Angiogenesis and White Blood Cell Proliferation Induced in Mice by Injection of a Prolactin-expressing Plasmid into Muscle

Prolactin (PRL) is a pituitary hormone involved in a broad spectrum of physiological processes, including lactation, development, and immune function. To further investigate the in vivo roles of PRL, rat PRL cDNA, fused to the cytomegalovirus promoter, was introduced into mouse muscle by direct injection. Prolactin mRNA and protein were detected in the muscle following injection. As a result the number of white blood cells (WBC) increased. When injection was combined with adrenalectomy there was an even greater increase. The augmentation of WBCs persisted for at least 20 days after injection of the rPRL plasmid either on its own and after injection combined with adrenalectomy. The increase in WBCs was accompanied in both cases by an increase in blood cell DNA content. We also observed an increase in heart volume, particularly of the left ventricle. Evidence of marked angiogenesis was found in the testis of rPRL-injected mice. New blood vessels were first found at 8 weeks of age and fully developed blood vessels with complex branching patterns were found after 11 weeks. When PRL fused with EGFP was introduced into mice by intramuscular injection, the EGFP localized to areas of the testis that corresponded to the sites of new blood vessel formation. PRL inhibited this binding. Taken together, our data reveal that intramuscularly expressed PRL augments WBC numbers and induces formation of new blood vessels in the testis, suggesting important roles for PRL in hematopoiesis and angiogenesis. They also indicate that direct intramuscular injection of naked DNA can be used effectively to study the function of secreted proteins, including endocrine signaling molecules.