What is the test for determining whether a challenged statement is a matter of public interest?

interest should be something of concern to a substantial number of people. [Citation.] Thus, a matter of concern to the speaker and a relatively small, specific audience is not a matter of public interest. [Citations.] Third, there should be some degree of closeness between the challenged statements and the asserted public interest [citation]; the assertion of a broad and amorphous public interest is not sufficient [citation]. Fourth, the focus of the speaker's conduct should be the public interest rather than a mere effort to gather ammunition for another round of [private] controversy.... [Citation.] ... A person cannot turn otherwise private information into a matter of public interest simply by communicating it to a large number of people. (Weinberg v. Feisel (2003) 110 Cal.App.4th 1122, 11321133, 2 Cal.Rptr.3d 385.)