in the inhibition of both ACh-evoked relaxation of coronary vessels in isolated perfused rat hearts and ACh-induced, endothelium-dependent relaxation of rat aortic segments. In conclusion, 16K-PRL can block the Ca(2+)-mediated activation of eNOS by three different vasoactive substances, and this action results in the inhibition of both angiogenesis and vasorelaxation.

Excessive accumulation of body fat triggers insulin resistance and features of the metabolic syndrome. Recently, evidence has accumulated that obesity, type 2 diabetes, and metabolic syndrome are associated with reduced levels of serum prolactin (PRL) in humans and rodents, raising the question of whether low PRL levels contribute to metabolic dysfunction. Here, we have addressed this question by investigating the role of PRL in insulin sensitivity and adipose tissue fitness in obese rodents and humans. In diet-induced obese rats, treatment with PRL delivered via osmotic mini-pumps, improved insulin sensitivity, prevented adipocyte hypertrophy, and reduced inflammatory cytokine expression in visceral fat. PRL also induced increased expression of Pparg and Xbp1s in visceral adipose tissue and elevated circulating adiponectin levels. Conversely, PRL receptor null mice challenged with a high-fat diet developed greater insulin resistance, glucose intolerance, and increased adipocyte hypertrophy compared with wild-type mice. In humans, serum PRL values correlated positively with systemic adiponectin levels and were reduced in insulin-resistant patients. Furthermore, PRL circulating levels and PRL produced by adipose tissue correlated directly with the expression of PPARG, ADIPOQ, and GLUT4 in human visceral and sc adipose tissue. Thus, PRL, acting through its cognate receptors, promotes healthy adipose tissue function and systemic insulin sensitivity. Increasing the levels of PRL in the circulation may have therapeutic potential against obesity-induced metabolic diseases.

These results demonstrate that PRL is synthesized and cleaved to antiangiogenic 16K-PRL by retinal tissue and that these molecules play a key role in preventing angiogenesis in the healthy retina.